Structural organization of microcystin biosynthesis in Microcystis aeruginosa PCC7806: an integrated peptide–polyketide synthetase system

Abstract

Background: Blooms of toxic cyanobacteria (blue-green algae) have become increasingly common in the surface waters of the world. Of the known toxins produced by cyanobacteria, the microcystins are the most significant threat to human and animal health. These cyclic peptides are potent inhibitors of eukaryotic protein phosphatases type 1 and 2A. Synthesized nonribosomally, the microcystins contain a number of unusual amino acid residues including the β-amino polyketide moiety Adda (3-amino-9-methoxy-2,6,8-trimethyl-10-phenyl-4,6-decadienoic acid). We have characterized the microcystin biosynthetic gene cluster from Microcystis aeruginosa PCC7806. Results: A cluster spanning 55 kb, composed of 10 bidirectionally transcribed open reading frames arranged in two putative operons (mcyA–C and mcyD–J), has been correlated with microcystin formation by gene disruption and mutant analysis. Of the 48 sequential catalytic reactions involved in microcystin synthesis, 45 have been assigned to catalytic domains within six large multienzyme synthases/synthetases (McyA–E, G), which incorporate the precursors phenylacetate, malonyl-CoA, S-adenosyl- L-methionine, glutamate, serine, alanine, leucine, D-methyl-isoaspartate, and arginine. The additional four monofunctional proteins are putatively involved in O-methylation (McyJ), epimerization (McyF), dehydration (McyI), and localization (McyH). The unusual polyketide amino acid Adda is formed by transamination of a polyketide precursor as enzyme-bound intermediate, and not released during the process. Conclusions: This report is the first complete description of the biosynthesis pathway of a complex cyanobacterial metabolite. The enzymatic organization of the microcystin assembly represents an integrated polyketide–peptide biosynthetic pathway with a number of unusual structural and enzymatic features. These include the integrated synthesis of a β-amino-pentaketide precursor and the formation of β- and γ-carboxyl-peptide bonds, respectively. Other features of this complex system also observed in diverse related biosynthetic clusters are integrated C- and N-methyltransferases, an integrated aminotransferase, and an associated O-methyltransferase and a racemase acting on acidic amino acids.