M.?3TII: a new monospecific DNA (cytosine-C5) methyltransf erase with pronounced ami no acid sequence similarity to a family of adenine-N6-DNA-methyltransferases

摘要

The temperate B.subtilis phages ?3T and ?11s code, in addition to the multispecific DNA (cytosine-C5) methyltransf erases (C5-MTases) M.?3TI and M.?11_sl, which were previously characterized, for the identical monospecific C5-MTases M.?3TII and M.?11_SII. These enzymes modify the C of TCGA sites, a novel target specificity among C5-MTases. The primary sequence of M.?3TII (326 amino acids) shows all conserved motifs typical of the building plan of C5-MTases. The degree of relatedness between M.?3TII and all other mono- or multispecific C5-MTases ranges from 30 - 40% amino acid identity. Particularly M.?3TII does not show pronounced similarity to M.?3TI indicating that both MTase genes were not generated from one another but were acquired independently by the phage. The amino terminal part of the M.?3TII (preceding the variable region ‘V'), which predominantly constitutes the catalytic domain of the enzyme, exhibits pronounced sequence similarity to the amino termini of a family of A-N6-MTases, which — like M.Taql — recognize the general sequence TNNA. This suggests that recently described similarities in the general three dimensional organization of C5- and A-N6-MTases imply divergent evolution of these enzymes originating from a common molecular ancestor.