CHAPTER 9
DNA Methyltransferases
(EC 2.1.1.72 and EC 2.1.1.73)David P. Hornby
- Introduction
DNA methyltransferases (Mtases) catalyze the transfer of the S-methyl
group of S-adenosylmethionine (SAM) to deoxycytosine (dC) or deoxy-
adenine (dA)bases within defined DNA sequences (1). Individual
enzymes are specific for one or the other base, and modify at the 6-NH 2
of dA (EC 2.1.1.72) or at the N 4 or 5-C position of dC (EC 2.1.1.73)
depending on the particular enzyme (2). The reaction is predominantly
irreversible. Enzymes, such as O6-methylguanine DNA Mtase, that
participate in DNA repair processes will not be discussed here.
Both mammalian and bacterial DNA Mtases have a sequence-speci-
ficity component that is an integral part of the reaction. However,
sequence recognition is much more relaxed in the case of mammalian
Mtases. Moreover, all known mammalian DNA Mtases are dC-specific,
whereas both dA and dC specificities are found in bacteria. A minimum
of four and a maximum of eight specific bases are found in the DNA
recognition sites of bacterial enzymes, whereas mammalian Mtases
require only a CpG doublet. (There is some evidence that CpN sequences
[3] are also modified in other higher organisms and that plant DNAMtases
can also methylate CpNpG motifs [4].) The biological consequences
of DNA methylation are diverse, but are usually the result of a modu-
lation of protein:nucleic acid interactions induced by methylation (1).
Thus, for example, in prokaryotes, site-specific DNAmethylation is suffi-
From: Methods in Molecular Biology, Vol. 16: Enzymes of Molecular Biology
Edited by: M. M. Burrell Copyright ©1993 Humana Press Inc., Totowa, NJ201