The tryptophan aminotransferase Tam1 catalyses the single biosynthetic step for tryptophan-dependent pigment synthesis in Ustilago maydis

摘要

Tryptophan is a precursor for many biologically active secondary metabolites. We have investigated the origin of indole pigments first described in the pityriasis versicolor-associated fungus Malassezia furfur. Some of the identified indole pigments have properties potentially explaining characteristics of the disease. As M. furfur is not amenable to genetic manipulation, we used Ustilago maydis to investigate the pathway leading to pigment production from tryptophan. We show by high-performance liquid chromatography, mass spectrometry and nuclear magnetic resonance analysis that the compounds produced by U. maydis include those putatively involved in the etiology of pityriasis versicolor. Using a reverse genetics approach, we demonstrate that the tryptophan aminotransferase Tam1 catalyses pigment biosynthesis by conversion of tryptophan into indolepyruvate. A forward genetics approach led to the identification of mutants incapable of producing the pigments. These mutants were affected in the sir1 gene, presumably encoding a sulphite reductase. In vitro experiments with purified Tam1 showed that 2-oxo 4-methylthio butanoate serves as a substrate linking tryptophan deamination to sulphur metabolism. We provide the first direct evidence that these indole pigments form spontaneously from indolepyruvate and tryptophan without any enzymatic activity. This suggests that compounds with a proposed function in M. furfur-associated disease consist of indolepyruvate-derived spontaneously generated metabolic by-products.