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Genome Res. 2007 May;17(5):625-31. Epub 2007 Apr 9.

CpG methylation is targeted to transcription units in an invertebrate genome.

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The Wellcome Trust Centre for Cell Biology, The University of Edinburgh, Michael Swann Building, The King's Buildings, Edinburgh EH9 3JR, UK.


DNA is methylated at the dinucleotide CpG in genomes of a wide range of plants and animals. Among animals, variable patterns of genomic CpG methylation have been described, ranging from undetectable levels (e.g., in Caenorhabditis elegans) to high levels of global methylation in the vertebrates. The most frequent pattern in invertebrate animals, however, is mosaic methylation, comprising domains of methylated DNA interspersed with unmethylated domains. To understand the origin of mosaic DNA methylation patterns, we examined the distribution of DNA methylation in the Ciona intestinalis genome. Bisulfite sequencing and computational analysis revealed methylated domains with sharp boundaries that strongly colocalize with approximately 60% of transcription units. By contrast, promoters, intergenic DNA, and transposons are not preferentially targeted by DNA methylation. Methylated transcription units include evolutionarily conserved genes, whereas the most highly expressed genes preferentially belong to the unmethylated fraction. The results lend support to the hypothesis that CpG methylation functions to suppress spurious transcriptional initiation within infrequently transcribed genes.

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