Under low osmolality , the ssrB genes are transcriptionally activated by the binding of PhoP ~ P to their promoters whose levels are in turn .
Under acidic pH , the ssrB genes are transcriptionally activated by the binding of PhoP ~ P to their promoters whose levels are in turn .
PhoP upregulates the expression of SPI-2 by directly activating transcription of ssrB , therefore PhoP is indispensible for Salmonella within phagosomes .