Positive and negative regulation of transferred nif genes mediated by indigenous GlnR in Gram-positive Paenibacillus polymyxa

Ammonia is a major signal that regulates nitrogen fixation in most diazotrophs. Regulation of nitrogen fixation by ammonia in the Gram-negative diazotrophs is well-characterized. In these bacteria, this regulation occurs mainly at the level of nif (nitrogen fixation) gene transcription, which requires a nif-specific activator, NifA. Although Gram-positive and diazotrophic Paenibacilli have been extensively used as a bacterial fertilizer in agriculture, how nitrogen fixation is regulated in response to nitrogen availability in these bacteria remains unclear. An indigenous GlnR and GlnR/TnrA-binding sites in the promoter region of the nif cluster are conserved in these strains, indicating the role of GlnR as a regulator of nitrogen fixation. In this study, we for the first time reveal that GlnR of Paenibacillus polymyxa WLY78 is essentially required for nif gene transcription under nitrogen limitation, whereas both GlnR and glutamine synthetase (GS) encoded by glnA within glnRA operon are required for repressing nif expression under excess nitrogen. Dimerization of GlnR is necessary for binding of GlnR to DNA. GlnR in P. polymyxa WLY78 exists in a mixture of dimers and monomers. The C-terminal region of GlnR monomer is an autoinhibitory domain that prevents GlnR from binding DNA. Two GlnR-biding sites flank the -35/-10 regions of the nif promoter of the nif operon (nifBHDKENXhesAnifV). The GlnR-binding site Ⅰ (located upstream of -35/-10 regions of the nif promoter) is specially required for activating nif transcription, while GlnR-binding siteⅡ (located downstream of -35/-10 regions of the nif promoter) is for repressing nif expression. Under nitrogen limitation, GlnR dimer binds to GlnR-binding siteⅠ in a weak and transient association way and then activates nif transcription. During excess nitrogen, glutamine binds to and feedback inhibits GS by forming the complex FBI-GS. The FBI-GS interacts with the C-terminal domain of GlnR and stabilizes the binding affinity of GlnR to GlnR-binding site Ⅱ and thus represses nif transcription.


Introduction
Biological nitrogen fixation, the conversion of atmospheric N 2 to ammonia (NH 3 ), is carried out by a specialized group of prokaryotes and plays an important role in world agriculture [1]. Yet the great demands for nitrogen in modern agriculture far outstrip this source of fixed nitrogen, and chemical nitrogen (N) fertilizer is used extensively in agriculture. Overuses of N fertilizer in many parts of the world have led to soil, water, and air pollution [2].
Ammonia is a major signal that regulates nitrogen fixation in most diazotrophs [3,4]. Regulation of nitrogen fixation in the Gram-negative diazotrophs is well-characterized. In these bacteria, this regulation occurs mainly at the level of nif gene transcription, which requires a nif-specific activator, NifA [5]. NifA acts as an enhancer binding protein (EBP) that recognizes sequences (TGT-N10-ACA), located upstream of the -24/-12 region of the promoters controlled by RNA polymerase containing the alternative σ 54 factor [3,[6][7][8].
Paenibacillus is a large genus of Gram-positive, facultative anaerobic, endospore-forming bacteria. The genus Paenibacillus currently comprises more than 150 named species, approximately 20 of which have nitrogen fixation ability, including eight novel species described by our laboratory [9]. Diazotrophic Paenibacilli has been extensively used as a bacterial fertilizer in agriculture [10]. However, the regulation mechanism of nitrogen fixation in response to nitrogen availability in Paenibacilli is not clarified, partially due to hardness in genetic transformation of these bacteria. Our recent studies by comparative genomic sequence analysis have revealed that a minimal and compact nif cluster comprising nine genes (nifB nifH nifD nifK nifE nifN nifX hesA nifV) encoding Mo-nitrogenase is conserved in 15 N 2 -fixing Paenibacillus strains [11]. Phylogeny analysis suggests that the ancestral Paenibacillus did not fix nitrogen. The N 2 -fixing Paenibacillus strains were generated by acquiring the nif cluster via horizontal gene transfer (HGT) from a source related to Frankia [11]. The 9 genes (nifBHD-KENXhesAnifV) within the nif cluster are organized as an operon under control of a σ A (σ 70 )dependent promoter located in front of nifB gene [12]. A global transcriptional profiling analysis revealed that nif gene transcription in P. polymyxa WLY78 was strongly regulated by ammonium and oxygen [13]. However, unlike Gram-negative diazotrophs, diazotrophic Paenibacilli have no nifA gene encoding transcriptional activator NifA and no NifA-binding site in the nif promoter region. But a glnR gene and GlnR/TnrA-binding sites in the promoter region of the nif operon are conserved in the 15 diazotrophic Paenibacillus strains by comparative genomics analyses [11], indicating the role of GlnR as a regulator of nitrogen fixation. GlnR is a central regulator of nitrogen metabolism in the class Bacilli, and the glnR gene in the diazotrophic Paenibacilli is not associated with the transferred nif gene cluster, indicating that Paenibacillus GlnR is indigenous. The recent studies with Surface Plasmon Resonance (SPR) experiments have demonstrated that GS stabilizes the binding of GlnR to nitrogen fixation gene operators in Paenibacillus riograndensis SBR5 [14]. However, these studies did not fully investigate the regulatory mechanism of GlnR in nitrogen fixation.
GlnR and TnrA are the two transcriptional regulators for the regulation of nitrogen metabolism in the Gram-positive model organism Bacillus subtilis [15,16]. They were previously recognized as the members of the MerR family regulators according to their common winged-HTH (helix-turn-helix) domains [17]. However, the recent studies have revealed that TnrA and GlnR are a new family of dimeric DNA-binding proteins with C-terminal, flexible, effector-binding sensors that modulate their dimerization that represents a separate branch of the MerR family proteins [18]. TnrA/GlnR form weak dimers by hydrophobic residues located on its winged-HTH and residues in its N-terminal helix [18], whereas MerR proteins form tight dimers via their extended C-terminal coiled coils [19]. Both of GlnR and TnrA proteins of B. subtilis have a high sequence similarity at their N terminal domains and bind a common consensus sequence (5'-TGTNAN7TNACA-3'), but the C terminal domains of these proteins differ completely [20][21][22][23][24]. GlnR of B. subtilis generally acts as a repressor repressing gene or operons required for ammonium assimilation like the glnRA operon, tnrA and ureABC (the urease gene cluster) under nitrogen-excess condition [15,16,25]. In contrast, TnrA serves in most cases as an activator, for instance activating ammonia transport (nrgAB = amtBglnK), ureABC, nitrate and nitrite reduction (nasABCDEF) and its own gene (tnrA) [23], whereas in a few cases, it acts like GlnR as a repressor repressing alsT (encoding an H + /Na + amino acid symporter) [26], gltAB (encoding glutamate synthase) [27,28] and ilvBHC-leuABCD (encoding branched-chain amino acid biosynthesis proteins) [29]. During excess nitrogen, glutamine (Gln) binds to and feedback inhibits glutamine synthetase (GS, the product of glnA)) by forming the complex FBI-GS. Formation of the feedback-inhibited GS (FBI-GS) signals the presence of excess nitrogen and transmits that signal by interacting with and affecting the DNAbinding and transcription programs of both GlnR and TnrA. Under nitrogen limitation, the C-terminal region of GlnR folds back and forms an autoinhibitory helix that prevents dimer formation and thus inhibits DNA binding [18,[20][21][22]. Under excess nitrogen, FBI-GS functions as a chaperone by a transient interaction with the GlnR autoinhibitory domain and relieves autoinhibition, shifting the equilibrium from the inhibited form to the DNA-binding active form and thus turning on GlnR repression [18,[20][21][22]. In contrast, FBI-GS forms a stable complex with TnrA, inhibiting its DNA-binding function under excess nitrogen, whereas TnrA is released from FBI-GS, allowing TnrA dimerization and activation of its transcription program under nitrogen limitatiom. GlnK appears to play an ancillary role in TnrA dimerization by acting as a templating agent for TnrA [25][26][27][28][29][30].
In this study, we fully investigate the regulation mechanisms of nitrogen fixation in P. polymyxa WLY78 by using comprehensive molecular methods. We reveal that during nitrogen limitation, GlnR binds to GlnR-binding site Ⅰ located upstream of -35/-10 regions of nif promoter of nif operon (nifBHDKENXhesAnifV) in a weak and transient association way and then activates nif transcription. During excess nitrogen, glutamine (Gln) binds to and feedback inhibits glutamine synthetase (GS) by forming the complex FBI-GS. FBI-GS interacts with Cterminal domain of GlnR and stabilizes the binding of GlnR to site Ⅱ located downstream of nifB transcription start codon and thus represses nif transcription. GS encoded by glnA within glnRA operon is involved in regulation of nif transcription. Also, overexpression of glnR and mutagenesis of glnA or GlnR-binding site Ⅱ led to constitutive nitrogen fixation in the absence or presence of ammonia. Our study not only reveals the novel regulation mechanisms of nif gene expression in Paenibacilli, but also provides insight into dual active and repressive functions of GlnR.

