Reverse Prenyltransferase in the Biosynthesis of Fumigaclavine C in Aspergillus fumigatus: Gene Expression, Purification, and Characterization of Fumigaclavine C Synthase FGAPT1

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Abstract
A putative prenyltransferase gene—fgaPT1—has been identified in the biosynthetic gene cluster of fumigaclavines in Aspergillus fumigatus AF293. The gene was cloned and overexpressed in Escherichia coli, and the His6‐fusion FgaPT1 was purified to near homogeneity and characterized biochemically. The enzyme was found to convert fumigaclavine A into fumigaclavine C by attaching a dimethylallyl moiety to C‐2 of the indole nucleus in a “reverse” manner, that is, by connection of C‐3 of the dimethylallyl moiety to an aromatic nucleus. FgaPT1 is a soluble, dimeric protein with a subunit size of 50 kDa. Km(app) values for fumigaclavine A and dimethylallyl diphosphate were determined to be 6 and 13 μM, respectively, while the turnover number was 0.8 s−1. Metal ions such as Mg2+ and Ca2+ are not essential for the enzymatic activity. FgaPT1 showed relatively strict substrate specificity towards fumigaclavine A, with only dimethylallyl diphosphate being accepted as a donor under our conditions. FgaPT1 is the first reverse prenyltransferase from fungi to have been purified and characterized in homogenous form after heterologous overproduction. Surprisingly, it shows very low sequence similarity to the recently identified prenyltransferase LtxC from cyanobacteria, which also catalyzes the reverse prenylation of an indole nucleus.