A pair of single-strand and double-strand DNA cytosine-N4 methyltransferases from Bacillus centrosporus

Sequence analysis of the BcnI restriction-modification system revealed the presence of an open reading frame encoding a second cytosine-N4 methyltransferase, M.BcnIA, in the vicinity of the genes specifying the previously characterized cytosine-N4 methyltransferase M.BcnIB and restriction endonuclease R.BcnI. Both methyltransferases were purified from the E. coli cells expressing the individual genes, and their enzymatic efficiencies in vitro were compared with a variety of DNA substrates. Both enzymes act on 5'-CC(C/G)GG-3' sites in double-stranded DNA, however, M.BcnIA can also, with a comparable efficiency, modify the specific targets in single-stranded DNA. The biological significance of the presence of the tandem methyltransferases in the BcnI system is discussed.