Degradation of the mycotoxin fusaric acid in burkholderia ambifaria t16: genes and metabolic pathways involved

Abstract

Fusaric acid (FA, 5-butylpyridine, 2-carboxylic acid) is a secondary metabolite produced by several Fusarium species, which is toxic for bacteria, plants, animals and humans. This mycotoxin contributes to the virulence of phytopathogenic Fusarium in several crops, causing important economic losses. Moreover, FA reduces survival and competition abilities of bacterial species able to antagonize Fusarium spp. due to its negative effects on viability and production of antibiotics effective against these fungi. Burkholderia ambifaria T16 is a bacterial strain isolated from the rhizosphere of barley that showed the interesting ability to degrade FA and detoxify this mycotoxin from barley seedlings. The genes and metabolic pathways involved in FA degradation have not been identified so far in any bacterial species. By screening of a transposon insertion library and proteomic analysis we were able to identify genes and metabolic pathways that would be involved in FA degradation. A functional 2-methylcitrate cycle (2-MCC), a carbon anaplerotic pathway widely distributed among bacteria and fungi where propionyl-CoA is converted to pyruvate and succinate, was shown to be essential for the growth of B. ambifaria T16 in the presence of FA. Propionyl-CoA and its derived catabolites are lethally toxic to cells when accumulate. For that reason, besides providing succinate and pyruvate, the 2-MCC also has a very important role in the detoxification of propionyl-CoA and its catabolites. The comparison of the proteomic profile of B. ambifaria T16 growing with FA or citrate as sole carbon sources showed that more than 50 enzymes were significantly overexpressed during growth with FA, including 2-MCC enzymes and enzymes that convert butyryl-CoA to propanoyl-CoA, suggesting that propanoyl-CoA is produced during FA degradation. Moreover, several proteins, including an AraC-type transcriptional regulator, a FMN-dependent two-component luciferase like monooxygenase (LLM) system, an amidohydrolase, two enoyl-CoA hydratases and a long-chain fatty acid ligase, encoded in the same gene cluster, were highly over-expressed during growth with FA (>10 fold up-regulation). In the last years, two-component LLMs were shown to catalyze the pyridine-ring cleavage of several N-heterocyclic compounds, suggesting that the mentioned gene cluster is a good candidate to be involved in the initial steps of FA degradation in B. ambifaria T16.