Arabidopsis AtMORC4 and AtMORC7 Form Nuclear Bodies and Repress a Large Number of Protein-Coding Genes.

C Jake Harris,Guillaume Moissiard,Ajay A Vashisht,Farid El Kasmi,Steven E Jacobsen,Jeffery L Dangl,James A Wohlschlegel,Haifeng Wang,Ashot Papikian,Dylan Husmann,William A Pastor,Wanlu Liu

doi:10.1371/journal.pgen.1005998

Abstract

The MORC family of GHKL ATPases are an enigmatic class of proteins with diverse chromatin related functions. In Arabidopsis, AtMORC1, AtMORC2, and AtMORC6 act together in heterodimeric complexes to mediate transcriptional silencing of methylated DNA elements. Here, we studied Arabidopsis AtMORC4 and AtMORC7. We found that, in contrast to AtMORC1,2,6, they act to suppress a wide set of non-methylated protein-coding genes that are enriched for those involved in pathogen response. Furthermore, atmorc4 atmorc7 double mutants show a pathogen response phenotype. We found that AtMORC4 and AtMORC7 form homomeric complexes in vivo and are concentrated in discrete nuclear bodies adjacent to chromocenters. Analysis of an atmorc1,2,4,5,6,7 hextuple mutant demonstrates that transcriptional de-repression is largely uncoupled from changes in DNA methylation in plants devoid of MORC function. However, we also uncover a requirement for MORC in both DNA methylation and silencing at a small but distinct subset of RNA-directed DNA methylation target loci. These regions are characterized by poised transcriptional potential and a low density of sites for symmetric cytosine methylation. These results provide insight into the biological function of MORC proteins in higher eukaryotes.

Highlights

Maintaining regulatory access to genes while repressing the expression of potentially deleterious transposable elements is a fundamental challenge for living organisms
Asymmetric CHH methylation must be continuously re-established. This is mostly mediated by CMT2 [7,8]; while in small patches of heterochromatin in the otherwise euchromatic arms, CHH methylation is mostly maintained by the action of DRM2 in the RNA-directed DNA methylation (RdDM) pathway [9,10,11]
We found that AtMORC4 and AtMORC7, like AtMORC1 and AtMORC6 [25], form nuclear bodies that are adjacent to chromocenters

Summary

Introduction

Maintaining regulatory access to genes while repressing the expression of potentially deleterious transposable elements is a fundamental challenge for living organisms. Eukaryotes achieve this in part by parsing their genomes into functional units characterized by distinct chromatin features [1,2]. Asymmetric CHH methylation must be continuously re-established. In pericentromeric heterochromatin, this is mostly mediated by CMT2 [7,8]; while in small patches of heterochromatin in the otherwise euchromatic arms, CHH methylation is mostly maintained by the action of DRM2 in the RNA-directed DNA methylation (RdDM) pathway [9,10,11]

Methods

Results

Conclusion