Interplay Between DNA Methylation and Demethylation

DNA methylation patterns are a product of antagonistic methylation and demethylation activities. In plants, enzymatic DNA demethylation is mediated by DNA glycosylases that remove 5-methylcytosine via base excision repair. We are studying how the glycosylases are regulated at the transcriptional level and seek to understand how they find their specific targets among a vast sea of methylated cytosines. We have recently shown that expression of the 5-methylcytosine DNA glycosylase ROS1 is under the control of RNA-directed DNA methylation (RdDM) and active DNA demethylation (Williams et al, 2015). However, the expression outcomes at ROS1 in relation to DNA methylation are opposite to most other loci in the genome; RdDM promotes ROS1 expression and DNA demethylation (by the ROS1 protein itself) represses it. We think the ROS1 locus thus functions as an epigenetic rheostat, allowing the genome to maintain DNA methylation homeostasis such that genes are expressed and transposable elements are silent.