A) Aligned structures of the E. coli apo-CusF and CusF-Cu(I) adapted from PDB:1ZEQ and 2VB2 respectively [24,33] with stick and surface representations of CusF. Residues His 36 (green), Met 47, Met 49 (blue with sulphur atoms highlighted in yellow) and Trp44 (purple) are shown. The Cu(I) ion is shown in red. B) Plating efficiency of the ΔcopA ΔcueO ΔcusF strain carrying empty vector, pCusF, pCusF^M47I/M49I or pCusF^M47Q/M49Q vectors onto M9 plates in the presence of CuSO₄ (25 μM). The same protocol as described for Fig 2 was used, except plates contained ampicillin (50 μg/ml) and IPTG (50 μM). The images are representative of experiments carried out at least three times. C) Gel shift assay and mass spectrometry relative quantification by LFQ of the oxidation of Met47 and Met49. D) Silver binding analysed by quenching of intrinsic tryptophan fluorescence. Increasing concentrations of AgNO₃ (0, 0.2, 0.4, 0.6, 0.8, 1, 1.5, 2, 3, 4, and 5 μM) were added to 1 μM CusF, CusF^M47Q/M49Q, CusF^ox and CusF^rep. The emission spectrum of CusF was recorded after each addition as described in the Materials and Methods. The integrated fluorescence peak (between 300 and 384 nm) in the presence of AgNO₃ (F) was compared with the peak obtained in its absence (F₀). The F/ F₀ ratio was plotted against the concentration of AgNO₃, after correction for the inner filter effect of AgNO₃ measured on N-acetyltryptophanamide (NATA). The maximal fluorescence quenching for each variant of CusF was reported as a percentage in the table.

https://doi.org/10.1371/journal.pgen.1010382.g001