Non-CG methylation is more potent than CG methylation in moss genome regulation [Ppatens BS-Seq]

Cytosine methylation is commonly targeted to symmetrical CG sites, as well as to non-CGs, such as the symmetrical-CHG and asymmetric-CHH sites in plants (H= A, C or T). Thus far, depletion of CG methylation in plants was associated with ample transcriptional activation of transposons. Here, we profiled transcription in various context specific methylation mutants in the early-diverged plant, Physcomitrella patens. We discovered that specific elimination of CG methylation is fully complemented by non-CG methylation but not vice versa. Between the symmetrically-methylated sites, CHG methylation silenced transposons stronger than CG methylation did. Finally, non-CG methylation revealed as crucial for the silencing of CG depleted transposons. Our results suggest that non-CG methylation evolved to silence transposons due to functional limitations and/or rapid mutability of methylated-CGs. Whole-genome bisulfite sequencing of two P. patens DNMT double mutants: met/dnmt3b and cmt/dnmt3b