Models on the feedback regulation between Pol IV transcription and epigenetic marks at D2 and C2 loci. At both D2 and C2 loci, P4siRNA biogenesis requires Pol IV, RDR2, and DCL3. At D2 loci with high levels of methylated CHH and relatively low levels of H3K9me2 or H3K27me1, P4siRNAs and CHH methylation are in a tight feedback loop in which P4siRNAs guide CHH methylation, and CHH methylation in turn promotes siRNA biogenesis, probably by recruiting Pol IV. At C2 loci with relatively low levels of methylated CHH and extensive overlap with H3K9me2 or H3K27me1, P4siRNA biogenesis is only moderately affected by the absence of CHH methylation (in drm12cmt23 and cmt2) or H3K9me2 (in suvh456). The high percentage of C2 siRNA loci containing both H3K9me2 and H3K27me1 suggests that both epigenetic marks may contribute to Pol IV recruitment at C2 loci.
