Figure 1.

Classes of amino-MTases. (A) A duplication model of SAM-binding motif I, with either of the duplicated regions diverging prior to introduction of the target recognition domain (TRD) and serving as the ancestor for either the α and γ or the β classes. (B) Schematic of three classes of MTases with altered orders of motifs responsible for SAM binding (motif I), methylation substrate binding (TRD) and catalysis (motif IV). The regions containing motifs I and IV are folded into a seven-stranded sheet. This linear representation does not reflect the fact that, in the three-dimensional structure of just class β MTases, the TRD is oriented oppositely to motifs I/IV. (C) Relative numbers of each class of MTases were obtained from the REBASE database as of spring 2020 (44).