The LysR-type PcaQ protein regulates expression of a protocatechuate-inducible ABC-type transport system in Sinorhizobium meliloti

Abstract

The LysR protein PcaQ regulates the expression of genes encoding products relevant to the degradation of the aromatic acid protocatechuate (3,4-dihydroxybenzoate), and we have previously defined a PcaQ DNA-binding site located upstream of the targetpcaDCHGBoperon inSinorhizobium meliloti. In this work, we show that PcaQ also regulates the expression of theS. meliloti smb20568-smb20787-smb20786-smb20785-smb20784gene cluster, which is predicted to encode an ABC transport system. ABC transport systems have not been shown before to transport protocatechuate, and we have designated this gene clusterpcaMNVWX. The transcriptional start site ofpcaMwas mapped, and the predicted PcaQ DNA-binding site was located at −73 to −58 relative to this site. Results from electrophoretic mobility shift assays with purified PcaQ and from expression assays indicated that PcaQ activates expression of the transport system in the presence of protocatechuate. To investigate this transport system further, we generated apcaMdeletion mutant (predicted to encode the substrate-binding protein) and introduced a polar insertion mutation intopcaN, a gene that is predicted to encode a permease. These mutants grew poorly on protocatechuate, presumably because they fail to transport protocatechuate. Genome analyses revealed PcaQ-like DNA-binding sites encoded upstream of ABC transport systems in other members of the α-proteobacteria, and thus it appears likely that these systems are involved in the uptake of protocatechuate.