FIG 8.

Model for the regulation of ssrAB by SlyA, HilD, OmpR, and H-NS. H-NS binds a region spanning the promoter upstream of ssrA and thus constitutively represses the expression of ssrAB, probably by blocking the access of both OmpR and the RNA polymerase. Under inducing conditions, SlyA and HilD (as during growth in LB) or only SlyA (as during growth in N-MM) displace the H-NS complex bound to the promoter upstream of ssrA. This allows binding of OmpR that recruits the RNA polymerase on this promoter, which finally induces the transcription of the ssrAB operon. The previously reported transcription of ssrB, from an additional promoter upstream of this gene, which involves the OmpR, PhoP, and SsrB regulators (23, 25, 26), is also indicated.