Fig. 4.

Model for SqrR sulfide-responsive gene regulation. In the absence of hydrogen sulfide, C41 and C107 of SqrR are in the reduced form. This form of SqrR binds the promoter region and represses the expression of sulfide-responsive genes (SRGs). In the presence of hydrogen sulfide, the resultant cellular RSS promotes the formation of a di-, tri-, or tetrasulfide bond between C41 and C107, inhibiting the ability of SqrR to bind to the promoter region. RNA polymerase binds DNA and subsequently induces gene expression.