Project description:Epigenetic gene silencing is of central importance to maintain genome integrity and is mediated by an elaborate interplay between DNA methylation, histone posttranslational modifications and chromatin remodeling complexes. DNA methylation and repressive histone marks usually correlate with transcriptionally silent heterochromatin, however there are exceptions to this interdependence. In Arabidopsis, mutation of MORPHEUS MOLECULE 1 (MOM1) causes transcriptional derepression of heterochromatin independently of changes in DNA methylation. More recently, two Arabidopsis homologs of mouse Microrchidia (MORC) have also been implicated in gene silencing and heterochromatin condensation without altering genome-wide DNA methylation patterns. In this study, we show that AtMORC6 physically interacts with AtMORC1 and with its close homologue AtMORC2 in two mutually exclusive protein complexes. RNA-seq analysis of high-order mutants indicates that AtMORC1 and AtMORC2 act redundantly to repress a common set of loci. We also examined the genetic interactions between AtMORC6 and MOM1 pathways. Although AtMORC6 and MOM1 control the silencing of a very similar set of genomic loci, we observed synergistic transcriptional regulation in the mom1/atmorc6 double mutant, suggesting that these epigenetic regulators act mainly by independent silencing mechanisms.
Project description:Heterochromatin is an inert region in the genome and composed of mainly remnants of transposons and repetitive elements. In Arabidopsis, the major heterchromatin regions are present at around centromeres (pericentromeric regions) and at a region on the short arm of chromosome 4 (heterochromatin knob). Histones and DNAs in heterochromatin have characteristic features with abundant H3H9me2 and cytosine methylation, respectively. Here, by using a genome tiling array, we showed that a subset of heterochromatin loci are silenced by the action of Morpheus' molecule 1 (MOM1) that is an epigeneic regulator for transcriptional gene silencing independent of global DNA and histone modification. Most of the up-regulated loci in the mom1 mutant carried sequences related to transposable elements but none of them was annotated as functional transposons. No specific subclass of transposons was targeted by MOM1 and loci that were unrelated to transposons but flanked by short tandem repeats were also shown to be under the control of MOM1. The results suggest the presence of an unknown level of regulatory network maintaining the silent state of heterochromain in the genome. Keywords: Epigenetic regulation of endogenous loci by Morpheus' Molecule 1 (MOM1)