Two regulatory levels of transcriptional gene silencing in Arabidopsis.

In mammals, some fungi, and plants, DNA methylation plays a central role in the epigenetic control of gene transcription. Recently, however, a subclass of Arabidopsis mutants revealed that the release of transcriptional gene silencing (TGS) does not necessarily require DNA demethylation. Here, we address the fundamental question of whether these mutants delineate a previously uncharacterized, methylation-independent level of epigenetic regulation, or whether they just act downstream of DNA methylation signals. Two mutants described earlier, ddm1 and mom1, reactivate previously silent loci: ddm1 impairs TGS by reducing chromosomal DNA methylation, and mom1 releases TGS without affecting DNA methylation. We examined the epistatic relationship between ddm1 and mom1 by constructing double mutant strains. The synergistic release of TGS revealed by gene expression patterns from silent loci, drastic developmental abnormalities, and characteristic changes in nuclear architecture in these double mutants implies that DDM1 and MOM are likely to operate at independent levels in TGS control. Our results indicate that the methylation-independent silencing mechanism reinforces the methylation-based system and prevents extremely rapid epigenetic deregulation in plants with DNA methylation deficiencies.