Methylation of the promoter for an immediate-early frog virus 3 gene does not inhibit transcription.

Methylation of critical sites within the promoter region of eucaryotic genes has been shown to inhibit transcription by RNA polymerase II. However, although the large DNA virus frog virus 3 (FV3) has a highly methylated genome, it uses host RNA polymerase II for at least the immediate-early stage of transcription. We have previously shown that an FV3-induced trans-acting protein allows transcription from adenovirus promoters inactivated by methylation. Since FV3 immediate-early genes are transcribed in the absence of de novo protein synthesis, it appears that the virus-induced trans-acting protein that allows transcription from methylated templates is not required for transcription of the immediate-early FV3 genes, possibly because they are not methylated in critical regulatory sequences. In this study, we used site-directed mutagenesis to alter the three CpG dinucleotide sequences in the promoter region of an immediate-early FV3 gene and thereby created sites recognized by bacterial methylases. Transient-expression assays demonstrated that neither the mutations nor methylation of the mutated sites inhibited transcription from the FV3 promoter in FV3-infected cells. These findings support the hypothesis that the immediate-early genes of FV3 do not contain methylatable sites in regions critical for transcription. The function of the virus-induced trans-acting protein that can override the inhibitory effect of methylation may therefore be to facilitate the transcription of methylated delayed-early or late FV3 genes.