Epigenetic silencing of endogenous repetitive sequences by MORPHEUS' MOLECULE1 in Arabidopsis thaliana.

Morpheus' molecule1 (MOM1) is a plant-specific epigenetic regulator of transcriptional gene silencing. Mutants of MOM1 release silencing of subsets of endogenous repetitive elements and transgenes without affecting their cytosine methylation status. Although MOM1 is evolutionarily related to chromodomain helicase DNA binding protein3 (CHD3), a family of chromatin remodeling proteins involved in repression of gene expression, MOM1 does not carry the functional ATPase/helicase domain essential for chromatin remodeling activity, and therefore, its mode of action is unknown. We recently performed a genome-wide survey for endogenous targets silenced by MOM1 and identified loci that are concentrated around centromeres and rich in sequences homologous to the 24-nt small interfering RNAs (siRNAs) that accumulate in wild type plants. Further and independent analyses indicated that the degree of contribution of MOM1 to maintenance of the silent states varies in different loci and that other silencing machineries, including those in the RNA-directed DNA methylation (RdDM) pathway, interact genetically with MOM1. In this short article, I review what we know about MOM1 and discuss its possible functions in silencing through examination of other silencing factors that interact genetically with MOM1.

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