GlnR is essentially required for nitrogen fixation under nitrogen limitation
The genome of P. polymyxa WLY78 contains a glnR gene and two paralogs of glnA, but it lacks a tnrA gene [11]. We found that of the two glnA genes, one was linked to glnR as a dicistronic glnRA operon and the other (here designated as glnA1) was elsewhere in the genome. The current analysis by using BLAST alignment showed that GS and GS1 proteins encoded by the glnA and glnA1 genes had 39% identity.
To elucidate the function of GlnR in nitrogen fixation of Paenibacillus, we constructed an in-frame deletion mutant ΔglnR, a complemention strain (ΔglnR/glnR) for the mutated glnR and an overexpression strain (WT/glnR), as described in S1 Fig. In comparison with wild-type P. polymyxa WLY78 which exhibited the highest nitrogenase activity in the absence of NH 4 + and no activity in the presence of more than 5 mM NH 4 + , activity in ΔglnR mutant was at basal constitutive levels under all conditions ( Fig 1A). Deletion of glnR resulted to nearly loss GlnR controls the nitrogenase activity and nif transcription in P. polymyxa WLY78. A. Nitrogenase activities of WT (the wild-type), ΔglnR (deletion mutant), ΔglnR/glnR (complementation strain) and WT/glnR (overexpression strain). These strains were grown anaerobically in nitrogen-deficient medium containing 2 mM glutamate supplemented with different concentration of NH 4 Cl at 0, 1, 5, 10, 50 and 100 mM. The nitrogenase activities of these strains were assayed by C 2 H 4 reduction method and expressed at nmol C 2 H 4 /mg protein/hr. B. qRT-PCR analysis of the relative mRNA levels of the nifHDK genes in the WT and ΔglnR strains grown in nitrogen-limited and -excess media. N-: nitrogen-limited condition (2 mM glutamate as the only nitrogen source). N+: nitrogen-excess condition (2 mM glutamate + 100 mM NH 4 + ). The relative expression level was calculated using ΔΔCt method.The transcription levels of genes in the WT strain under nitrogen-excess condition were arbitrarily set to 1.0. C. β-galactosidase activity of a Pnif-lacZ fusion in the WT and ΔglnR strains grown in nitrogen-limited (N-) and -excess (N+) conditions. D. qRT-PCR analysis of the relative transcription levels of glnR gene under nitrogen-limited (N-) and -excess (N+) conditions. The transcription levels of glnR gene under nitrogen-excess condition were arbitrarily set to 1.0. E. qRT-PCR analysis of the transcription profiles of glnR and nifH under nitrogen limitation. The transcription levels of genes at time 0 hr were arbitrarily set to 1.0. Results are representative of at least three independent experiments. Error bars indicate SD. �� P < 0.01; � P < 0.05. https://doi.org/10.1371/journal.pgen.1007629.g001 Novel regulation mechanisms of nitrogen fixation in Paenibacillus polymyxa mediated by GlnR of activity, indicating that GlnR is essentially required for nitrogen fixation under nitrogen limitation. Somewhat higher activity was observed in the ΔglnR mutant at high ammonia than in the ammonia-repressed wild-type strain. Complementation of ΔglnR with a single copy of glnR integrated on the amyE site of its genome restored nitrogenase activity to the wild-type level in complemented strain (ΔglnR/glnR), suggesting that change of nitrogenase activity was due solely to deletion of glnR. Overexpression of glnR by introduction of glnR carried on multicopy vector pHY300PLK into wild-type strain led to enhancement of activity in the presence of NH 4 + . The ΔglnR and the wild-type strains exhibited similar growth phenotypes on minimal media with glutamine, glutamate and ammonium as sole nitrogen sources (S2 Fig). Taken together, these results indicate for the first time that GlnR positively regulates nitrogen fixation under nitrogen-limited condition.
To examine the effect of glnR mutation on the transcription of nif genes in P. polymyxa WLY78, the transcription levels of nifH, nifD and nifK were determined by qRT-PCR. As shown in Fig 1B, the transcription levels of the nifHDK in wild-type strain exhibited more than 1000-fold of increase under nitrogen-limited condition (2 mM glutamate as sole nitrogen) compared to nitrogen-excess condition (2 mM glutamate + 100 mM NH 4 + ). However, the nifHDK genes in ΔglnR mutant were expressed constitutively under both conditions at very low level which was approximately 2.7% of that observed in wild-type strain under nitrogenlimited condition ( Fig 1B). These results are consistent with nitrogenase activity in this ΔglnR mutant, indicating that GlnR activates nif transcription under nitrogen-limited condition.
To further examine the effect of GlnR on regulation of nif expression, a transcriptional lacZ fusion to nif promoter region was constructed and then this Pnif-lacZ fusion was introduced into wild-type and ΔglnR mutant, respectively. As shown in Fig 1C, the β-galactosidase levels produced by Pnif-lacZ fusion in wild-type strain were 5000-fold higher in nitrogen-limited condition than in nitrogen-excess condition. However, the β-galactosidase levels produced by Pnif-lacZ fusion in ΔglnR mutant were similar in both conditions. The data are consistent with the above described qRT-PCR results and nitrogenase activities.
Furthermore, qRT-PCR analysis demonstrated that glnR transcription was highly induced under nitrogen-limited condition compared to under nitrogen-excess condition (Fig 1D), suggesting that glnR expression itselfis nitrogen-dependent. Also, the transcription profiles of glnR and nifH were similar under nitrogen limitation ( Fig 1E). The current results are consistent with our previous global transcriptional profiling analysis that the expressions of glnR and nif genes were significantly up-regulated when P. polymyxa WLY78 was grown in N 2 -fixing condition (without O 2 and NH 4 + ) compared to non-N 2 -fixing condition (air and 100 mM NH 4 + ) [13]. These results indicate that the expressions of glnR and nif genes are highly coordinated.

