The SNF2 and RING domain proteins FRG1 and 2 are required for RNA-directed DNA methylation in Arabidopsis [mRNA-Seq]

DNA methylation in Arabidopsis thaliana is maintained by at least four different enzymes: MET1, CMT3, DRM2, and CMT2. However, DNA methylation is established exclusively by the enzyme DRM2, which acts in the RNA-directed DNA methylation (RdDM) pathway. Some RdDM components belong to gene families and have partially redundant functions, such as DICER-LIKE 2, 3 and 4, and IDN2- LIKE 1 and 2. Traditional mutagenesis screens usually fail to detect genes if they are redundant, since the loss of one gene can be compensated by a related gene. In an effort to circumvent this issue, we utilized co-expression data to identify closely related genes that are co-regulated with genes in the RdDM pathway. Here we report the discovery of two redundant proteins, SNF2-RING-HELICASE-LIKE1 and 2 (FRG1 and 2) that are putative chromatin-related paralogous proteins. Analysis of genome–wide bisulfite sequencing shows that simultaneous mutations of FRG1 and 2 cause broad defects in methylation at RdDM targeted loci. We also show that FRG1 stably associates with Su(var)3-9-related SUVR2, a known RdDM component, in vivo. Combined, our results identify FRG1 and FRG2 as novel components of the RdDM machinery. Overall design: mRNA profiles of 14 days old seedlings of Columbia 0 wild type and frg1-1 frg2-1 double mutants, 3 biological replicates each.