On the Evolution of Nonribosomal Peptide Synthetase Gene Clusters in Cyanobacteria

Abstract

Cyanobacteria from genera Anabaena, Microcystis and Planktothrix produce a wide selection of nonribosomal peptides, e.g. microcystins and cyanopeptolins. Nonribosomal peptides are produced by peptide synthetases or polyketide synthase/peptide synthetase hybrids. In this study, microcystin and cyanopeptolin synthetase gene clusters were analyzed in Anabaena, Microcystis and Planktothrix. Cyanopeptolin synthetase gene clusters were characterized from Microcystis and Planktothrix. Comparison of these two gene clusters with a previously described cyanopeptolin synthetase gene cluster from Anabaena shows similar gene and domain arrangements, while adenylation domains and tailoring domains vary. This suggests independent loss and gain of tailoring domains within each genus as major events leading to the present diversity. Although recombination clearly has been a major factor in the evolution of nonribosomal peptide synthetases, no clear evidence of recombination involving transfer of genetic material across genera was found, neither within the the cyanopeptolin gene clusters nor between the cyanopeptolin and microcystin gene clusters. Microcystin synthetase genes were investigated in a number of naturally occurring Anabaena, Microcystis and Planktothrix strains. Production of solely unmethylated microcystin isoforms correlated either with lack of the N-methyltransferase domain in Anabaena strains, or, in Microcystis, with point mutations in functionally important sites of the domain. The analysis of adenylation domains encoded by mcyB and mcyC in the three genera revealed group-specific differences in recombination rates and selectional forces. Analysis of regions flanking the mcy gene cluster in Microcystis spp. revealed that the genomic location of the mcy gene cluster is the same in all strains examined.