Characterization of the β-Ketoadipate Pathway in Sinorhizobium meliloti

Aromatic compounds represent an important source of energy for soil-dwelling organisms. The β-ketoadipate pathway is a key metabolic pathway involved in the catabolism of the aromatic compounds protocatechuate and catechol, and here we show through enzymatic analysis and mutant analysis that genes required for growth and catabolism of protocatechuate in the soil-dwelling bacteriumSinorhizobium meliloti are organized on the pSymB megaplasmid in two transcriptional units designatedpcaDCHGB andpcaIJF . ThepcaD promoter was mapped by primer extension, and expression from this promoter is demonstrated to be regulated by the LysR-type protein PcaQ. β-Ketoadipate succinyl-coenzyme A (CoA) transferase activity inS. meliloti was shown to be encoded by SMb20587 and SMb20588, and these genes have been renamedpcaI andpcaJ , respectively. These genes are organized in an operon with a putative β-ketoadipyl-CoA thiolase gene (pcaF ), and expression of thepcaIJF operon is shown to be regulated by an IclR-type transcriptional regulator, SMb20586, which we have namedpcaR . We show thatpcaR transcription is negatively autoregulated and that PcaR is a positive regulator ofpcaIJF expression and is required for growth ofS. meliloti on protocatechuate as the carbon source. The characterization of the protocatechuate catabolic pathway inS. meliloti offers an opportunity for comparison with related species, includingAgrobacterium tumefaciens . Differences observed betweenS. meliloti andA. tumefaciens pcaIJ offer the first evidence ofpca genes that may have been acquired after speciation in these closely related species.