LAUSR

One of the most commonly prescribed antibiotics against Burkholderia infections is co‐trimoxazole, a cocktail of trimethoprim and sulfamethoxazole. Trimethoprim elicits an upregulation of the mal gene cluster, which encodes proteins involved in synthesis of the cytotoxic polyketide malleilactone; trimethoprim does so by increasing expression of the malR gene, which encodes the activator MalR. We report that B. thailandensis grown on trimethoprim exhibited increased virulence against Caenorhabditis elegans. This enhanced virulence correlated with an increase in expression of the mal gene cluster. Notably, inhibition of xanthine dehydrogenase by addition of allopurinol led to similar upregulation of malA and malR, with addition of trimethoprim or allopurinol also resulting in an equivalent intracellular accumulation of xanthine. Xanthine is a ligand for the transcription factor MftR that leads to attenuated DNA binding, and we show using chromatin immunoprecipitation that MftR binds directly to malR. Our gene expression data suggest that malR expression is repressed by both MftR and by a separate transcription factor, which also responds to a metabolite that accumulates on exposure to trimethoprim. Since allopurinol elicits a similar increase in malR/malA expression as trimethoprim, we suggest that impaired purine homeostasis plays a primary role in trimethoprim‐mediated induction of malR and in turn malA.