The cAMP receptor protein CRP can function as an osmoregulator of transcription in Escherichia coli

Transcription of the pi promoter of the Escherichia coli proP gene, which e ncodes a transporter of osmoprotectants, is strongly induced by a shift to hyperosmotic media. Unlike most other osmotically regulated promoters, the induction occurs for a brief period of time, corresponding to the replaceme nt of intracellular K+ glutamate with osmoprotecting compounds. This burst of proP transcription is correlated with the osmolarity-dependent binding o f the cAMP receptor protein CRP to a site within the proP pi promoter. We s how that CRP-cAMP functions as an osmotically sensitive repressor of proP p i transcription in vitro. Binding of CRP to the proP promoter in vivo is tr ansiently destabilized after a hyperosmotic shift with kinetics that corres pond to the derepression of transcription, whereas Fis and Lac repressor bi nding is not osmotically sensitive, Similar osmotic regulation of proP P1 t ranscription by the CRP* mutant implies that binding of cAMP is not respons ible for the unusual osmotic sensitivity of CRP activity. Osmotic regulatio n of CRP activity is not limited to proP. Activation of the lac promoter by CRP is also transiently inhibited after an osmotic upshift, as is the bind ing of CRP to the gal Delta 4 P1 promoter. These findings suggest that CRP functions in certain contexts to regulate gene expression in response to os motic changes, in addition to its role in catabolite control.