Site specificity of the Arabidopsis METI DNA methyltransferase demonstrated through hypermethylation of the superman locus

Abstract

Plants with low levels of DNA methylation show a range of developmental abnormalities including homeotic transformation of floral organs. Two independent DNA METHYLTRANSFERASEI (METI) antisense transformants with low levels of DNA methylation had flowers with increased numbers of stamens which resembled flowers seen on the loss-of-function superman (sup) mutant plants and on transgenic plants that ectopically express APETALA3 (AP3). These METI antisense plants have both increased and decreased methylation in and around the sup gene, compared with untransformed controls. DNA from the antisense plants was demethylated at least 4 kb upstream of the sup gene, while there was dense methylation around the start of transcription and within the coding region of this gene; these regions were unmethylated in control DNA. Methylation within the sup gene was correlated with an absence of SUP transcripts. The pattern and density of methylation was heterogeneous among different DNA molecules from the same plant, with some molecules being completely unmethylated. Methylcytosine occurred in asymmetric sites and in symmetric CpA/TpG but rarely in CpG dinucleotides in the antisense plants. In contrast, segregants lacking the METI antisense construct and epimutants with a hypermethylated allele of sup (clark kent 3), both of which have active METI genes, showed a higher frequency of methylation of CpG dinucleotides and of asymmetric cytosines. We conclude that METI is the predominant CpG methyltransferase and directly or indirectly affects asymmetric methylation.