New insights into polyene macrolide biosynthesis in Couchioplanes caeruleus

Couchioplanes caeruleus DSM43634 synthesises 67-121C, an aromatic heptaene macrolide that contains a mannosyl-mycosaminyl disaccharide. An improved draft genome sequence was used to obtain the biosynthetic gene cluster for this antifungal. Bioinformatic analysis of the polyketide synthase indicated that extension modules 7 and 8 contain A-type ketoreductase and dehydratase domains. These modules are therefore predicted to form cis double bonds. The deduced stereostructure of the 67-121C macrolactone is identical to that experimentally determined for the partricin subgroup of aromatic heptaenes. Some of these polyenes are N-methylated on the aminoacetophenone moiety. The C. caeruleus AceS protein was shown to methylate 4-aminoacetophenone and esters of 4-aminobenzoate, but not 4-aminobenzoate. This suggests that the substrate specificity of AceS prevents it from interfering with folate biosynthesis. The methyltransferase should be valuable for chemoenzymatic alkylation of compounds that contain aminobenzoyl moieties.