Cytosine methylation of repeated sequences in eukaryotes: the role of DNA pairing.

In eukaryotic genomes, methylation of cytosine residues commonly occurs in repetitive sequences. This methylation correlates with reduced gene expression and suppression of recombination, and is thus thought to serve as a genome-defense mechanism that guards against the deleterious effects of multicopy transposable elements and aberrant gene duplications. Analysis of methylation in fungi and plants suggests that the ability of DNA repeats to pair with one another is a key to their selection for methylation. Recent data have outlined the substrate requirements for the establishment and maintenance of methylation in eukaryotic repeated sequences. Substrate-methylation patterns could help us to understand the way in which methyltransferase enzymes recognize their substrates.