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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.

Evolutionary history of Methyltransferase 1 genes in hexaploid wheat

“…This observation resembles a phenomenon observed after METI gene duplication in Arabidopsis where METI transcripts accumulate to 10,000 fold higher levels than those of the duplicated METIIa and b, while METIII is considered to be a pseudogene [39]. Expression of a specific member of a given gene family is referred to as predominance [40] or transcriptional dominance [16] and in our case occurs for TaMET1 genes at homoeologous group 2.…”

“…Low evolution rate in the MET-1b lineage matches with a predominant expression of homoeologous group 2 over group 5 and 7. Predominant expression of one member of the MET1 gene family was already observed in other species such as Arabidopsis [39] and rice [37] suggesting that MET1 expression level and pattern needs to be carefully controlled. Interestingly, we mapped TaMET1 homoeologous group 2 to peri-centric (proximal) position while group 5 and 7 were located at more sub-telomeric (distal) regions.…”

Strand-biased DNA methylation associated with centromeric regions in Arabidopsis

“…Arabidopsis enzymes that methylate cytosine have been identified (40), yet methyltransferases capable of biased modification of complementary strands are unknown. Such enzymes must transfer methyl groups to only one strand of the helix in a pattern that extends over hundreds of bases.…”

The gene for domains rearranged methyltransferase (DRM2) in Arabidopsis thaliana plants is methylated at both cytosine and adenine residues

“…The first gene family, METI , encodes four proteins closely related to mouse maintenance cytosine DNA‐methyltransferase Dnmt1 [2]. METI encodes, in particular, the predominant DNA‐methyltransferase that is most highly expressed in meristematic cells in both vegetative and floral plant tissues [3–5]. METIIa / b are also transcribed in all tissues but the level of transcripts is about 10 000‐fold lower than that for METI [5].…”

“…METI encodes, in particular, the predominant DNA‐methyltransferase that is most highly expressed in meristematic cells in both vegetative and floral plant tissues [3–5]. METIIa / b are also transcribed in all tissues but the level of transcripts is about 10 000‐fold lower than that for METI [5]. The expression of METIII has not yet been studied.…”