Figure 4.

A Hypothetical Explanation for Loss of RdDM in ddm1 and cmt Mutants.

In wild-type conditions, heterochromatin is maintained in an inaccessible state that excludes RdDM. All the components required for RdDM are then concentrated at a small set of loci in the genome (mCHH islands) where they function in concert to methylate DNA. In the absence of chromomethylases or DDM1-type nucleosome remodelers, heterochromatin no longer excludes RdDM, and RdDM components are scattered over a larger area of repetitive DNA.