A PAIR OF SINGLE-STRAND AND DOUBLE-STRAND DNA CYTOSINE N4 METHYLTRANSFERASES FROM BACILLUS-CENTROSPORUS

Sequence analysis of the Bcnl restriction-modification system revealed the presence of an open reading frame encoding a second cytosine-N4 m ethyltransferase, M.BcnlA, in the vicinity of the genes specifying the previously characterized cytosine-N4 methyltransferase M.BcnlB and re striction endonuclease R.Bcnl. Both methyltransferases were purified f rom the E. coil cells expressing the individual genes, and their enzym atic efficiencies in vitro were compared with a variety of DNA substra tes. Both enzymes act on 5'-CC(C/G)GG-3' sites in double-stranded DNA, however, M.BcnlA can also, with a comparable efficiency, modify the s pecific targets in single-stranded DNA, The biological significance of the presence of the tandem methyltransferases in the Bcnl system is d iscussed.