DNA methylase induced by bacteriophage phiX174.

A cytosine-specific DNA methylase activity, which is normally absent in the Escherichia coli B strain, was found to be induced in these cells by infection with bacteriophage varphiX174. In vivo experiments revealed a single 5-methylcytosine residue in the phage DNA molecule and 5-methylcytosine residues in the infected host DNA, in addition to the 6-methylaminopurine residues present in the uninfected cells. In vitro, a partially purified enzyme from infected cells methylated DNA from uninfected cells, but showed no activity with cellular DNA from infected cells. The partially purified methylase derived from uninfected cells lacks this activity.