The mosaic structure of the mcyABC operon in Microcystis

1 University of Oslo, Department of Molecular Biosciences, 0316 Oslo, Norway 2 University of Oslo, Department of Biology, Centre for Ecological and Evolutionary Synthesis (CEES), 0316 Oslo, Norway 3 University of Oslo, Microbial Evolution Research Group (MERG), 0316 Oslo, Norway Correspondence Kjetill S. Jakobsen kjetill.jakobsen{at}bio.uio.no An extensive study of the mcyABC genes and regions flanking the mcy gene cluster was performed in naturally occurring Microcystis strains. Lack of methylation in strains producing only desmethyl 7 -microcystin was found to be associated with point mutations in substrate-binding sequence motifs of the N -methyltransferase (NMT) domain in McyA. Multiple recombination events giving rise to ‘phylogenetic mosaics’ were detected within the NMT -domain-encoding mcyA sequences and the adenylation (A) domain sequences of mcyB and mcyC . Recombination leading to exchanges between the mcyB and mcyC regions encoding A domains in modules McyB1 and McyC was also detected. A previously reported replacement of the A domain in McyB1 was found to involve the region between the conserved motifs A3 and A8/A9. In all microcystin-producing strains the mcy gene cluster was flanked by the genes uma1 and dnaN . Clear indications of recombination, an insertion element and footprints of IS elements were found in the dnaN – mcyJ intergenic region. Among the non-microcystin producers, uma1 and dnaN were linked in some, but not all strains. Most non-producing strains lacked all mcy genes, while one strain possessed a partially deleted mcy operon. Our results show that frequent horizontal gene transfer events in addition to point mutations and insertions/deletions contribute to variation in the mcy gene cluster. Abbreviations: A, adenylation (domain); C, condensation (domain); ML, maximum likelihood; NJ, neighbour joining; NMT, N -methyltransferase (domain); SAM, S -adenosylmethionine Present address: Division of Pathology, Rikshospitalet University Hospital, 0027 Oslo, Norway. The GenBank/EMBL/DDBJ accession numbers for the sequences determined in this work are shown in Table 1. A supplementary table of primers and three supplementary figures are available with the online version of this paper.