Arabidopsis EDM2 promotes IBM1 distal polyadenylation and regulates genome DNA methylation patterns.

DNA methylation is important for the silencing of transposons and other repetitive elements in many higher eukaryotes. However, plant and mammalian genomes have evolved to contain repetitive elements near or inside their genes. How these genes are kept from being silenced by DNA methylation is not well understood. A forward genetics screen led to the identification of the putative chromatin regulator Enhanced Downy Mildew 2 (EDM2) as a cellular antisilencing factor and regulator of genome DNA methylation patterns. EDM2 contains a composite Plant Homeo Domain that recognizes both active and repressive histone methylation marks at the intronic repeat elements in genes such as the Histone 3 lysine 9 demethylase gene Increase in BONSAI Methylation 1 (IBM1) and is necessary for maintaining the expression of these genes by promoting mRNA distal polyadenylation. Because of its role in maintaining IBM1 expression, EDM2 is required for preventing CHG methylation in the bodies of thousands of genes. Our results thus increase the understanding of antisilencing, genome methylation patterns, and regulation of alternative RNA processing by intronic heterochromatin.