Characterization of redox-active cysteine residues of persulfide-responsive transcriptional repressor SqrR

We recently identified the novel persulfide sensor SqrR that functions as a master regulator of sulfide-dependent gene expression in the purple photosynthetic bacterium Rhodobacter capsulatus. SqrR binds to the promoter regions of target genes to repress their expression in the absence of sulfide, and the repressor activity is negated by sulfide treatment. SqrR has 3 cysteine residues, 2 of which are conserved in SqrR homologs from other bacteria: Cys41 and Cys107. SqrR forms an intramolecular tetrasulfide bond between Cys41 and Cys107 when exposed to persulfide, which results in loss of the DNA-binding activity in vitro. Here, we address the mechanism through which these cysteine residues are modified by persulfides. We show that the predicted pK(a) value of Cys107, as revealed by a putative SqrR structural model, is lower than that of Cys41. Furthermore, C41S SqrR in which Cys41 was changed to serine forms an intermolecular disulfide-bond between Cys107 of 2 SqrRs, suggesting high nucleophilic reactivity of Cy107. These data suggest that Cys107 and Cys41 function as attacking Cys and resolving Cys, respectively; this occurs during tetrasulfide-bond formation of WT SqrR, when it is exposed to persulfide.