Multiple DNA methyltransferase genes in Arabidopsis thaliana

Abstract

Methylation of plant DNA occurs at cytosines in any sequence context, and as the Arabidopsis methyltransferase, METI, preferentially methylates cytosines in CG dinucleotides, it is likely that Arabidopsis has other methyltransferases with different target specificities. We have identified five additional 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 vegetative and floral organs and the level of transcripts is not affected by the introduction of a METI antisense transgene, nor do the METII enzymes substitute 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 characteristic of cytosine methyltransferases, but with little homology to the METI-like methyltransferases through the remainder of the protein. These two methyltransferases are characterized by the presence of a chromodomain inserted within the methyltransferase domain, suggesting that they may be associated with heterochromatin. Both these genes are transcribed at low levels in vegetative and reproductive tissues.