Natural Variation in the Microcystin Synthetase Operon mcyABC and Impact on Microcystin Production in Microcystis Strains

Abstract

Toxic Microcystis strains often produce several isoforms of the cyclic hepatotoxin microcystin, and more than 65 isoforms are known. This has been attributed to relaxed substrate specificity of the adenylation domain. Our results show that in addition to this, variability is also caused by genetic variation in the microcystin synthetase genes. Genetic characterization of a region of the adenylation domain in module mcyB1 resulted in identification of two groups of genetic variants in closely related Microcystis strains. Sequence analyses suggested that the genetic variation is due to recombination events between mcyB1 and the corresponding domains in mcyC. Each variant could be correlated to a particular microcystin isoform profile, as identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry. Among the Microcystis species studied, we found 11 strains containing different variants of the mcyABC gene cluster and 7 strains lacking the genes. Furthermore, there is no concordance between the phylogenies generated with mcyB1, 16S ribosomal DNA, and DNA fingerprinting. Collectively, these results suggest that recombination between imperfect repeats, gene loss, and horizontal gene transfer can explain the distribution and variation within the mcyABC operon.Cyanobacteria are phototrophic organisms that often form water blooms in eutrophic or estuarine waters. These water blooms undergo fluctuations and may exhibit toxic states. One common genus in such water blooms, Microcystis, produces the hepatotoxin microcystin (6). There are approximately 65 known isoforms of microcystin, representing a family of cyclic heptapeptides having the common structure cyclo(D-Ala-L-X-D-MeAsp-L-Z-Adda-D-Glu-Mdha), where L-X and L-Z are variable L amino acids, Adda is 3-amino-9-methoxy-2,6,8,-trimethyl-10-phenyl-4,6-decandienoic acid, D-MeAsp is 3-methylaspartic acid, and Mdha is N-methyl-dehydroalanine (Fig.

The cylindrospermopsin gene cluster of Aphanizomenon sp. strain 10E6: organization and recombination

“…For the mcy genes, it has been shown that the operon structure and the order of genes varies between the genera Planktothrix, Anabaena and Microcystis (reviewed by Dittmann & Börner, 2005). In addition, recombination between different domains within the mcy gene clusters, point mutations and deletion of gene regions, as well as lack of substrate specificity of A-domains have been shown to contribute to the synthesis of different microcystin isoforms (Fewer et al, 2008;Mikalsen et al, 2003;Tooming-Klunderud et al, 2008). A deletion within the second module of mcyA and the first module of mcyB resulted in the synthesis of another cyanotoxin, nodularin (Moffitt & Neilan, 2004;Rantala et al, 2004).…”

Diversity of cyanobacteria and cyanotoxins in Hartbeespoort Dam, South Africa

“…Other genes of the microcystin encoding gene cluster, e.g. mcyA, mcyB, and mcyT, have been reported in non-MC-producing Microcystis and Planktothrix strains (Mikalsen et al 2003;Kurmayer et al 2004;Christiansen et al 2008). Genes encoding the biosynthesis of other cyanobacterial toxins, e.g.…”

Characterization of the Biocidal Spectrum of Extracellular Filtrates of Microcystis aeruginosa

“…Also, the genetic aspect of toxin production i.e. the enzyme responsible for toxin production, enzyme structure and genes involved in toxin production by this cyanobacterial genus has also been investigated by several workers [19][20][21][22]. Studies have revealed that there is significant influence of culture age on microcystin production.…”