Genetic dissection of the degradation pathways for the mycotoxin fusaric acid in Burkholderia ambifaria T16

Fusaric acid (FA) is a mycotoxin produced by several Fusarium species. Burkholderia ambifaria T16 is a rhizosphere bacterium, able to use FA as sole nitrogen, carbon, and energy source. By screening a transposon insertional library, combined with proteomic analysis, genes and enzymes involved in the microbial degradation of FA were identified for the first time. A functional 2-methylcitrate cycle, an anaplerotic pathway where propionyl-coenzyme A (CoA) is converted to pyruvate and succinate, was shown to be essential for growth in the presence of FA. The proteomic profile of B. ambifaria T16 showed that more than 50 enzymes (including those belonging to the 2-methylcitrate cycle, fatty acid metabolism, valine catabolism, and flavin biosynthesis) were significantly more abundant when growing on FA than on citrate. Flavin mononucleotide (FMN)-dependent luciferases like monooxygenase (LLM) are shown to catalyze the pyridine-ring cleavage reaction of several N-heterocyclic compounds. Deletion of a gene encoding a predicted LLM enzyme that was highly upregulated during growth on FA, completely abolished the capability of B. ambifaria T16 to grow with this mycotoxin as sole nitrogen, carbon, and energy source. Re-introduction of the wild type gene was able to restore growth. The mentioned gene is part of a gene cluster of unknown function that we termed fua, due to its probable role in fusaric acid catabolism. Our results suggest that the LLM encoded in the fua cluster catalyzes the pyridine-ring opening reaction during FA degradation, and that propionyl-CoA is one of the intermediates of FA catabolism in B. ambifaria T16.