AuaA, a Membrane-Bound Farnesyltransferase from Stigmatella aurantiaca, Catalyzes the Prenylation of 2-Methyl-4-hydroxyquinoline in the Biosynthesis of Aurachins

Aurachins are quinoline alkaloids isolated from the myxobacterium Stigmatella aurantiaca. They are substituted with an isoprenoid side chain and act as potent inhibitors in the electron transport chain. A biosynthetic gene cluster that contains at least five genes (auaA-auaE) has been identified for aurachin biosynthesis. In this study, auaA, the gene encoding a putative prenyltransferase of 326 amino acids, was cloned and overexpressed in Escherichia coli. Biochemical investigations showed that AuaA catalyzes the prenylation of 2-methyl-4-hydroxyquinoline in the presence of farnesyl diphosphate (FPP), thereby resulting in the formation of aurachin D. The hydroxyl group at position C4 of the quinoline ring is essential for an acceptance by AuaA; this was concluded by testing 18 quinoline derivatives or analogues with AuaA and FPP. H-1 NMR and HR-EI-MS analyses of six isolated enzyme products revealed the presence of a farnesyl moiety at position C3 of the quinoline ring. K-M values of 43 and 270 mm were determined for FPP and 2-methyl-4-hydroxyquinoline, respectively. Like other known membrane-bound prenyltransferases, the reaction catalyzed by AuaA is dependent on the presence of metal ions such as Mg2+, Mn2+ and Co2+, although no typical (N/D) DXXD binding motif was found in the sequence.