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

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RESUMO

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.

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