Characterization of an acyl-CoA:: carboxylate CoA-transferase from Aspergillus nidulans involved in propionyl-CoA detoxification

Filamentous fungi metabolize toxic propionyl-CoA via the methylcitrate cycle. Disruption of the methylcitrate synthase gene leads to an accumulation of propionyl-CoA and attenuates virulence of Aspergillus fumigatus. However, addition of acetate, but not ethanol, to propionate-containing medium strongly reduces the accumulation of propionyl-CoA and restores growth of the methylcitrate synthase mutant. Therefore, the existence of a CoA-transferase was postulated, which transfers the CoASH moiety from propionyl-CoA to acetate and, thereby, detoxifying the cell. In this study, we purified the responsible protein from Aspergillus nidulans and characterized its biochemical properties. The enzyme used succinyl-, propionyl- and acetyl-CoA as CoASH donors and the corresponding acids as acceptor molecules. Although the protein displayed high sequence similarity to acetyl-CoA hydrolases this activity was hardly detectable. We additionally identified and deleted the coding DNA sequence of the CoA-transferase. The mutant displayed weak phenotypes in the presence of propionate and behaved like the wild type when no propionate was present. However, when a double-deletion mutant defective in both methylcitrate synthase and CoA-transferase was constructed, the resulting strain was unable to grow on media containing acetate and propionate as sole carbon sources, which confirmed the in vivo activity of the CoA-transferase.