Integration of chemotaxis, transport and catabolism in Pseudomonas putida and identification of the aromatic acid chemoreceptor PcaY

Abstract

Summary: Aromatic and hydroaromatic compounds that are metabolized through the β-ketoadipate catabolic pathway serve as chemoattractants for Pseudomonas putidaF1. A screen of P.putidaF1 mutants, each lacking one of the genes encoding the 18 putative methyl-accepting chemotaxis proteins (MCPs), revealed that pcaY encodes the MCP required for metabolism-independent chemotaxis to vanillate, vanillin, 4-hydroxybenzoate, benzoate, protocatechuate, quinate, shikimate, as well as 10 substituted benzoates that do not serve as growth substrates for P.putidaF1. Chemotaxis was induced during growth on aromatic compounds, and an analysis of a pcaY-lacZ fusion revealed that pcaY is expressed in the presence of β-ketoadipate, a common intermediate in the pathway. pcaY expression also required the transcriptional activator PcaR, indicating that pcaY is a member of the pca regulon, which includes three unlinked gene clusters that encode five enzymes required for the conversion of 4-hydroxybenzoate to tricarboxylic acid cycle intermediates as well as the major facilitator superfamily transport protein PcaK. The 4-hydroxybenzoate permease PcaK was shown to modulate the chemotactic response by facilitating the uptake of 4-hydroxybenzoate, which leads to the accumulation of β-ketoadipate, thereby increasing pcaY expression. The results show that chemotaxis, transport and metabolism of aromatic compounds are intimately linked in P.putida. This study demonstrated that the pcaY gene encodes a methyl-accepting chemotaxis protein that mediates metabolism-independent chemotaxis to both metabolizable and nonmetabolizable aromatic acids in Pseudomonas putida F1. PcaK, the major facilitator superfamily transporter of 4-hydroxybenzoate was shown to modulate the chemotactic response by facilitating the uptake of 4-hydroxybenzoate, leading to the accumulation of the inducer molecule ß-ketoadipate, which interacts with the transcriptional activator PcaR to stimulate pcaY expression.