The tumor suppressor protein Rb (retinoblastoma) is a chromatin factor that functions in transcriptional repression of cell cycle regulatory genes as well as other genes. As a transcriptional repressor, Rb functions in a core complex (dREAM/Muv B) that binds to specific promoters and recruits a crew of repressive chromatin cofactors to inhibit the expression of target genes 
. Rb-recruited factors include repressive histone methyltransferases (Suv39, Suv420), repressive heterochromatin proteins that bind to methylated histones (HP-1, L3MBT), and the Nucleosome Remodeling and Deacetylase complex (NuRD complex) 
. Beyond transcription, Rb also interacts with other chromatin factors (e.g.
, condensin II) and participates in other chromatin functions such as chromosome condensation and maintaining genome stability 
. Even though it has been studied as a cell cycle regulator for two decades, the functions of Rb are clearly much broader.
Rb pathway genes have been studied in the nematode C. elegans
because Rb and many of its interacting proteins identified biochemically in flies and mammals are conserved in C. elegans
and null alleles in the corresponding C. elegans
genes cause similar developmental phenotypes 
. Genes encoding Rb (lin-35
), the rest of the core DRM/dREAM complex, and Rb-recruited repressive chromatin factors all belong to the class of synMuv B (synthetic multivulva B) genes, mutations in which cause a Muv (Multivulva) phenotype when combined with a mutation in a synMuv A gene. synMuv B genes, along with synMuv A genes, repress the expression of the growth factor gene lin-3
/EGF in the developing hypodermis 
. Excess EGF (Epidermal Growth Factor) signaling from the hypodermis in synMuv AB double mutant animals induces multiple vulva precursor cells to adopt the cell division patterns normally specified for only one vulval precursor cell, causing the Muv phenotype. Rb pathway chromatin factors comprise the bulk of the synMuv B genes as revealed by saturation genetic analysis and whole genome RNAi screens in synMuv A mutant strains, from which a few additional synMuv B genes have been identified, some of which also encode probable chromatin factors 
. The genetic pathways necessary for the Muv phenotype in synMuv AB double mutant worms have been revealed by whole genome RNAi screens for gene inactivations that suppress the Muv phenotype of synMuv AB mutants, which also identified distinct chromatin factors 
. Therefore, Rb pathway proteins function with particular chromatin factors while antagonizing others to specify the production of the LIN-3/EGF signal from particular cells during vulval development.
Mutations in several synMuv B genes, especially those that encode the Rb core complex also cause a dramatic enhancement of response to dsRNA, that is enhanced RNAi (Eri) 
. Inactivation of lin-35/Rb
also causes enhanced transgene silencing, a process that depends on some RNAi factors 
. Eri and enhanced transgene silencing are also caused by mutations in distinct RNAi regulatory factors, for example, the genes eri-1
, that do not interact with synMuv A mutations to induce a Muv phenotype 
likely inhibits RNAi using a distinct mechanism from these other eri
genes because null alleles in lin-35
genes are genetically additive, have different genetic requirements for canonical RNAi factors, and display specificity in gene inactivation tests involving distinct dsRNAs 
. One potential mechanism of the enhanced RNAi response in synMuv B mutants is the somatic misexpression of germline-specific genes observed in these animals, given that many RNAi factors are preferentially expressed in the C elegans
germline which is also more proficient at RNAi 
Many synMuv B mutants misexpress germline-specific P granules in their somatic tissue 
. Homologous to nuage and polar granules of insects and mammals, P granules mark the germline of C. elegans
from the very first cell divisions and are essential to the function and maintenance of the germline 
. These perinuclear RNP granules harbor processing and binding proteins for mRNAs as well as endogenous small RNAs, and are thought to be the site of nascent mRNA export and endogenous small RNA biogenesis and storage 
. The somatic misexpression of P granule components was first observed in mep-1
mutants which were found to also function in the synMuv B pathway 
. Unlike null mutants of most synMuv B genes, which are viable, null mutations of mep-1
cause L1 arrest or sterility, suggesting that the expression of germline P granules in somatic cells can have severe developmental consequences or that these genes also regulate other essential functions in contrast to lin-35/Rb
. The somatic misexpression of germline P granules as well as the developmental arrest phenotypes of mep-1
are fully suppressed by inactivation of three synMuv B suppressor genes, which encode germline chromatin factors (mes-4
), pointing to antagonistic chromatin factors in regulating the expression of germline genes in the soma 
. It was proposed that the NuRD complex, containing LET-418, prevents somatic misexpression through chromatin modification or remodeling and antagonizing the function of germline specific chromatin factors 
. Later, the misexpression of P granules in somatic cells was shown in several other synMuv B mutants, including lin-35/Rb
, indicating a broader involvement of synMuv B genes in this process 
. While these synMuv B mutants are viable, all of them display high temperature larval arrest, again suggesting severe developmental consequences from such misexpression of normally germline-specific genes 
. Loss of the germline chromatin factors mes-4, mrg-1 and isw-1
, which suppresses the somatic misexpression and high temperature larval arrest in these other synMuv B mutants, was also shown to suppress the enhanced transgene silencing and vulval specification phenotypes, suggesting that the somatic misexpression of germline components contributes to the enhanced RNAi and the vulval cell fate change in these animals 
. The inappropriate expression of germline genes including small RNA pathway components could also contribute to the oncogenicity of loss of Rb in mammalian cells.
Both the enhanced RNAi and the expression of germline-specific genes in soma uniquely reflect the function of the synMuv B pathway, as they are not present in mutants of other synMuv classes. However, not all synMuv B mutants are Eri or show PGL-1 misexpression, suggesting functional specialization. Moreover, only some of the Muv suppressing chromatin factors affect RNAi and only three (mes-4, mrg-1, isw-1
) suppress PGL-1 misexpression 
. Thus, the genetic pathways underlying Eri and the expression of germline genes in somatic cells remain to be further investigated.
Here we use response to RNAi, subcellular analysis of somatic P granules, and characterization of somatic misexpression of germline-specific genes to reveal that synMuv B genes constitute distinct intersecting axes for the repression of germline genes in somatic cells. We find that these chromatin factors regulate the expression of partially overlapping sets of germline target genes, including genes that encode annotated small RNA cofactors such as Argonaute proteins, and P granule components such as helicases and RNA binding proteins. The three synMuv B chromatin complexes we find are the LIN-35/Rb-containing core complex (DRM), the SUMO-mediated Mec complex (rather than the previously suspected NuRD complex), and a synMuv B heterochromatin complex, representing distinct classes of synMuv B mutants. We show that the DRM and synMuv B heterochromatin complexes each have distinct requirements for MES-4 function, suggesting different placements in the genetic pathway. We present a model of how synMuv B complexes collectively inhibit RNAi and prevent germline gene expression in the soma.