The styrene-responsive StyS/StyR regulation system controls expression of an auxiliary phenylacetyl-coenzyme A ligase: implications for rapid metabolic coupling of the styrene upper- and lower-degradative pathways

Pseudomonas sp. strain Y2 degrades styrene through oxidation to phenylacetic acid via the styABCD operon-encoded enzymes, whose expression is induced in response to styrene by the StyS/StyR two-component regulatory system. Further transformation of phenylacetic acid to tricarboxylic acid cycle intermediates is mediated by the enzymes of paa catabolic genes, whose expression is regulated by the PaaX repressor. The first step of this paa degradation pathway is catalysed by paaF-encoded phenylacetyl-coenzyme A ligases that produce phenylacetyl-coenzyme A. This metabolic intermediate, upon being bound by PaaX, inactivates PaaX-mediated repression of both the paa genes and the styABCD operon. Strain Y2 is unique in having three paaF genes located within two complete copies of the paa gene clusters. Expression of both paaF and paaF3 is controlled by the PaaX repressor. Here we use specific mutants in combination with in vivo and in vitro assays to demonstrate that paaF2, adjacent to the StyS/StyR regulatory genes, belongs to the StyR regulon and is not subject to repression by PaaX. We propose that this unexpected styrene-responsive regulatory strategy for the otherwise metabolically redundant PaaF2 auxiliary enzyme provides a system for rapid co-ordinate de-repression of the two sets of catabolic genes required for styrene degradation.