Fig 1.
(p)ppGpp reprograms cell metabolism by transcriptional and post-translational regulatory mechanisms.
In response to environmental stress, RelA and SpoT synthesize ppGpp and pppGpp by catalyzing the transfer of pyrophosphate from ATP to GDP or GTP, respectively, and generating AMP and GMP or GDP as byproducts. In contrast to RelA, SpoT is also able to hydrolyze (p)ppGpp to produce GTP or GDP and inorganic phosphate. ppGpp binds to RNA polymerase with DksA to modulate transcription, and to effector proteins to regulate their biochemical activity. AMP, adenosine monophosphate; DksA, DnaK suppressor A; GDP, guanosine diphosphate; GMP, guanosine monophosphate; GTP, guanosine triphosphate; ppGpp, guanosine tetraphosphate; pppGpp, guanosine pentaphosphate.
Fig 2.
(p)ppGpp levels are regulated by RSH proteins.
(A) Rel, RelA, and SpoT are long, multidomain RSH enzymes that synthesize (p)ppGpp. Rel and SpoT also have an active hydrolase domain that degrades (p)ppGpp. The domains of each RSH enzyme are depicted with the N-terminus comprised of the catalytic synthetase and hydrolase domains, and the C-terminus consisting of a TGS, a helical domain, a CC domain, and an ACT domain. (B) RelA monitors the translational status of the cell by directly associating with the ribosome. Accumulation of uncharged tRNAs in the ribosomal A-site triggers (p)ppGpp production. (p)ppGpp mediates positive feedback regulation of RelA, stimulating ribosome-independent (p)ppGpp synthesis. ACT, aspartokinase, chorismate mutase, and TyrA; A-site, acceptor site; CC, conserved cysteine; CTD, C-terminal domain; GDP, guanosine diphosphate; GTP, guanosine triphosphate; HD, hydrolysis domain; NTD, N-terminal domain; RSH, RelA-SpoT homologue; SYN, synthesis domain; TGS, ThrRS, GTPase, and SpoT; ThrRS, threonyl-tRNA synthetase.
Fig 3.
(p)ppGpp regulates gene expression during host-bacterial interactions, whereas (p)ppApp mediates bacterial competition.
(A) (p)ppGpp contributes to activating genes required for host cell invasion and intracellular survival in Salmonella Typhimurium. Virulence gene regulation by (p)ppGpp also allows S. Typhimurium to evade different components of innate immunity including reactive nitrogen species, low pH, and the complement system. (B) (p)ppApp is used in interbacterial warfare and potentially in bacterial defense against phage superinfection. Pseudomonas aeruginosa delivers the toxin, Tas1, via a type VI secretion system into competitor bacteria. Tas1 produces (p)ppApp resulting in significant metabolic dysregulation and cell death. Prophage-encoded toxins such as PhRel may also produce (p)ppApp to confer protection against phage superinfection by reducing the metabolic potential of their host. MAC, membrane attack complex; T3SS, type III secretion system; T6SS, type VI secretion system; gDNA, genomic DNA.