Escherichia coli mhpR gene expression is regulated by catabolite repression mediated by the cAMP-CRP complex.

The expression of the mhp genes involved in the degradation of the aromatic compound 3-(3-hydroxyphenyl)propionic acid (3HPP) in Escherichia coli is dependent on the MhpR transcriptional activator at the Pa promoter. This catabolic promoter is also subject to catabolic repression in the presence of glucose mediated by the cAMP-CRP complex. The Pr promoter drives the MhpR-independent expression of the regulatory gene. In vivo and in vitro experiments have shown that transcription from the Pr promoter is downregulated by the addition of glucose and this catabolic repression is also mediated by the cAMP-CRP complex. The activation role of the cAMP-CRP regulatory system was further investigated by DNase I footprinting assays, which showed that the cAMP-CRP complex binds to the Pr promoter sequence, protecting a region centred at position -40.5, which allowed the classification of Pr as a class II CRP-dependent promoter. Open complex formation at the Pr promoter is observed only when RNA polymerase and cAMP-CRP are present. Finally, by in vitro transcription assays we have demonstrated the absolute requirement of the cAMP-CRP complex for the activation of the Pr promoter.