Detection of toxigenicity by a probe for the microcystin synthetase A gene(mcyA) of the cyanobacterial genus Microcystis, comparison of toxicities with 16S rRNA and phycocyanin operon (phycocyanin intergenic spacer) phylogenies

The relationship between toxigenicity and phylogeny within the cyanobacteri al genus Microcystis is unclear. To investigate this issue, we have designe d PCR primers for the N-methyltransferase (NMT) domain of the microcystin s ynthetase gene mcyA and have probed 37 Microcystis sp, cultures as well as several field samples, The NMT region was present in all 18 laboratory stra ins that gave positive reactions in the protein phosphatase inhibition assa y for microcystin but was absent in 17 nontoxic strains. Two other nontoxic strains, one of which had previously been reported to produce microcystin, possessed the NMT region. Detection of NMT-specific DNA in field samples c orresponded to periods of toxicity as assessed by protein phosphatase inhib ition, The Microcystis strains formed a monophyletic cluster based on 16S r RNA gene sequences but comprised two groups with respect to phycocyanin int ergenic spacer (PC-IGS) sequences. Toxic and nontoxic strains appeared to b e erratically distributed within the PC-ICS and 16S rRNA trees, Sequence an alysis of the NMT domain revealed two coherent groups. The genomic region i mmediately downstream of the mcyABC cluster in all 20 NMT-positive strains contained an open reading frame of unknown function (uma1) at a conserved d istance from mcyC. All nontoxic strains also contained uma1, which is not c otranscribed with mcyABC, The consistent linkage of mcyC to umal suggests t hat mcyC has not been frequently transferred into nontoxic strains via any mechanism involving insertion at random chromosomal locations. These result s are discussed with respect to various mechanisms that could explain the p atchy distribution of toxigenicity among the various Microcystis clades.