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Natural occurrence of microcystin synthetase deletion mutants capable of producing microcystins in strains of the genus Anabaena (Cyanobacteria)

¹ Department of Applied Chemistry and Microbiology, PO Box 56, Viikki Biocenter, Viikinkaari 9, FIN-00014, University of Helsinki, Finland
² Institute of Molecular Biosciences, PO Box 1041, Blindern, University of Oslo, Norway
³ NIVA – Norwegian Institute for Water Research, 0441 Oslo, Norway
⁴ Centre for Ecological and Evolutionary Synthesis, Department of Biology, University of Oslo, Norway

Correspondence
Kaarina Sivonen
kaarina.sivonen{at}helsinki.fi

Microcystins form a large family of small cyclic heptapeptides harbouring extensive modifications in amino acid residue composition and functional group chemistry. These peptide hepatotoxins contain a range of non-proteinogenic amino acids and unusual peptide bonds, and are typically N-methylated. They are synthesized on large enzyme complexes consisting of non-ribosomal peptide synthetases and polyketide synthases in a variety of distantly related cyanobacterial genera. Here we report a 1236 bp in-frame deletion mutation in the mcyA gene of the microcystin biosynthetic pathway in nine strains of the genus Anabaena. The deletion removed almost the entire N-methyltransferase (NMT) domain. Strains of Anabaena carrying the in-frame deletion mutation incorporated mainly dehydroalanine (Dha) into the microcystins they produce while strains with full-length mcyA genes incorporated mainly N-methyldehydroalanine (Mdha). Interestingly, the strains of Anabaena lacking the NMT domain also incorporated elevated amounts of L-Ser, the precursor of Mdha and Dha, into the microcystin they produced relative to strains carrying functional NMT domains. We provide evidence for the in-frame deletion of the NMT domain without the co-conversion of the flanking adenylation domain. Our results demonstrate a further example of the strategies employed by cyanobacteria in the biosynthesis of microcystin variants.

A supplementary table with details of the microcystin variants produced by the 26 strains of Anabaena studied is available with the online version of this paper.