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Trends Genet. 1997 Aug;13(8):335-40.

Cytosine methylation and the ecology of intragenomic parasites.

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  • 1Department of Genetics and Development, College of Physicians and Surgeons of Columbia University, New York, NY 10032, USA. jay14@columbia.edu

Abstract

Most of the 5-methylcytosine in mammalian DNA resides in transposons, which are specialized intragenomic parasites that represent at least 35% of the genome. Transposon promoters are inactive when methylated and, over time, C-->T transition mutations at methylated sites destroy many transposons. Apart from that subset of genes subject to X inactivation and genomic imprinting, no cellular gene in a non-expressing tissue has been proven to be methylated in a pattern that prevents transcription. It has become increasingly difficult to hold that reversible promoter methylation is commonly involved in developmental gene control; instead, suppression of parasitic sequence elements appears to be the primary function of cytosine methylation, with crucial secondary roles in allele-specific gene expression as seen in X inactivation and genomic imprinting.

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PMID:
9260521
[PubMed - indexed for MEDLINE]
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