Isolation and nucleotide sequence of the gene encoding the XamI DNA methyltransferase of Xanthomonas campestris pv. amaranthicola

The gene (xainIM) encoding the DNA methyltransferase of the XamI restriction-modification system from Xanthomonas campestris pv. amaranthicola (M.XamI) has been cloned in Escherichia coli and its nucleotide sequence determined. The sequence predicts a protein of 527 amino acids that contains nine conserved motifs characteristic of DNA amino methyltransferases. In fact, M.XamI shows significant similarity with NG-adenine methyltransferases of the γ group of amino methyltransferases, including M.XamI (from the isoschizomeric SalI restriction-modification system) and M.TaqI (the only N6-adenine methyltransferase for which a three-dimensional structure is available). M.XamI and M.XamI share two highly conserved regions within the C-terminal domain, one of which aligns with one of the DNA recognition loops proposed for M.TaqI. Analysis of the chromosomal DNA adjacent to xamIM led to the identification of an additional ORF (275 codons), downstream, in the same transcriptional orientation. Although some limited similarities between the SalI restriction enzyme and the product deduced from this ORF were found, the clone carrying xamIM did not express the expected endonuclease function.