title: Evidence for positive selection acting on microcystin synthetase adenylation domains in three cyanobacterial genera.
abstract: BACKGROUND
  Cyanobacteria produce a wealth of secondary metabolites, including the group of small cyclic heptapeptide hepatotoxins that constitutes the microcystin family. The enzyme complex that directs the biosynthesis of microcystin is encoded in a single large gene cluster (mcy). mcy genes have a widespread distribution among cyanobacteria and are likely to have an ancient origin. The notable diversity within some of the Mcy modules is generated through various recombination events including horizontal gene transfer.
  
  
  RESULTS
  A comparative analysis of the adenylation domains from the first module of McyB (McyB1) and McyC in the microcystin synthetase complex was performed on a large number of microcystin-producing strains from the Anabaena, Microcystis and Planktothrix genera. We found no decisive evidence for recombination between strains from different genera. However, we detected frequent recombination events in the mcyB and mcyC genes between strains within the same genus. Frequent interdomain recombination events were also observed between mcyB and mcyC sequences in Anabaena and Microcystis. Recombination and mutation rate ratios suggest that the diversification of mcyB and mcyC genes is driven by recombination events as well as point mutations in all three genera. Sequence analysis suggests that generally the adenylation domains of the first domain of McyB and McyC are under purifying selection. However, we found clear evidence for positive selection acting on a number of amino acid residues within these adenylation domains. These include residues important for active site selectivity of the adenylation domain, strongly suggesting selection for novel microcystin variants.
  
  
  CONCLUSIONS
  We provide the first clear evidence for positive selection acting on amino acid residues involved directly in the recognition and activation of amino acids incorporated into microcystin, indicating that the microcystin complement of a given strain may influence the ability of a particular strain to interact with its environment.
authors.affiliation: University of Oslo, Department of Biology, Centre for Ecological and Evolutionary Synthesis, 0316 Oslo, Norway. ave.tooming-klunderud@bio.uio.no
authors.forename: Ave
authors.initials: A
authors.lastname: Tooming-Klunderud
authors.forename: David P
authors.initials: DP
authors.lastname: Fewer
authors.forename: Thomas
authors.initials: T
authors.lastname: Rohrlack
authors.forename: Jouni
authors.initials: J
authors.lastname: Jokela
authors.forename: Leo
authors.initials: L
authors.lastname: Rouhiainen
authors.forename: Kaarina
authors.initials: K
authors.lastname: Sivonen
authors.forename: Tom
authors.initials: T
authors.lastname: Kristensen
authors.forename: Kjetill S
authors.initials: KS
authors.lastname: Jakobsen
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics*; D000458:Cyanobacteria / Q000201:enzymology* / Q000235:genetics*; D004269:DNA, Bacterial / Q000235:genetics; D019143:Evolution, Molecular; D005798:Genes, Bacterial; D014644:Genetic Variation; D008969:Molecular Sequence Data; D005810:Multigene Family; D010453:Peptide Synthases / Q000235:genetics*; D010802:Phylogeny; D054730:Protein Interaction Domains and Motifs; D020033:Protein Isoforms; D011995:Recombination, Genetic; D012641:Selection, Genetic; D016415:Sequence Alignment; D020539:Sequence Analysis, Protein; D013379:Substrate Specificity
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; D020033:Protein Isoforms; D010453:Peptide Synthases; C432283:microcystin synthetase A