There is no unequivocal evidence of the existence of DNA M/R systems in eukaryotic cells. Enzymes resembling pairs of DNA methyltransferases and restriction endonucleases are, however, encoded by viruses infecting eukaryotic algae (Chlorella spp.); their biological functions remain still unclear. Other viruses, like Frog virus 3 and lymphocystis disease virus (members of the family lridoviridae), encode their own DNA methyltransferases, and their genomes carry high numbers of 5-methylcytosine residues. Functional consequences of the extensive methylation have to be further elucidated.
In the DNA of eukaryotic cells 5-methylcytosine is usually generated by cellular methyltransferase activities and mainly but not exclusively occurs in CpG dinucleotides. DNA methylation is known to be involved in processes like transcription inhibition, genomic imprinting, cell differentiation and dediffer-entiation (carcinogenesis), and hypermutability. Furthermore, DNA methylation is assumed to silence transcription from integrated retroviral genomes, whereas demethylation results in the reactivation of the provirus. For some human pathogenic viruses,
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