Both GS and GlnR are required for negative regulation of nitrogen fixation
To examine the role of GS proteins encoded by glnA and glnA1 in regulation of nitrogen fixation, a series of in-frame-deletion mutants, including ΔglnA1, ΔglnA and ΔglnRA mutants, and their complementary strains ΔglnA/glnA and ΔglnRA/glnRA were constructed as described in S1 Fig. We found that nitrogenase activities were similar in ΔglnA1 mutant and wild-type strain under both nitrogen-limited and -excess conditions, suggesting that glnA1 is not involved in regulation of nitrogen fixation (Fig 2A). However, nitrogenase activity in ΔglnA mutant was produced constitutively at modest level under both nitrogen-limited and -excess conditions. Complementation of ΔglnA with glnA gene (complementary strain ΔglnA/glnA) restored the nitrogenase activity to the wild-type level (basal nitrogenase activity) under nitrogen-excess condition and to 80% of wild-type level (high nitrogenase activity) under nitrogen-limited condition (Fig 2A). Nitrogenase activity in ΔglnRA double mutant was almost abolished just as observed in ΔglnR single mutant. Complementation study showed that glnRA could partially restored the activity of ΔglnRA double mutant (Fig 2A), suggesting that the role of GS is dependent on GlnR. These results indicate that both GS and GlnR are required for the repression of nitrogen fixation under nitrogen-excess condition.
qRT-PCR analysis showed that the transcription levels of nifH gene were similar in both ΔglnA1 and wild-type strains under both nitrogen-limited and -excess conditions, in agreement with nitrogenase activity in these strains and suggesting that GS encoded by glnA1 is not involved in regulation of nif gene expression ( Fig 2B). In contrast, the nifH gene in ΔglnA mutant was transcribed constitutively at modest level under both nitrogen-limited and -excess conditions, in agreement with nitrogenase activity in ΔglnA mutant. Transcription levels of the nifH gene in ΔglnRA double mutant were at basal low level under both nitrogen-limited and -excess conditions, in agreement with nitrogenase activity in this strain. These data suggest that GlnR and GS encoded by glnA within glnRA operon are responsible for negative regulation of nif gene expression according to nitrogen availability.

C-terminal deletion of GlnR, purification and interaction of GlnR with GS
GlnR protein of B. subtilis has a high sequence similarity at the N terminus with TnrA, but the C-terminal signal transduction domain of GlnR is sequentially distinct from TnrA and contains an extra 15 residues [20,23,24].
Here, sequence alignments showed that the 137-residue GlnR protein of P. polymyxa WLY78 exhibited 54% and 40% identity with GlnR and TnrA of B. subtilis 168, respectively (S3 Fig). GlnR, GS and GS1 from P. polymyxa WLY78 with His 6 -tag at the N-terminus were overexpressed and purified in Escherichia coli, respectively. Also, GlnR Δ25 , a truncated GlnR with a deletion of the last 25 C-terminal codons (aa 113-137) was overexpressed and purified in E. coli. Of these purified proteins, GlnR was further evaluated by size-exclusion chromatography analysis. The GlnR protein was eluted as a broad peak with two maxima. Judged from the elution positions of marker proteins, this profile could reflect the coexistence of His 6 -GlnR Our results are different from some reports that P. riograndensis SBR5 GlnR is mainly the dimeric form [14] and B. subtilis GlnR is mainly monomeric form [20].
Then, the interaction of GlnR with GS proteins was evaluated by surface plasmon resonance (SPR) assay. His 6 -tagged GlnR was immobilized on a Ni-nitrilotriacetic acid-activated chip sensor surface. Then different concentrations of GS and FBI-GS (GS and glutamine) were loaded onto the GlnR chip surface. In the absence of glutamine, only a weak interaction between GlnR and GS was observed, and the GlnR-GS complex also dissociated quickly even when the concentration of GS was increased from 200 nM to 3.2 mM (Fig 3A). In contrast, in the presence of glutamine, there was still strong interaction between GlnR and FBI-GS even when the concentration of GS protein was decreased from 200 nM to 6.25 nM (Fig 3B). These results indicated that GlnR of P. polymyxa WLY78 interacted with the feedback inhibited GS form (FBI-GS). Our results are consistent with the reports that GS, in its feedback inhibited form, interacts with GlnR of P. riograndensis SBR5 [14] and GlnR and TnrA of B. subtilis [18,20,21]. However, nearly no interaction between GlnR Δ25 and FBI-GS was observed ( Fig  3C), suggesting that the C-terminal domain is required for interaction between GlnR and FBI-GS.
In contrast, only a basal weak interaction between GlnR and GS1 was detected whether the feedback inhibitor glutamine was present or not ( Fig 3C), in agreement with the abovedescribed results that mutation of glnA1 did not affect regulation of nif transcription and nitrogenase activity.

