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The mosaic structure of the mcyABC operon in Microcystis

Bjørg Mikalsen^2,

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 desmethyl⁷-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.

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.