O-methyltransferase, family 2 <p>Methyl transfer from the ubiquitous S-adenosyl-L-methionine (AdoMet) to either nitrogen, oxygen or carbon atoms is frequently employed in diverse organisms ranging from bacteria to plants and mammals. The reaction is catalysed by methyltransferases (Mtases) and modifies DNA, RNA, proteins and small molecules, such as catechol for regulatory purposes. The various aspects of the role of DNA methylation in prokaryotic restriction-modification systems and in a number of cellular processes in eukaryotes including gene regulation and differentiation is well documented.</p><p>Three classes of DNA Mtases transfer the methyl group from AdoMet to the target base to form either N-6-methyladenine, or N-4-methylcytosine, or C-5- methylcytosine. In C-5-cytosine Mtases, ten conserved motifs are arranged in the same order [<cite idref="PUB00004446"/>]. Motif I (a glycine-rich or closely related consensus sequence; FAGxGG in M.HhaI [<cite idref="PUB00000896"/>]), shared by other AdoMet-Mtases [<cite idref="PUB00009714"/>], is part of the cofactor binding site and motif IV (PCQ) is part of the catalytic site. In contrast, sequence comparison among N-6-adenine and N-4-cytosine Mtases indicated two of the conserved segments [<cite idref="PUB00004370"/>], although more conserved segments may be present. One of them corresponds to motif I in C-5-cytosine Mtases, and the other is named (D/N/S)PP(Y/F). Crystal structures are known for a number of Mtases [<cite idref="PUB00009715"/>, <cite idref="PUB00000896"/>, <cite idref="PUB00004446"/>, <cite idref="PUB00004831"/>]. The cofactor binding sites are almost identical and the essential catalytic amino acids coincide. The comparable protein folding and the existence of equivalent amino acids in similar secondary and tertiary positions indicate that many (if not all) AdoMet-Mtases have a common catalytic domain structure. This permits tertiary structure prediction of other DNA, RNA, protein, and small-molecule AdoMet-Mtases from their amino acid sequences [<cite idref="PUB00006319"/>].</p><p>This domain includes a range of O-methyltransferases some of which utilise S-adenosyl methionine as substrate [<cite idref="PUB00000141"/>]. In prokaryotes, the major role of DNA methylation is to protect host DNA against degradation by restriction enzymes. In eukaryotes, DNA methylation has been implicated in the control of several cellular processes, including differentiation, gene regulation, and embryonic development. O-methyltransferases have a common catalytic domain structure, which might be universal among S-adenosyl-L-methionine (AdoMet)-dependent methyltransferases [<cite idref="PUB00001090"/>]. </p><p>Comparative analysis of the predicted amino acid sequences of a number of plant O-methyltransferase cDNA clones show that they share some 32-71% sequence identity, and can be grouped according to the different compounds they utilise as substrates [<cite idref="PUB00006400"/>].</p>