Methylation of viral DNA in Vivo and in Vitro

Abstract

Methylation of herpesvirus saimiri DNA and adenovirus type 5 DNA was examined by labelling with [6- 3H]uridine and [2- 3H]adenosine and separating the bases by thin layer chromatography. The number of methyl groups was less than one per genome in both DNAs, indicating that these viral DNAs are not substrates for methylation in vivo. In a long-term in vitro incubation with purified rat liver DNA methylase adenovirus type 5 DNA and herpes simplex virus type I DNA were methylated to about the same degree as M. luteus DNA (1 methyl group per 42 bases). In a short-term incubation the viral DNAs accepted considerably less methyl groups than M. luteus DNA. This suggests that viral DNAs have lower affinity for DNA methylases than bacterial DNAs. Rat liver DNA was a competitive inhibitor for the in vitro methylation of adenovirus type 5 DNA by rat liver DNA methylase. The higher affinity of the enzyme for cellular DNA may explain why viral DNA is not methylated in the cell.