A Point Mutation inftmDBlocks the Fumitremorgin Biosynthetic Pathway inAspergillus fumigatusStrain Af293

Fumitremorgins (FTMs), tremorgenic mycotoxins produced by the human pathogen Aspergillus fumigatus, are prenylated indole alkaloids that have been extensively studied in view of their diverse chemical structures and biological activities. Their biosynthetic gene (ftm) cluster was identified on the basis of the genome sequence of A. fumigatus. However, it has been reported that the ftm cluster in genome reference strain Af293 is inactive, which makes complete understanding of the FTM pathway difficult. Hence, we used an FTM-producing strain of A. fumigatus, BM939, to dissect the FTM pathway. Here, we delineate the genetic determinant for the observed defect in the FTM pathway in A. fumigatus Af293. Metabolite profiling and sequence comparison of the two strains revealed a point mutation in ftmD as a possible cause of altered metabolite production in strain Af293. FTM production in Af293 was restored when a DNA fragment containing ftmD from BM939 was introduced. Biochemical analysis indicated that FtmD is a methyltransferase that catalyzes the conversion of 6-hydroxytryprostatin B into tryprostatin A. The mutated FtmD retained enzymatic activity but did not function under physiological conditions, resulting in blockage of the FTM pathway in A. fumigatus Af293.