The Genetic Basis for O-Acetylation of the Microcystin Toxin in Cyanobacteria

•We report here the complete O-acetylated microcystin biosynthetic pathway•We show that this pathway encodes a 25 kDa O-acetyltransferase•We used this microcystin acetyltransferase to make unnatural microcystin variants•Microcystin acetyltransferase belongs to an enzyme family that inactivates antibiotics

SummaryMicrocystins are a family of cyclic peptide toxins produced by cyanobacteria. They are responsible for the toxicosis and death of wild and domestic animals throughout the world. They display extensive variation in amino acid composition and functional group chemistry. O-acetylated microcystins are frequently produced by free-living and symbiotic strains of the genus Nostoc. Here, we show that the production of acetylated microcystins is catalyzed by an acetyl-coenzyme A-dependent O-acetyltransferase (McyL) encoded in the 57 kb microcystin synthetase gene cluster of Nostoc sp. 152. Phylogenetic analysis demonstrates that McyL belongs to a family of enzymes that inactivate antibiotics through O-acetylation. The McyL enzyme has a relaxed substrate specificity, allowing the preparation of semisynthetic microcystins. This study sheds light on the evolutionary origins and genetic diversity of an important class of enzymes involved in antibiotic resistance.