FBI-GS enhances the in vitro and in vivo DNA-binding activity of GlnR and the C-terminal domain of GlnR affects DNA-binding
We predicted that the promoter region of nif operon of P. polymyxa WLY78 contained two GlnR-binding sites: GlnR-binding site Ⅰ and GlnR-binding site Ⅱ (Fig 4A and S5 Fig) by using MEME/MAST software [31]. The two sites were 118 bp separate. Site Ⅰ was located 58 bp upstream of -35 regions of nif promoter, and site Ⅱ was seated 24 bp downstream of the nifB transcription start site. The binding motifs of the two sites resembled the common consensus sequences (5'-TGTNAN7TNACA-3') of the GlnR/TnrA-binding site [15,32,33]. Novel regulation mechanisms of nitrogen fixation in Paenibacillus polymyxa mediated by GlnR To determine whether the two GlnR-binding sites are direct targets of GlnR, the in vitro and in vivo binding of GlnR protein to the two GlnR-binding sites were performed by using electrophoretic mobility shift assays (EMSA), surface plasmon resonance (SPR) spectroscopy ChIP-qPCR assays revealed in vivo biding of GlnR to both GlnR-binding sites under both nitrogen-limited (N-) and -excess (N+) conditions. The binding levels of control (ΔglnR) were arbitrarily set to 1.0. Error bars indicate SD from three independent experiments. �� P < 0.01; � P < 0.05. D. SPR analysis of GlnR binding to both GlnR-binding sites. 500 nM GlnR alone or with FBI-GS (GS + 1 mM glutamine) was injected onto the chip surface-immobilized DNA fragments harboring GlnR-binding site Ⅰ or site Ⅱ. GlnR (Site Ⅰ) indicates the binding of GlnR alone to site Ⅰ; GlnR (Site Ⅱ) indicates the binding of GlnR alone to site Ⅱ; GlnR+FBI-GS (Site Ⅰ) indicates the binding of GlnR plus FBI-GS to site Ⅰ; GlnR+FBI-GS (Site Ⅱ) indicates the binding of GlnR plus FBI-GS to site Ⅱ. E. SPR analysis of the binding affinity of the truncated GlnR (GlnR Δ25 ) protein to the two GlnR-binding sites. 500 nM GlnR or GlnR Δ25 alone or with FBI-GS (GS + 1 mM glutamine) was injected onto the chip surface-immobilized DNA fragments harboring GlnR-binding site Ⅰ or site Ⅱ. In comparison with wild-type GlnR, GlnR Δ25 protein has the increased binding affinity to both sites, especially site Ⅱ. The addition of FBI-GS does not obviously increase the binding affinity of GlnR. and chromatin immunoprecipitation-quantitative PCR (ChIP-qPCR). EMSA experiments revealed that in vitro GlnR bound to the two sites ( Fig 4B). Addition of both GS and glutamine (the feedback-inhibited GS) enhanced the binding affinity of GlnR to the two sites ( Fig 4B). Also, the addition of both GS and glutamine did not change the band positions of the DNA-GlnR protein complex, supporting that FBI-GS did not directly bind to DNA and it functioned as a chaperon to activate the DNA-binding activity of GlnR [20,21].
Then, ChIP-qPCR experiments were performed to investigate the in vivo binding of GlnR to the two GlnR-binding sites. GlnR polyclonal antibody was used to measure binding of GlnR to its target and qRT-PCR with primers corresponding to the GlnR-binding site Ⅰ and site Ⅱ was performed. As shown in Fig 4C, GlnR bound to the both sites under both nitrogen limitation and nitrogen excess conditions, but the binding levels of GlnR to both sites were much higher under excess nitrogen than under nitrogen limitation. These findings agree with the results obtained by EMSA. Also, Fig 4C shows that the binding level of GlnR to site Ⅱ was higher than to site Ⅰ under both conditions. Furthermore, we tested the in vitro affinity of GlnR to the two GlnR-binding sites by SPR spectroscopy. This SPR assay demonstrated that GlnR alone could specifically bind to the two GlnR-binding sites, but the affinity of GlnR for site Ⅱ was much stronger than for site Ⅰ. Regardless of the interaction intensity with each site, in the absence of glutamine, GlnR-DNA binding was transient and unstable due to quick dissociation ( Fig 4D). However, addition of both GS and glutamine (the feedback-inhibited GS) greatly stabilized the DNA-protein complex. Especially, FBI-GS significantly stabilized the DNA (site Ⅱ)-protein complex, consistent with the classical function of GlnR as a repressor [15,16].
The affinity of the truncated GlnR (GlnR Δ25 ) protein to the two GlnR-binding sites was also investigated by SPR assay. As shown in Fig 4E, in comparison with wild-type GlnR, GlnR Δ25 protein had higher affinity for both sites. The addition of FBI-GS greatly stabilized the DNA-GlnR complex, but it did not have obvious effect on the DNA-GlnR Δ25 complex. Our results indicate that the C-terminal region of P. polymyxa GlnR is an autoinhibitory domain that inhibits DNA-binding ability of GlnR and that the C-terminal domain is also required for the interaction between FBI-GS and GlnR, consistent with the observations in B. subtilis GlnR [20,21].
To clarify the affinity of GlnR to both sites, quantitative evaluation was carried out with SPR. A double-stranded DNA oligomer that contained the sequence of site Ⅰ or site Ⅱ was fixed onto the chip as described in Material and Methods. Different concentrations of GlnR protein were loaded onto the DNA chip surface. As shown in Fig 5A and 5B, there was no binding signal in the absence of GlnR and the binding signals became strong with the increase of concentrations of GlnR protein. When the concentration of GlnR was increased to 500 nm and 1000 nM, an obvious binding of GlnR to site Ⅰ was found, but it dissociated quickly, indicating that the binding of GlnR to site Ⅰ is transient and unstable. In contrast, the binding of GlnR to site Ⅱ was stronger and it dissociated slowly, indicating that the binding of GlnR to site Ⅱ is stronger than to site Ⅰ due to slow dissociation. The corresponding K A and K D values for site Ⅰ were calculated to be 1.09×10 6 and 9.16×10 −7 , respectively. Whereas the K A and K D values for site Ⅱ were 1.86×10 7 and 5.37×10 −8 , respectively (Fig 5C). The values of K A for site Ⅱ was consistently higher than that for site Ⅰ, and the values of K D for site Ⅱ was much lower than that for site Ⅰ, indicating that affinity of site Ⅱ for GlnR is higher than site Ⅰ. Our current results are different from those obtained in P. riograndensis SBR5 where GlnR bound to the two GlnR-binding sites [PnifM(1) and PnifM (2)] of the main nif gene cluster at similar levels and whose GlnR affinity for site Ⅰ was slightly higher than for site Ⅱ [14].

GlnR-binding site Ⅰ and Site Ⅱ are involved in positively and negatively regulating nif gene transcription, respectively
Since GlnR was positively and negatively involved in the regulation of nitrogen fixation according to nitrogen availability, both increase and decrease of nitrogenase activity could be expected through mutations of the two GlnR-binding sites in the nif promoter region. Thus, the site-specific mutagenesis of the two GlnR-binding sites was performed. As shown in Fig  6A, the consensus sequence TGACGT in site Ⅰ region was replaced with a restriction site of Kpn Ⅰ (GGTACC) via homologous recombination, generating mutant MPnif1. The consensus motif ATAACG in site Ⅱ was replaced by a restriction site of Cla Ⅰ (ATCGAT), which generated the mutant MPnif2. A double mutant MPnif3 with mutations of both GlnR-binding sites was generated. Also, the mutant MPnif97 with deletion of site Ⅰ was also constructed. EMSA confirmed that GlnR did not bind to the mutated sites (Fig 6B).
In comparision with wild-type strain, only basal nitrogenase activity was observed in both mutants MPnif1 and MPnif97 under both nitrogen-limited and -excess conditions (Fig 6C), suggesting that site Ⅰ is essentially required for nitrogen fixation. The data are consistent with nitrogenase activity in ΔglnR mutant, indicating that site Ⅰ is the target of GlnR. In contrast, nitrogenase activity in mutant MPnif2 was derepressed partially under nitrogen-excess condition, suggesting that site Ⅱ is involved in repressing nitrogen fixation. The data are consistent with nitrogenase activity in ΔglnA mutant, suggesting that site Ⅱ is the target of GS encoded by glnA. Nitrogenase activity in the double mutant MPnif3 was nearly abolished under both nitrogen-limited and excess conditions, in agreement with nitrogenase activities in ΔglnRA double mutant. The nif gene transcription levels determined by qRT-PCR (Fig 6D) were consistent with the nitrogenase activities in mutants MPnif97, MPnif1, MPnif2 and MPnif3. Taken together, these results indicate that GlnR binds to site Ⅰ to activate nif expression under nitrogen-limited condition and binds to site Ⅱ to repress nif transcription under Novel regulation mechanisms of nitrogen fixation in Paenibacillus polymyxa mediated by GlnR nitrogen-excess condition. FBI-GS is involved in repressing nif transcription by its interaction with GlnR under excess nitrogen.

