MORC proteins and epigenetic regulation

Abstract

Two recent studies in Arabidopsis implicated MORC proteins, which contain a GHKL ATPase domain, in transcriptional gene silencing. Here, these studies are compared and contrasted to discuss the roles of MORC proteins in epigenetic regulation. Although MORC proteins are likely to catalyze changes in chromatin structure in response to epigenetic signals, their precise mode of action and target site-specificity still remain unclear.

Keywords: Arabidopsis; DNA methylation; GHKL ATPase; H3K27 monomethylation (H3K27me); MORC; RNA directed DNA methylation; histone modification.

Figure 1. DMS11 is required for establishment of repressive histone modifications at some RdDM loci. Analysis of histone modifications (A) and RNA Pol II occupancy (B) at endogenous RdDM targets and downsteam region of the target transgene enhancer. (C) Histone modifications and RNA Pol II occupancy at the control actin locus. Chromatin immunoprecipitations were performed with chromatin from Col-O wild type (containing transgenic target and silencer loci; T+S), nrpe1 and dms11 mutants according to published protocol. Precipitations without antibody (mock) were used to assess background levels for ChIP samples. Values for H3K9me2, H3K27me and H3ac were normalized to the total amount of precipitated H3. Antibodies used were H3K9me2 (Abcam, ab1220), H3K27me (Millipore, 17–643), H3ac (Millipore, 17–615), H3 (Abcam, ab1791) and Pol II (Millipore, 4H8). Bars and error bars are mean values and standard deviations respectively from six amplifications (two independent biological replicates). Note that there is no enrichment of RNA Pol II at IGN loci in RdDM mutants (i.e., nrpe1 mutant used here). Oligonucleotides used for real-time PCR were as published elsewhere.^,,

Figure 2. Chromosomal locations of upregulated protein coding genes in atmorc1 and atmorc2 mutants. Asterisks denote genes upregulated in both mutants, those starting with capital AT specific for atmorc1 whereas the others are atmorc6 specific. GFP target transgene and soloLTR are also indicated.

Figure 3. A model depicting different pathways for heterochromatin formation. The DDM1, RdDM and H3K27me1 pathways control constitutive heterochromatin formation. The DDM1-dependent pathway is required for maintenance of symmetric DNA methylation and H3K9me2 independently of RdDM. In the RdDM pathway, H3K9 methyltransferases are recruited to the methylated CHG target regions, to form a self-reinforcing loop between H3K9me2 and maintenance of CHG DNA methylation. The H3K27me1 mark, another histone methylation mark characteristic of silent chromatin, is set by histone methyltransferases ATXR5 and ATXR6. The siRNA-mediated DNA methylation–dependent (RdDM) and the DNA methylation–independent H3K27me1 pathways have been proposed, based on genetic and biochemical evidence, to act in parallel.^, AtMORC6 is likely to contribute to both RdDM and the H3K27me1 pathways (this work).^, However, it is also possible that, at least at the subset of loci, AtMORC6 integrates these two pathways (red arrows) to achieve their full silencing.