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 bacterium
Sinorhizobium melilotiare organized on the pSymB megaplasmid in two transcriptional units designated pcaDCHGBand pcaIJF. The pcaDpromoter 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 in S. melilotiwas shown to be encoded by SMb20587 and SMb20588, and these genes have been renamed pcaIand pcaJ, respectively. These genes are organized in an operon with a putative β-ketoadipyl-CoA thiolase gene ( pcaF), and expression of the pcaIJFoperon is shown to be regulated by an IclR-type transcriptional regulator, SMb20586, which we have named pcaR. We show that pcaRtranscription is negatively autoregulated and that PcaR is a positive regulator of pcaIJFexpression and is required for growth of S. melilotion protocatechuate as the carbon source. The characterization of the protocatechuate catabolic pathway in S. melilotioffers an opportunity for comparison with related species, including Agrobacterium tumefaciens. Differences observed between S. melilotiand A. tumefaciens pcaIJoffer the first evidence of pcagenes that may have been acquired after speciation in these closely related species.