Under low osmolality , the ssrA genes are transcriptionally activated by the binding of PhoP ~ P to their promoters whose levels are in turn .
Under acidic pH , the ssrA genes are transcriptionally activated by the binding of PhoP ~ P to their promoters whose levels are in turn .
Since PhoP regulates expression of ssrA , it also regulates expression of genes in the SsrB regulon .