FIG. 1.

Microcystin synthetase gene cluster of Anabaena strain 90, a linear model for the assembly and the structure of microcystin-LR. Abbreviations for domains: A, adenylation; C, condensation; T, thiolation; NMT, N-methyltransferase; EP, epimerase; TE, thioesterase; KR, β-ketoacyl reductase. Adda is 3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid, Mdha is N-methyldehydroalanine, and d-Me-Asp is d-erythro-β-methylaspartic acid. The thick arrows point to three methyl groups, which are putatively introduced by the CM domains. The way of cyclization of the microcystin precursor is shown with an arrow on the right of the picture.