The pimFABCDE operon from Rhodopseudomonas palustris mediates dicarboxylic acid degradation and participates in anaerobic benzoate degradation

Abstract

Bacteria in anoxic environments typically convert aromatic compounds derived from pollutants or green plants to benzoyl-CoA, and then to the C7dicarboxylic acid derivative 3-hydroxypimelyl-CoA. Inspection of the recently completed genome sequence of the purple nonsulfur phototrophRhodopseudomonas palustrisrevealed one predicted cluster of genes for theβ-oxidation of dicarboxylic acids. These genes, annotated aspimFABCDE, are predicted to encode acyl-CoA ligase, enoyl-CoA hydratase, acyl-CoA dehydrogenase and acyl-CoA transferase enzymes, which should allow the conversion of odd-chain dicarboxylic acids to glutaryl-CoA, and even-chain dicarboxylic acids to succinyl-CoA. A mutant strain that was deleted in thepimgene cluster grew at about half the rate of the wild-type parent when benzoate or pimelate was supplied as the sole carbon source. The mutant grew five times more slowly than the wild-type on the C14dicarboxylic acid tetradecanedioate. The mutant was unimpaired in growth on the C8-fatty acid caprylate. The acyl-CoA ligase predicted to be encoded by thepimAgene was purified, and found to be active with C7–C14dicarboxylic and fatty acids. The expression of apimA–lacZchromosomal gene fusion increased twofold when cells were grown in the presence of straight-chain C7–C14dicarboxylic and fatty acids. These results suggest that theβ-oxidation enzymes encoded by thepimgene cluster are active with medium-chain-length dicarboxylic acids, including pimelate. However, the finding that thepimoperon deletion mutant is still able to grow on dicarboxylic acids, albeit at a slower rate, indicates thatR. palustrishas additional genes that can also specify the degradation of these compounds.