Substrate specificity and protein stability drive the divergence of plant-specific DNA methyltransferases [Bisulfite-seq]
DNA methylation is an evolutionarily conserved epigenetic mechanism essential for transposon silencing and heterochromatin assembly. In plants, Chromomethylase3 (CMT3) and CMT2 are the major DNA methyltransferases mediating DNA methylation on CHG and CHH (H = A, C or T), respectively, but the underlying mechanisms of how CMT3 and CMT2 methylate different contexts during evolution is poorly known. Here we reveal the mechanisms of CMT3 and CMT2 divergence in flowering plants. CMT2 originates from a duplication of the evolutionarily more ancient CMT3 and change of a key arginine residue recognizing CHG in CMT2 makes it lose CHG methylation activity in most flowering plants. Mutation of the residue to arginine in Arabidopsis (V1200R) greatly increases its methylation activity on CHG DNA. Besides different substrate specificity, CMT2 contains a long and disordered N-terminus compared with a short one in CMT3. The long N-terminus reduces protein stability, especially under high temperature. Moreover, CMT2 N-terminus is plastic to various natural mutations. Together, our study reveals the mechanisms of chromomethylase divergence for methylating different DNA contexts in plants and sheds light on understanding of plant evolution. Overall design: To investigate genome-wide CHG and CHH methylation change in various CMT2 and CMT3 modifications, and to study the effects of DNA methylation change in plants.