Transcriptional regulation of the divergent paa catabolic operons for phenylacetic acid degradation in Escherichia coli

The expression of the divergently transcribed paaZ and paaABCDEFGHIJK: cata bolic operons, which are responsible for phenylacetic acid (PA) degradation in Escherichia coli, is driven by the Pt and Pa promoters, respectively. T o study the transcriptional regulation of the inducible paa catabolic genes , genetic and biochemical approaches were used. Gel retardation assays show ing that the PaaX regulator binds specifically to the Pa and Pt promoters w ere complemented with in vivo experiments that indicated a PaaX-mediated re pression effect on the expression of Pa-lacZ and Pz-lacZ reporter fusions. The region within the Pa and Pt promoters that is protected by the PaaX rep ressor in DNase I footprinting assays contains a conserved 15-base pair imp erfect palindromic sequence motif that was shown, through mutational analys is, to be indispensable for PaaX binding and repression. PA-coenzyme A (PA- CoA), but not PA, specifically inhibited binding of PaaX to the target sequ ences, thus confirming the first intermediate of the pathway as the true in ducer and PaaX as the only bacterial regulatory protein described so far th at responds to an aryl-CoA compound. Superimposed in the specific PaaX-medi ated regulation is transcriptional activation by the cAMP receptor protein and the integration host factor protein. These global regulators may adjust the transcriptional output from Pa and Pt promoters to the overall growth status of the cell.