ArticleThe C . elegans Polycomb Gene sop - 2 Encodes an RNA Binding Protein yeast

heterochromatic-like structures, including deacetyla-tion and methylation of histone tails (Francis and Kings-ton, 2001). Of the two distinct complexes formed by the PcG proteins, the ESC-E(Z) complex associates with Summary histone deacetylases and the SET domain of E(Z) possesses H3 specific methyltransferase activity, while the Epigenetic silencing of Hox cluster genes by Polycomb group (PcG) proteins is thought to involve the forma-PRC1 complex prevents access of the SWI/SNF nucleo-some-remodeling factors (Otte and Kwaks, 2003). Com-tion of a stably inherited repressive chromatin structure. Here we show that the C. elegans-specific PcG ponents of the PRC1 complex are also remarkable for their localization to distinct nuclear speckles, called PcG protein SOP-2 directly binds to RNA through three nonoverlapping regions, each of which is essential for bodies, whose role in transcriptional repression is not well understood (Francis and Kingston, 2001). Recently its localization to characteristic nuclear bodies and for its in vivo function in the repression of Hox genes. a link between the PcG complex and some RNA-dependent silencing processes has emerged in contexts other Functional studies indicate that the RNA involved in SOP-2 binding is distinct from either siRNA or mi-than Hox gene regulation. For example, the mammalian ESC-E(Z) complex, EED/ENX1, is recruited to the inacti-croRNA. Remarkably, the vertebrate PcG protein Rae28, which is functionally and structurally related vated X chromosome in an Xist RNA-dependent manner, and it is required for X chromosome inactivation (Plath to SOP-2, also binds to RNA through an FCS finger domain. Substitution of the Rae28 FCS finger for the et al., 2003; Silva et al., 2003). Silencing of tandem trans-genic arrays in C. elegans germ cells and cosuppression essential RNA binding region of SOP-2 partially restores localization to nuclear bodies. These observa-in Drosophila require both the PcG complex and components of the RNAi pathway (Kelly and Fire, 1998; Tabara tions suggest that direct binding to RNA is an evolu-tionarily conserved and potentially important property et al., 1999; Pal-Bhadra et al., 2002). However, it is unclear whether these effects result from direct RNA-PcG of PcG proteins. protein interactions and whether they contribute to Hox gene repression, a primary property of PcG proteins Introduction that is conserved across the phylogenetic spectrum. We have recently isolated the C. elegans-specific PcG Distinct RNA components have recently been recognized as essential for epigenetic silencing of genes gene, sop-2, which is involved in maintaining the repressed state of Hox genes (Zhang et al., 2003). …