Discussion
GlnR is a global transcription regulator of nitrogen metabolisms found extensively in Bacillus and other Gram-positive bacteria. It generally acts as a repressor repressing the transcription of glnRA operon, tnrA and ureABC in B. subtilis under excess nitrogen [15,16,25]. TnrA is another transcription regulator of nitrogen metabolisms found mainly in Bacillus and it serves in most cases as an activator under nitrogen limitation [26]. In the present work, we reveal that P. polymyxa GlnR simultaneously acts as an activator and a repressor for nitrogen fixation by binding to different loci of the single nif promoter region according to nitrogen availability. GS is ne cessarily required for nif repression mediated by GlnR.
In this study, two GlnR-binding sites flanking the -35/-10 regions of the promoter of nif operon in P. polymyxa WLY78 is predicted by software and then confirmed by in vitro EMAS and SPR experiments and by in vivo ChIP-qPCR. The two sites are 118 bp separated. Site Ⅰ is located 58 bp upstream of -35 region of nif promoter, and site Ⅱ is seated 24 bp downstream of the nifB transcription start site. The location of site Ⅰ is an indicative of activation site, since regulator, such as TnrA, bound at this position most likely activates gene transcription [23,25,26]. Site Ⅱ located just downstream of promoter is an indicative of repression site, since regulator bound at this site will represses gene transcription by sterically hindering RNA extension [34]. The binding motif (5'-TGTAAGGGAATATAACG-3') of site Ⅱ possesses the common consensus sequence (5'-TGTNAN7TNACA-3') of the GlnR-binding site (S5 Fig), while the consensus sequences (5'-CGATATATTACTTGACG-3') of site Ⅰ fit the TnrA-specific motif (5'-NGNNAN7TNACN-3') which clearly lacks the conserved A and T at the 3 0 and 5 0 end [15,32,33]. Our studies of deletion or mutagenesis of GlnR-binding site Ⅰ, site Ⅱ and both sites demonstrated that site Ⅰ is responsible for activating nif expression and site Ⅱ is required for repressing nif transcription, in agreement with the locations of the two sites. Two GlnR-binding sites flanking the -35/-10 nif promoter region were also found in P. riograndensis SBR5 [14], but they exhibit a little difference with those of P. polymyxa WLY78 in the precise consensus sequences and locations. As shown in S6 Fig, the site Ⅱ in P. polymyxa WLY78 is located 16 bp upstream of ATG (translation start site), while the O A -nifB (site Ⅱ) in P. riograndensis SBR5 is located 60 bp upstream of ATG. A common consensus sequence (TGTNAN7TNACA) of GlnR-binding motif in the class Bacillus (S5 Fig) is more conserved in the two GlnR-binding sites of P. riograndensis SBR5 than in those of P. polymyxa WLY78. SPR assay demonstrated that GlnR-binding site Ⅰ of P. riograndensis SBR5 displayed higher affinity for GlnR, whereas the second site had lower affinity and dissociated faster [14]. In contrast, GlnR-binding site Ⅰ of P. polymyxa WLY78 exhibited lower affinity for GlnR and dissociated faster, while site Ⅱ displayed higher affinity due to slow dissociation, especially in the presence of FBI-GS.
Based on the two binding sites in the nif promoter region of P. riograndensis SBR5, a DNAlooping model that represents a strong and strict regulation for nif genes was proposed [14]. In this model, DNA loop formation was induced by two GlnR dimers bound to both GlnR-binding sites and bridged by feedback-inhibited GS. However, our results from deletion and complementation analyses of glnR, glnA and glnRA and from mutation analyses of GlnR-binding sites did not support the DNA-looping model. Our data demonstrate evidently that GlnR bound to site Ⅰ in a weak and transient way and then activated nif gene transcription under nitrogen limitation, and the FBI-GS stabilized the binding affinity of GlnR to binding site Ⅱ and the strong binding of GlnR to site Ⅱ repressed nif gene transcription by interfering RNA extension under nitrogen-excess condition. Our studies that mutation of the 4-5 nucleotides in the half-sequences within the GlnR-binding site Ⅰ (ACGATATATTACTTGACGT) or site Ⅱ (ATGTAAGGGAATATAACGT) resulted to no binding of GlnR are consistent with that 4 nucleotides in each operator half-site of the DNA consensus sequence (TGTNAN7TNACA) were required for GlnR/TnrA specific DNA binding [18,23,26,35]. Our data also support that TnrA/GlnR form a weak symmetric dimer by binding their palindromic cognate sites [18,21]. Thus we think that it is unlikely for GlnR to be a tetramer formed by interacting between two dimers bound on the two GlnR-binding sites. There are also two GlnR-binding sites in the promoter region of B. subtilis glnRA, one of which lies immediately upstream of the -35 promoter element and the other site overlaps the -35 region [36]. It was previously reported that GlnR bond to these sites in a cooperative manner, and both sites were required for full repression of B. subtilis glnRA [37].
Mutation of GlnR-binding site Ⅱ made the mutant MPnif2 have nitrogenase activities and express nif genes under both nitrogen-limited and -excess conditions (Fig 6C and 6D), supporting that GlnR bound to site Ⅰ to activate nif gene transcription under nitrogen limitation. However, the nitrogenase activity and nif gene transcription in mutant MPnif2 did not reach similar levels under both nitrogen-limited and -excess conditions. We deduce that perhaps the mutation of site Ⅱ made the mutant MPnif2 have more FBI-GS proteins to strength the binding of GlnR to site Ⅰ and then interfere nif gene transcription under nitrogen excess. However, under normal physiological condition, since there are two GlnR-binding sites and the affinity of GlnR for site Ⅱ was much stronger than for site Ⅰ, repression of nif gene transcription was mediated by site Ⅱ. Whether site Ⅰ, together with site Ⅱ was involved in repressing nif gene transcription under nitrogen-rich condition needs to be determined in the future.
Our study by deletion, complementation and overexpression of glnR, glnA and glnRA and by mutagenesis or deletion of GlnR-binding sites reveals that GlnR bound to GlnR-binding site Ⅰ and activated nif transcription under nitrogen limitation, and GlnR bound to GlnR-binding site Ⅱ and repressed nif transcription under excess nitrogen. The novel, dual positive and negative regulatory mechanism is for the first time reported in nitrogen fixation. Although dual function of GlnR in Streptomyces hygroscopicus var. jinggangensis 5008 was reported [38], Streptomyces GlnR is an OmpR-like response regulator which does not display any similarity to the Paenibacillus/Bacillus GlnR regulator belonging to the MerR family [39].
