Evidence That Microcystin Is a Thio‐template Product1

The hepatotoxic cyclic heptapeptide toxins of cyanobacteria, collectively termed microcystins, are potent inhibitors of protein phosphatases PP1 and PP2A. The structure of microcystins resembles small, cyclic peptide secondary metabolites from fungi and eubacteria. Many of these metabolites are manufactured via a nonribosomal thio‐template mechanism. We submit evidence that microcystin is synthesized by a similar mechanism. The organism used in this study was Microcystis aeruginosa PCC7820. Using the traditional ATP‐32PPi exchange assay for thio‐template activity, we found activity in the presence of the substrate d‐amino acids occurring in microcystin. Thio‐template mechanisms are known to be unaffected by protein synthesis inhibitors such as chloramphenicol. We subjected cultures in exponential and stationary growth to chloramphenicol and monitored culture health versus toxicity. Although the health of the treated cultures declined, the toxicity of the remaining cells increased. We developed an in vitro assay to measure microcystin synthesis in cell lysates in the presence of chloramphenicol. By supplementing the lysates with ATP and the substrate amino acids present in microcystin, we detected a fourfold increase in total microcystins over the course of 20 min.