Regulation of the divergent guaBA and xseA promoters of Escherichia coli by the cyclic AMP receptor protein.

The gua promoter (guaP) of Escherichia coli resembles those for ribosomal RNA (rrn) operons and lies in a close back-to-back arrangement with the promoter for xseA (xseP). Transcription from guaP is subject to stringent control and growth-rate-dependent regulation, and to repression by DnaA and PurR. In addition, transcription from guaP is regulated by the cyclic AMP receptor protein (CRP). Plasmid-borne promoter fusions to the receptor gene for chloramphenicol acetyl transferase were used to assess the role of CRP in controlling transcription from guaP and xseP following a downshift of cultures from rich into minimal medium. CRP is required to activate guaBA transcription and repress xseA transcription following downshift. Bandshift assays with a DNA fragment carrying the divergent promoters revealed specific binding of CRP. We propose that CRP, binding to a near-consensus site centred at -117.5, activates transcription from guaP and obstructs transcription from the xseA promoter.