Although GlnR protein alone could bind to the two sites in nif promoter of P. polymyxa WLY78, this binding was transient and unstable. We deduce that the transient GlnR-DNA interaction is sufficient for GlnR to act as an active regulator. Interestingly, under nitrogenlimited condition, TnrA (but not GlnR) of B. subtilis is further stabilized by an interaction with GlnK [40,41]. It also reported that GlnR protein exhibited an increased affinity for the glnRA operon promoter when bound to GlnK in Streptococcus mutans [42]. Whether GlnR dimer is stabilized by GlnK in P. polymyxa WLY78 under nitrogen-limited condition needs to be investigated in the future.
It is well characterized that the C-terminal domain of B. subtilis GlnR protein is sequentially distinct from TnrA and contains an extra 15 residues [23]. This region acts as an autoinhibitory domain that prevents GlnR dimerization and thus inhibits DNA binding [20][21][22]. FBI-GS acts as a chaperone to stabilize dimerization and subsequent DNA binding of GlnR [20][21][22]. In this study, the P. polymyxa GlnR Δ25 , a truncated GlnR with a deletion of the last 25 C-terminal codons was overexpressed and purified in E. coli. SPR analyses show that the interaction between GlnR Δ25 and GS is greatly decreased compared to wild-type GlnR. Also, GlnR Δ25 had higher binding affinity to both GlnR-binding sites than wild-type GlnR (Fig 4E). The addition of FBI-GS greatly enhanced the DNA-binding affinity of wild-type GlnR, but it did not obviously increase the DNA-binding affinity of GlnR Δ25 protein. FBI-GS also stabilized the DNA-GlnR complex, but it had no effect on the DNA-GlnR Δ25 complex. These results reveal that the C-terminal region of P. polymyxa GlnR is an autoinhibitory domain and it is also involved in the interaction between FBI-GS and GlnR, in agreement with the results obtained in B. subtilis GlnR. Our results demonstrate that FBI-GS stabilizes the binding of GlnR to the two site, especially site Ⅱ, consistent with the observations in B. subtilis GlnR [20,21]. We deduce that the monomers in the mixture of dimers and monomers of P. polymyxa GlnR protein were shifted to dimers by the interaction of FBI-GS with GlnR under excess nitrogen. Consequently, the strong binding of GlnR to site Ⅱ led to the represstion of nif transcription by interfering RNA extension. This mode of repressing nif gene transcription of P. polymyxa during excess nitrogen is a classical function of GlnR regulator found extensively in B. subtilis and some other Gram-positive bacteria. Although the activity of TnrA is also controlled by FBI-GS, the mechanisms of regulation are different between TnrA and GlnR. Under excess nitrogen, FBI-GS forms a stable complex with TnrA, which inhibits its DNA-binding activity [20,24].
TnrA and GlnR are generally recognized as the members of the MerR family regulators according to their common winged-HTH (helix-turn-helix) domains. However, TnrA and GlnR may regulate transcription using molecular mechanisms distinct from MerR proteins. MerR proteins activate transcription by distorting and realigning DNA promoters with nonoptimal spacing between the -10 and -35 boxes [43]. Unlike MerR members, the promoters bound by TnrA and GlnR are optimally arranged and a 17-bp inverted repeat sequences with the consensus TGTNAN7TNACA constitutes the minimal binding site for these proteins [15,32]. It was previously suggested that TnrA functions primarily as an activator by binding operator DNA sites and recruiting RNA polymerase (RNAP) [24,26], whereas GlnR does not bind RNAP and hence functions as a repressor. The recent study on structures has revealed that GlnR induces bend and conformational changes in the DNA similar to those in TnrA [18], supporting our results that GlnR functions as an activator just as TnrA does under nitrogen limitation.
The wild-type P. polymyxa WLY78 has the highest nitrogenase activity in the absence of NH 4 + and has no activity in the presence of more than 5 mM NH 4 + (Fig 1A). Deletion of glnR leads to loss of both nitrogenase activity and nif gene transcription under nitrogen limitation, suggesting that GlnR is essential required for activating nif gene expression. Deletion of glnA makes the ΔglnA mutant have both nitrogenase activity and nif gene transcription under both nitrogen-limited and -excess conditions, suggesting that GS encoded by glnA is involved in repressing nif gene transcription under nitrogen-excess condition. Mutation of GlnR-binding site Ⅰ results to loss of both nitrogenase activity and nif transcription under nitrogen limitation, suggesting that site Ⅰ is responsible for activating nif gene transcription. However, mutation of GlnR-binding site Ⅱ makes the mutant MPnif2 have both nitrogenase activities and nif gene transcriptions under both condition, consistent with the nitrogenase and nif gene ranscription in ΔglnA mutant. Our study with SPR also demonstrates that the affinity of GlnR for site Ⅱ is stronger than for site Ⅰ. Under nitrogen-excess condition, glutamine is synthesized and it feedbacks the GS, yielding FBI-GS, EMSA, SPR and Chip-PCR reveal that the presence of FBI-GS (GS and glutamine) greatly stabilizes the GlnR-DNA complex and decreases the dissociation of GlnR from binding site Ⅱ. According to our results, we proposed a regulatory model of GlnR involved in nitrogen fixation in P. polymyxa WLY78 (Fig 7). GlnR exists in a mixture of dimers and monomers. Monomer of GlnR is an autoinhibitory form whose C-terminal region folds back and inhibits dimer formation. Under nitrogen-limited condition, GlnR dimer binds to site Ⅰ in a weak and transient association way and then activates nif expression (Fig 7A). Although GlnR also sequentially or simultaneously binds to site Ⅱ, binding of GlnR to this site does not repress nif transcription due to GlnR having only a weak and transient association with DNA during this condition. Also, the large amounts of GlnR produced under this condition enable nif transcription to carry on, since expression of glnR itself is nitrogen-dependent. Under nitrogenexcess condition (Fig 7B), glutamine is in excess and it binds to and feedback inhibits GS by forming the complex FBI-GS. The FBI-GS interacts with the C-terminal tail of GlnR and relieves autoinhibition, shifting the monomer to the DNA-binding active form. The FBI-GS further stabilizes the binding affinity of GlnR to both sites, especially site Ⅱ. The stable binding of GlnR to site Ⅱ blocks the RNA extension and thus represses nif transcription.
In conclusion, our combined data reveal a novel molecular regulatory mechanism of nitrogen fixation in P. polymyxa WLY78. GlnR binds to site Ⅰ to activate nif gene transcription under nitrogen-limited condition, and it binds to site Ⅱ to repress nif gene transcription under nitrogenexcess condition. The activity of GlnR is controlled by GS in response to nitrogen availability.

