In the June 2000 issue of Plant Mol. Biol (43(2-3):189-201), Jean Finnegan and KA Kovac of the CSIRO, Australia, have reviewed the role of Plant DNA methyltransferases in DNA methylation. Methylation is an essential component of genome management and regulation of gene expression during development. DNA methyltransferases act by catalysing the transfer of a methyl group to bases within the DNA helix. On the basis of protein structure and function, methyltransferases fall into at least three classes: the Dnmt1/METI family of cytosine methyltransferase which are probably maintenance methyltransferases, putative de novo methyltransferases and the chromomethylases. The latter class of methyl transferases is only found in plants.
These various methyltransferases appear to target cytosines in different sequence contexts; for example, in maize, cytosines in CpNpG sequences are methylated by chromomethylases while in Arabidopsis, the METI protein has a preference for cytosines as shown in CpG sequences. It has been shown that DDM1, a member of the SNF2/SWI2 family of chromatin remodelling proteins, is also essential for the maintenance of normal plant DNA methylation patterns.