MORC Family ATPases Required for Heterochromatin Condensation and Gene Silencing

To Silence or Not to Silence Repressed genes commonly have methylated DNA, and/or covalent histone modifications associated with silent chromatin, and/or associated small interfering (si)RNAs. All three features are components of gene-silencing systems (see the Perspective by Jacob and Martienssen ). In a screen for components of DNA methylation gene-silencing systems in the flowering plant, Moissiard et al. (p. 1448 , published online 3 May) identified the genes AtMoRC1 and AtMORC6 , which are homologs of the mouse Microrchidia1 gene. AtMORC1 and AtMORC6 are involved in silencing transposable elements and genes corresponding to DNA-methylated loci, and yet neither gene is required for maintenance of DNA methylation. Instead, AtMoRC1 and AtMORC6 are related to proteins that remodel chromatin superstructure, and they seem to control gene-silencing through the higher-order compaction of methylated and silent chromatin. Qian et al. (p. 1445 ) identified an Arabidopsis gene, IDM1 (increased DNA methylation 1), that is involved in regulating DNA methylation at loci enriched for repeats and multigene families containing highly homologous genes. IDM1 protects target genes from DNA silencing and recognizes both histone H3 and methylated DNA at target loci and is able to acetylate histone H3.