Strains, plasmids and growth conditions
Bacterial strains and plasmids used in this study are summarized in S1 Table. P. polymyxa strains were grown in nitrogen-limited medium (2 mM glutamate) or nitrogen-excess medium (2 mM glutamate +100 mM NH 4 + ) under anaerobic condition [12]. For assays of nitrogenase activity, β-galactosidase assays and nif expression, P. polymyxa strains were grown in nitrogenlimited medium or nitrogen-excess medium under anaerobic condition. Escherichia coli strains JM109 and BL21 (DE3) were used as routine cloning and protein expression hosts, respectively. Thermo-sensitive vector pRN5101 [44] was used for gene disruption in P. polymyxa. Shuttle vector pHY300PLK was used for complementation experiment and transcriptional fusion construction. pET-28b(+) (Novagen) was used for expressing recombinant His6-tagged protein in E. coli. When appropriate, antibiotics were added in the following concentrations: 100 μg/ml ampicillin, 25 μg/ml chloramphenicol, 12.5 μg/ml tetracycline, 50 μg/ ml kanamycin, and 5μg/ml erythromycin for maintenance of plasmids.

Construction of ΔglnR, ΔglnA1, ΔglnA and ΔglnRA mutants and their complementation and overexpression strains
The four in-frame-deletion mutants: ΔglnR, ΔglnA1, ΔglnA and ΔglnRA, were constructed by a homologous recombination method. The upstream (ca. 1 kb) and downstream fragments (ca. 0.5 kb) flanking the coding region of glnR, glnA1, glnA and glnRA were PCR amplified from the genomic DNA of P. polymyxa WLY78, respectively. The primers used for these PCR amplifications were listed in S2 Table. The two fragments flanking each coding region of glnR, glnA1, glnA and glnRA were then fused with SalⅠ/BamHⅠ digested pRN5101 vector using Gibson assembly master mix (New England Biolabs), generating the four recombinant plasmids pRDglnR, pRDglnA1, pRDglnA and pRDglnRA, respectively. Then, each of these recombinant plasmids was transformed into P. polymyxa WLY78 as described by [45], and the singlecrossover transformants were selected for erythromycin resistance (Em r ). Subsequently, marker-free deletion mutants (the double-crossover transformants) ΔglnR, ΔglnA1, ΔglnA and ΔglnRA were selected from the initial Em r transformants after several rounds of nonselective growth at 39˚C and confirmed by PCR amplification and sequencing analysis.
Complementation for ΔglnR, ΔglnA and ΔglnRA was performed. For complementation of ΔglnR, the glnR gene and its promoter was inserted into the amyE site on genome of ΔglnR strain. To do this, two fragments: an 1161 bp DNA fragment and an 1017 bp fragment flanking the amyE gene, were PCR amplified from the genomic DNA of P. polymyxa WLY78, respectively. An 803 bp DNA fragment carrying the glnR ORF (414 bp) and its own promoter (389 bp) was also PCR amplified. Then, three fragments and the vector pRN5101 digested with BamHⅠ and HindⅢ were fused together using Gibson assembly master mix, generating the recombinant plasmid pRCglnR. The recombinant plasmid pRCglnR was transformed into the cells of ΔglnR strain and then double-crossover transformants were selected after several rounds of growth at 39˚C. Finally, the complementation strain CglnR which contains an 803 bp DNA fragment carrying glnR ORF and its promoter integrated on the amyE site was obtained and confirmed by PCR and DNA sequencing. For complementation of ΔglnRA mutant, a 2116 bp DNA fragment containing the complete glnRA operon and its own promoter was PCR amplified from the genomic DNA of P. polymyxa WLY78. For complementation of ΔglnA, a 1419 bp DNA fragment containing the coding region of glnA and a 280 bp promoter region of glnRA operon were PCR amplified, respectively, and then the two fragments were fused together using Gibson assembly master mix. These fragments were digested with BamHⅠ/SalⅠ, and ligated into vector pHY300PLK, generating glnA-complemented vector pHYglnA and glnRA-complemented vector pHYglnRA, respectively. Each of these recombinant plasmids was correspondingly transformed into ΔglnA and ΔglnRA mutants, and tetracycline-resistant (Tet r ) transformants were selected and confirmed by PCR and sequencing.
The strain WT/glnR in which glnR is overexpressed was also constructed. An 803 bp DNA fragment carrying the glnR ORF (414 bp) and its own promoter (389 bp) was PCR amplified and then ligated to multicopy vector pHY300PLK and then transformed to P. polymyxa WLY78, generating the glnR overexpression strain. The primers used here are listed in S2 Table.

Construction of mutants with deletion or mutagenesis of the GlnR-binding site(s)
Four mutants with deletion or mutagenesis of the GlnR-binding site(s) were performed via homologous recombination. A 313 bp nif promoter region (from -253 to +60 relative to the nifB transcription start codon) containing both of the GlnR-binding sites Ⅰ and Ⅱ was used as a target for mutation. Thus, three 313-bp DNA fragments PnifM1, PnifM2 and PnifM3 (S3 Table) were synthesized based on the sequences of nif promoter region. Notably, PnifM1 contains the mutated GlnR-binding site Ⅰ where the last six base pairs TGACGT within the 19-bp consensus sequences (ACGATATATTACT TGACGT) of the GlnR-binding site Ⅰ were replaced by a restriction site of KpnⅠ (GGTACC). PnifM2 carries the mutated GlnR-binding site Ⅱ where the last six base pairs ATAACG within the 19-bp consensus sequences (ATGTAAGGGAAT ATAACG) was replaced by a restriction site of ClaⅠ (ATCGAT). PnifM3 contains both of mutated site Ⅰ and site Ⅱ where the consensus motifs TGACGT and ATAACG were simultaneously replaced by restriction sites KpnⅠ and ClaⅠ. The three DNA fragments PnifM1, PnifM2 and PnifM3 were then cloned to plasmid pUC19, respectively. Then, each of the three fragments PnifM1, PnifM2 and PnifM3 (S3 Table) was PCR amplified from the recombinant plasmids. Two homologous arms (1205 bp and 1101 bp) flanking the 313 bp region in nif promoter were amplified from the genomic DNA of P. polymyxa WLY78 using the primers MPnif1/MPnif2 and primers MPnif5/ MPnif6 (S4 Table), respectively. Each of the two arms contains ca. 20 bp overlap with the abovedescribed 313 bp DNA fragments (PnifM1, PnifM2 and PnifM3). Then, the two arms and the DNA fragments PnifM1, PnifM2 and PnifM3 were assembled to the BamHⅠ/HindⅢ digested plasmid vector pRN5101, yielding the recombinant plasmids pRMP1, pRMP2, pRMP3. Each of these recombinant plasmids was introduced into P. polymyxa WLY78 by transformation. The single-crossover transformants were selected for erythromycin resistance (Em r ). Subsequently, the double-crossover transformants were selected from the initial Erythromycin resistance transformants after several rounds of nonselective growth at 39˚C. These mutants were confirmed by PCR amplification using the primers and subsequent digestion with KpnⅠ or ClaⅠ and then by DNA sequencing. The mutant with deletion of site Ⅰ was also constructed as follows. A 1418 bp DNA upstream fragment and an 1101 bp downstream fragment were PCR amplified from the genomic DNA of P. polymyxa WLY78. The two fragments were assembled to vector pRN5101, yielding the recombinant plasmid pRMP100 and then the plasmid was transformed into P. polymyxa WLY78. The mutant with deletion of 213 bp fragment (from -40 bp to -253 bp relative to the nifB transcription start codon) containing GlnR-binding site Ⅰ was obtained as described above.

Construction of a nif promoter-lacZ fusion (Pnif-lacZ fusion)
A 313 bp of the native nif promoter (Pnif) (from -253 to +60 relative to the nifB transcription start codon) containing both of the GlnR-binding sites Ⅰ and Ⅱ was amplified from the genomic DNA of P. polymyxa WLY78 using primers LPnif1 and LPnif2 (S5 Table). The lacZ coding region was PCR amplified with primers LPnif3 and LPnif4 from the plasmid pPR9TT. The two PCR-amplified fragments were fused together with vector pHY300PLK and then it was transformed into P. polymyxa WLY78.

Expression and purification of GlnR and GS proteins in E. coli
The glnR, truncated glnR (GlnR Δ25 , for C-terminal deletion of GlnR, removing the last 25 amino acid residues), glnA within glnRA operon and glnA1 were PCR amplified from the genomic DNA of P. polymyxa WLY78, respectively. These PCR products were cloned into pET-28b(+) (Novagen) to construct tagged proteins with His-tag at the N-terminus and then transformed into E. coli BL21 (DE3). The recombinant E. coli strains were cultivated at 37˚C in LB broth supplemented with 50 μg/ml kanamycin until mid-log phase, when 0.2 mM IPTG was added and incubation continued at 20˚C for 8 hours. Cells were collected and disrupted in a lysis buffer (50 mM NaH 2 PO 4 , 300 mM NaCl, 10 mM Imidazole) by sonication on ice. Recombinant His 6 -tagged proteins in the supernatant were purified on Ni 2 -NTA resin (Qiagen, Germany) according to the manufacturer's protocol. Fractions eluted with 250 mM imidazole were dialyzed into storage buffer (10 mM Tris-HCl pH7.5, 1 mM EDTA, 80 mM NaCl, 20% (v/v) glycerol) for antibody production or binding buffer (20 mM HEPES pH 7.6, 1 mM EDTA, 10 mM (NH 4 ) 2 SO 4 , 1 mM DTT, 0.2% Tween 20, 30 mM KCl) for electrophoretic mobility shift assays (EMSA) and HBS-Mg buffer (10 mM HEPES pH 7.4, 300 mM NaCl, 3 mM MgCl 2 , and 0.005% Nonidet P-40) for surface plasmon resonance spectroscopy (SPR). Purified His-GlnR was used to raise polyclonal rabbit antibody (Beijing Protein Innovation) and for size-exclusion chromatography. Primers used here are listed in S5 Table. RNA preparation and qRT-PCR analysis Transcription levels of genes were compared among P. polymyxa WLY78 strain and ΔglnR, ΔglnA and ΔglnRA mutants by quantitative real-time RT-PCR (qRT-PCR) analysis. At each experimental time point, 50 ml of culture were harvested and rapidly frozen under liquid nitrogen. Total RNAs were extracted with RNAiso Plus (Takara, Japan) according to the manufacturer's protocol. Remove of genome DNA and synthesis of cDNA were performed using PrimeScript RT reagent Kit with gDNA Eraser (Takara, Japan). qRT-PCR was performed on Applied Biosystems 7500 Real-Time System (Life Technologies) and detected by the SYBR Green detection system with the following program: 95˚C for 15 min, 1 cycle; 95˚C for 10 s and 65˚C for 30 s, 40 cycles. Primers used for qRT-PCR are listed in S6 Table. The relative expression level was calculated using ΔΔCt method. 16S rRNA was set as internal control and the expression levels of genes in WT strain under nitrogen-excess condition were arbitrarily set to 1.0. Each experiment was performed in triplicate.

