Cyclic AMP receptor protein functions as a repressor of the osmotically inducible promoter proP P1 in Escherichia coli

Transcription of the proP gene, encoding a transporter of the osmoprotectants proline and glycine betaine, is controlled from two promoters, P1 and P2, that respond primarily to osmotic and stationary-phase signals, respectively, The P1 promoter is normally expressed at a very low level under low or normal medium osmolarity, We demonstrate that the binding of the cyclic AMP (cAMP) receptor protein (CRP) to a site centered at -34.5 within the promoter is responsible for the low promoter activity under these conditions, A brief period of reduced CRP binding in early log phase corresponds to a transient burst of P1 transcription upon resumption of growth in Luria-Bertani broth, A CRP binding-site mutation or the absence of a functional crp gene leads to high constitutive expression of P1, We show that the binding of CRP-cAMP inhibits transcription by purified RNA polymerase in vitro at P1, but this repression is relieved at moderately high potassium glutamate concentrations. Likewise, open-complex formation at P1 in vivo is inhibited by the presence of CRP under low-osmolarity conditions, Because P1 expression can be further induced by osmotic upshifts in a Delta crp strain or in the presence of the CRP binding-site mutation, additional controls exist to osmotically regulate P1 expression.