Multiple DNA methyltransferase genes in Arabidopsis thaliana

Methylation of plant DNA occurs at cytosines in any sequence context, and a s the Arabidopsis methyltransferase, METI, preferentially methylates cytosi nes in CG dinucleotides, it is likely that Arabidopsis has other methyltran sferases with different target specificities. We have identified five addit ional genes encoding putative DNA methyltransferases. Three of these genes are very similar to METI throughout the coding region; these genes probably arose by a series of gene duplication events, the most recent giving rise to METIIa and METIIb. METIIa and b are expressed at low levels in vegetativ e and floral organs and the level of transcripts is not affected by the int roduction of a METI antisense transgene, nor do the METII enzymes substitut e for the reduced activity of METI in methylating CG dinucleotides. METIII is not essential as it encodes a truncated protein. Two other genes encode a second class of DNA methyltransferase with the conserved motifs character istic of cytosine methyltransferases, but with little homology to the METI- like methyltransferases through the remainder of the protein. These two met hyltransferases are characterized by the presence of a chromodomain inserte d within the methyltransferase domain, suggesting that they may be associat ed with heterochromatin. Both these genes are transcribed at low levels in vegetative and reproductive tissues.