Characterization of Methyltransferase AlmCII in Chalcomycin Biosynthesis: The First TylF Family O-Methyltransferase Works on a 4'-Deoxysugar.

Sugar O-methylation is a ubiquitous modification in natural products and plays diverse roles. This realization has inspired many attempts to search for novel methyltransferases. Chalcomycins are a group of 16-membered macrolides containing two methylated sugars that require three methyltransferases for their biosynthesis. Here, we identified that AlmCII, a sugar O-methyltransferase belonging to the TylF family that was previously only known to methylate sugars with a 4'-hydroxy group, can methylate a 4',6'-dideoxysugar during the biosynthesis of chalcomycins. An in vitro enzymatic assay revealed that AlmCII is divalent metal-dependent with an optimal pH of 8.0 and optimal temperature of 42 °C. Moreover, the 3'-O-demethylated chalcomycins exhibit less than 6 % of the antibacterial activity of their parent compounds. This is the first report demonstrating that a TylF family O-methyltransferase can use a 4'-deoxy sugar as a substrate and highlighting the importance of this methylation for the antibacterial activity of chalcomycins.