Regulation of the early steps of 3-phenylpropionate catabolism in Escherichia coli.

Microbial catabolism of phenylpropanoid compounds plays a key role in the degradation of aromatic molecules originating from the degradation of proteins and plant constituents. In this study, the regulation of the early steps in the utilisation of 3-phenylpropionate, a phenylpropanoid compound, was investigated. Expression of the hcaA gene product, which is involved in 3-phenylpropionate catabolism in Escherichia coli, was positively regulated by HcaR, a regulatory protein similar to members of the LysR regulators family. Remarkably, the expression of hcaA in the presence of 3-phenylpropionate was sharply and transiently induced at the end of the exponential growth phase. This occurred in a rpoS-independent manner. This transient induction was also mediated by HcaR. The expression of this positive regulator is negatively autoregulated, as for other members of the LysR family. The expression of hcaR is strongly repressed in the presence of glucose. Glucose-dependent repression of hcaR expression could only be partially overcome by adding exogenous cAMP.