title: Cytosine methylation and the ecology of intragenomic parasites.
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.
authors.affiliation: Department of Genetics and Development, College of Physicians and Surgeons of Columbia University, New York, NY 10032, USA. jay14@columbia.edu
authors.forename: J A
authors.initials: JA
authors.lastname: Yoder
authors.forename: C P
authors.initials: CP
authors.lastname: Walsh
authors.forename: T H
authors.initials: TH
authors.lastname: Bestor
mesh_terms: D000818:Animals; D003596:Cytosine / Q000378:metabolism*; D019175:DNA Methylation; D004251:DNA Transposable Elements; D005718:Gametogenesis; D018507:Gene Expression Regulation, Developmental; D008322:Mammals / Q000235:genetics; D009369:Neoplasms / Q000235:genetics; D012190:Retroviridae / Q000235:genetics
chemical_list: D004251:DNA Transposable Elements; D003596:Cytosine