Electrophoretic mobility shift assays (EMSAs)
EMSAs were performed as described previously using a DIG Gel Shift Kit (2nd Generation; Roche, USA) [12]. The promoter fragments of nif operon were synthesized by Sangon Biotech Co., Ltd (Shanghai). Two DNA fragments corresponding to the sequences of the first strand and the complementary DNA strand were synthesized. The two strands were annealed and then labeled at the 30 end with digoxigenin (DIG) using terminal transferase, and used as probes in EMSAs. Each binding reaction (20 μl) consisted of 1 μg poly [d(A-T)], 0.3 nM labelled probe, and various concentrations of purified His6-GlnR in the binding buffer. Reaction mixtures were incubated for 30 min at 25˚C, analyzed by electrophoresis using native 5% polyacrylamide gel run at 4˚C with 0.5×TBE as running buffer, and electrophoretically transferred to a positively charged nylon membrane (GE healthcare, UK). Labelled DNAs were detected by chemiluminescence according to the manufacturer's instructions, and recorded on X-ray film. The primers used here are listed in S6 Table.

Chromatin immunoprecipitation-quantitative PCR (ChIP-qPCR)
Chromatin immunoprecipitation-quantitative PCR (ChIP-qPCR) was performed as described by [46]. 100 ml of culture of WT or ΔglnR grown in nitrogen-limited or -excess media at 30˚C were harvested and immersed in cross-linked buffer (0.4 M sucrose, 1 mM EDTA, 10 mM Tris-Cl, pH 8.0) with 1% formaldehyde and 1% PMSF for 20 min at 28˚C. Cross-linking was stopped by addition of glycine (final concentration 125 mM) and incubation for another 5 min. After cross-linking, cells were sonicated to break chromosomal DNA into 200-500 bp fragments. Supernatant containing 2 mg total protein was diluted in 1 ml lysis buffer. 5 μl GlnR polyclonal antibody was added into precleared supernatant and incubated overnight at 4˚C. Purified immunoprecipitated DNA was resuspended in 120 μl double-distilled water. 2 μl DNA was used for qPCR, using the primer pairs listed in S6 Table. Relative levels of GlnRcoprecipitated DNAs were determined by comparison with negative controls.
Protein-DNA interaction assays were performed with Sensor Chip SA. First, a biotinylated single-stranded DNA capture linker (biotin-GCAGGAGGACGTAGGGTAGG) was irreversibly bound to the chip. DNA oligomer used for SPR assays (S7 Table) were designed and synthesized based on nif promoter region harboring GlnR-binding sites and containing a singlestranded overhang complementary to the linker. Then a partially double-stranded DNA oligomer that contained the GlnR-binding site Ⅰ or site Ⅱ in the double-stranded region with a single-stranded overhang complementary to the capture linker was fixed onto the chip, reaching a signal of 250 RU. Control DNA was fixed onto flow cell 1 (FC1), and DNA containing GlnR binding sites were fixed onto flow cell 2 and 3 (FC2, FC3). GlnR with or without 25 nM GS protein was injected at a flow rate of 30 ul/min.
Protein-Protein interaction assays were performed with Sensor Chip CM5. GlnR was immobilized via amine groups onto all four flow cells, receiving a signal of 1000 RU. Purified GS with or without 1 mM glutamine were injected separately at a flow rate of 30 μl/min.

Size-exclusion chromatography
Purified His 6 -GlnR from E. coli was used for size-exclusion chromatography. Analytical sizeexclusion chromatography was carried out on an Akta purifier system equipped with a Superdex 200 column 10/300 (geometric column volume of 24 mL GE Healthcare). The running buffer contains 50 mM Tris-HCl (pH 7.4) and 300 mM NaCl. His 6 -tagged GlnR was diluted on running buffer to reach a concentration of 2 mg/ml. 1 mL filtered and centrifuged sample was injected at a flow rate of 0.3 ml/min. Purified His 6 -GlnR from E. coli was used for sizeexclusion chromatography. Analytical size-exclusion chromatography was carried out on an Akta purifier system equipped with a Superdex 200 column 10/300 (geometric column volume of 24 mL GE Healthcare). The running buffer contains 50 mM Tris-HCl (pH 7.4) and 300 mM NaCl. His 6 -GlnR was diluted on running buffer to reach a concentration of 2 mg/ml. 1 mL filtered and centrifuged sample was injected at a flow rate of 0.3 ml/min. The apparent molecular weights of proteins were estimated after calibration of the column with standard proteins: thyroglobulin (670 kDa), globulin (158 kDa), ovalbumin (44 kDa), myoglobin (17 kDa), vitamin B12 (1.35 kDa) (Bio-Rad gel filtration standard).

Acetylene reduction assays of nitrogenase activity
Acetylene reduction assays were performed as described previously to measure nitrogenase activity [12]. P. polymyxa WLY78 and its mutant strains were grown in 5 ml of LD media (supplemented with antibiotics) in 50 ml flasks shaken at 250 rpm for 16 h at 30˚C. The cultures were collected by centrifugation, washed three times with sterilized water and then resuspended in nitrogen-deficient medium containing 2 mM glutamate containing 2 mM glutamate plus 0-100 mM NH 4 Cl as nitrogen source under anaerobic condition to a final OD600 of 0.2-0.4. Here, nitrogen-deficient medium containing 2 mM glutamate as nitrogen source and nitrogen-excess medium containing 2 mM glutamate and 100 mM NH4Cl as nitrogen source are generally used. Then, 1 ml of the culture was transferred to a 25-ml test tube and the test tube was sealed with robber stopper. The headspace in the tube was then evacuated and replaced with argon gas. After incubating the cultures for 6-8 h at 30˚C with shaking at 250 rpm, C 2 H 2 (10% of the headspace volume) was injected into the test tubes. After incubating the cultures for a further 3 h, 100 ml of culture was withdrawn through the rubber stopper with a gas tight syringe and manually injected into a HP6890 gas chromatograph to quantify ethylene (C 2 H 4 ) production. The nitrogenae activity was expressed in nmol C 2 H 4 /mg protein/hr. All treatments were in three replicates and all the experiments were repeated three or more times. β-galactosidase assays β-galactosidase activity was assayed according to the method described by [48]. Each experiment was performed in quintuplicate.