Abstract
BackgroundMicrocystins are small cyclic heptapeptide toxins produced by a range of distantly related cyanobacteria. Microcystins are synthesized on large NRPS-PKS enzyme complexes. Many structural variants of microcystins are produced simulatenously. A recombination event between the first module of mcyB (mcyB1) and mcyC in the microcystin synthetase gene cluster is linked to the simultaneous production of microcystin variants in strains of the genus Microcystis.ResultsHere we undertook a phylogenetic study to investigate the order and timing of recombination between the mcyB1 and mcyC genes in a diverse selection of microcystin producing cyanobacteria. Our results provide support for complex evolutionary processes taking place at the mcyB1 and mcyC adenylation domains which recognize and activate the amino acids found at X and Z positions. We find evidence for recent recombination between mcyB1 and mcyC in strains of the genera Anabaena, Microcystis, and Hapalosiphon. We also find clear evidence for independent adenylation domain conversion of mcyB1 by unrelated peptide synthetase modules in strains of the genera Nostoc and Microcystis. The recombination events replace only the adenylation domain in each case and the condensation domains of mcyB1 and mcyC are not transferred together with the adenylation domain. Our findings demonstrate that the mcyB1 and mcyC adenylation domains are recombination hotspots in the microcystin synthetase gene cluster.ConclusionRecombination is thought to be one of the main mechanisms driving the diversification of NRPSs. However, there is very little information on how recombination takes place in nature. This study demonstrates that functional peptide synthetases are created in nature through transfer of adenylation domains without the concomitant transfer of condensation domains.
Highlights
Microcystins are small cyclic heptapeptide toxins produced by a range of distantly related cyanobacteria
Microcystins are a diverse group of low molecular weight cyclic heptapeptides and are the most common hepatotoxins produced by cyanobacteria
The microcystin variants produced by these strains differed in the methylation of the α-amino group of Mdha, the β-carboxyl of D-erythro-β-methyl-aspartic acid (D-MeAsp) and the C9 hydroxyl of Adda
Summary
Microcystins are small cyclic heptapeptide toxins produced by a range of distantly related cyanobacteria. Microcystins are a diverse group of low molecular weight cyclic heptapeptides and are the most common hepatotoxins produced by cyanobacteria. They are potent inhibitors of eukaryotic protein phosphatases 1 and 2A [2] and are linked to the deaths of wild animals and livestock worldwide [1]. The general structure of microcystins can be summarized as cyclo-D-Ala1-X2-D-MeAsp3-Z4-Adda5-D-Glu6Mdha7where X and Z are variable L-amino acids (Figure 1). Many of these microcystin variants are synthesized simultaneously by the producing cyanobacterium [1]. Structural variants of microcystin do not have the same toxicities and microcystin-LR is an order of magnitude more toxic than microcystin-RR [1]
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