DNA methyltransferase induced by frog virus 3.

Over 20% of the cytosine bases in frog virus 3 DNA are methylated at the 5-carbon position. To determine whether this high degree of methylation is the result of a virus-specific enzyme, we examined the kinetics of induction and the substrate specificity of a DNA methyltransferase from frog virus 3-infected fathead minnow cells. A novel DNA methyltransferase activity appeared in the cytoplasm of infected cells at 3 h postinfection. This activity was induced in the absence of viral DNA replication and was therefore probably an early viral enzyme. In contrast to the methyltransferase activity extracted from uninfected cell nuclei, the cytoplasmic enzyme showed a strong template preference for double-stranded over single-stranded and for unmethylated over hemimethylated DNA. The dinucleotide sequence dCpdG was a necessary and sufficient exogenous substrate for methylation in vitro. A mutant of frog virus 3, isolated as resistant to 5-azacytidine and having unmethylated virion DNA, did not induce cytoplasmic DNA methyltransferase, leading to the conclusion that this activity is coded for by the virus.