5-Methylcytosine is a methylated form of the DNA base cytosine that may be involved in the regulation of gene transcription. When cytosine is methylated, the DNA maintains the same sequence, but the expression of methylated genes can be altered (the study of this is part of the field of epigenetics). 5-Methylcytosine is incorporated in the nucleoside 5-methylcytidine.

In 5-methylcytosine, a methyl group, is attached to the 5th carbon atom (counting counterclockwise from the NH nitrogen at the two o'clock position, not the 6 o'clock). This methyl group distinguishes 5-methylcytosine from cytosine.

In vivo

5-Methylcytosine is an epigenetic modification formed by the action of DNA methyltransferases.

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The function of this chemical varies significantly among species:^[1]

• In bacteria, 5-methylcytosine can be found at a variety of sites, and is often used as a marker to protect DNA from being cut by native methylation-sensitive restriction enzymes.
• In plants, 5-methylcytosine occurs at CpG, CpHpG and CpHpH sequences (where H = A, C or T).
• In fungi and animals, 5-methylcytosine predominantly occurs at CpG dinucleotides. Most eukaryotes methylate only a small percentage of these sites, but 70-80% of CpG cytosines are methylated in vertebrates.

While spontaneous deamination of cytosine forms uracil, which is recognized and removed by DNA repair enzymes, deamination of 5-methylcytosine forms thymine. This conversion of a DNA base from cytosine (C) to thymine (T) can result in a transition mutation. In addition, active enzymatic deamination of cytosine or 5-methylcytosine by the APOBEC family of cytosine deaminases could have beneficial implications on various cellular processes as well as on organismal evolution.^[2] The implications of deamination on 5-hydroxymethylcytosine, on the other hand, remains less understood.

In vitro

The NH₂ group can be removed (deamination) from 5-methylcytosine to form thymine with use of reagents such as nitrous acid; cytosine deaminates to uracil under similar conditions.

Deamination of 5-methylcytosine to thymine

5-Methylcytosine is resistant to deamination by bisulfite treatment, which deaminates cytosine residues. This property is often exploited to analyze DNA cytosine methylation patterns with bisulfite sequencing.^[3]

See also

• Nucleobase

References

Literature

• (available online at the United States National Center for Biotechnology Information)

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