M.PHI-3TII - A NEW MONOSPECIFIC DNA (CYTOSINE-C5) METHYLTRANSFERASE WITH PRONOUNCED AMINO-ACID-SEQUENCE SIMILARITY TO A FAMILY OF ADENINE-N6-DNA-METHYLTRANSFERASES

The temperate B.subtilis phages phi 3T and rho 11(s) code, in addition to the multispecific DNA (cytosine-C5) methyltransferases (C5-MTases) M.phi 3TI and M.rho 11(s)I, which were previously characterized, for the identical monospecific C5-MTases M.phi 3TII and M.rho 11(s)II. The se enzymes modify the C of TCGA sites, a novel target specificity amon g C5-MTases. The primary sequence of M.phi 3TII (326 amino acids) show s all conserved motifs typical of the building plan of C5-MTases. The degree of relatedness between M.phi 3TII and all other mono- or multis pecific C5-MTases ranges from 30-40% amino acid identity. Particularly M.phi 3TII does not show pronounced similarity to M.phi 3TI indicatin g that both MTase genes were not generated from one another but were a cquired independently by the phage. The amino terminal part of the M.p hi 3TII (preceding the variable region 'V'), which predominantly const itutes the catalytic domain of the enzyme, exhibits pronounced sequenc e 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 organ ization of C5- and A-NG-MTases imply divergent evolution of these enzy mes originating from a common molecular ancestor.