Plant DNA methyltransferases

DNA methylation is an important modification of DNA that plays a role in ge nome management and in regulating gene expression during development. Methy lation is carried out by DNA methyltransferases which catalyse the transfer of a methyl group to bases within the DNA helix. Plants have at least thre e classes of cytosine methyltransferase which differ in protein structure a nd function. The METI family, homologues of the mouse Dnmt1 methyltransfera se, most likely function as maintenance methyltransferases, but may also pl ay a role in de novo methylation. The chromomethylases, which are unique to plants, may preferentially methylate DNA in heterochromatin; the remaining class, with similarity to Dnmt3 methyltransferases of mammals, are putativ e de novo methyltransferases. The various classes of methyltransferase may show differential activity on cytosines in different sequence contexts. Chr omomethylases may preferentially methylate cytosines in CpNpG sequences whi le the Arabidopsis METI methyltransferase shows a preference for cytosines in CpG sequences. Additional proteins, for example DDM1, a member of the SN F2/SWI2 family of chromatin remodelling proteins, are also required for met hylation of plant DNA.