【摘要】：DNA methylation is typically regarded as a repressive epigenetic marker for gene expression.Genome-wide DNA methylation patterns in plants are dynamically regulated by the opposing activities of DNA methylation and demethylation reactions. In Arabidopsis, a DNA methylation monitoring sequence(MEMS) in the promoter of the DNA demethylase gene ROS_1 functions as a methylstat that senses these opposing activities and regulates genome DNA methylation levels by adjusting ROS_1 expression. How DNA methylation in the MEMS region promotes ROS_1 expression is not known. Here, we show that several Su(var)3-9 homologs(SUVHs) can sense DNA methylation levels at the MEMS region and function redundantly to promote ROS_1 expression. The SUVHs bind to the MEMS region, and the extent of binding is correlated with the methylation level of the MEMS.Mutations in the SUVHs lead to decreased ROS_1 expression, causing DNA hypermethylation at more than 1,000 genomic regions. Thus, the SUVHs function to mediate the activation of gene transcription by DNA methylation.