Regulation of Transcription by Methylation

Most cells contain several enzymes that transfer a methyl group from S-adenosylmethionine (Chapter 17) to cytosine or adenine in DNA. These DNA methylases are both base-specific and sequence-specific. Another enzyme that methylates cytosine only in particular CpG sequences also is important for transcription. In a few cases, particularly in vertebrates, methylation of CpG sequences prevents transcription of some genes. 

Certain genes are heavily methylated in cells in which the gene is not expressed and unmethylated in cells in which the gene is expressed. 

In vitro methylation of the upstream site of a cloned y-globulin gene prevents transcription. Methylation outside the upstream sequence does not inhibit transcription. 

The housekeeping genes, which provide for general cell function and which are continuously transcribed, are rarely methylated in or near their initiation regions. 

Undifferentiated and precursor cells often replicate. If methylation actually prevents gene expression in some types of cell, an inhibitory methylated site must be inherited as a methylated site in a daughter strand during DNA 

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