Small RNA pathways have emerged as key players in maintaining a balance between silencing the expression of exogenous or deleterious (nonself) nucleic acid and licensing the expression of endogenous (self) genes in various species. These functions are especially critical in the germline, which is an immortal cell lineage in organisms such as the nematode C. elegans. In the C. elegans germline, the piRNA-mediated surveillance system encodes tens of thousands of unique 21-nucleotide piRNAs, which set off a cascade of events to silence a variety of deleterious nucleic acids. If left unchecked, the piRNA pathway would have the potential to recognize and silence nearly the entire C. elegans germline transcriptome, thus counter measures are also in place to promote germline gene expression.
My lab and others recently demonstrated that the Argonaute CSR-1 is guided by its 22G-RNA partners to nascent transcripts, where it promotes germline transcription to counteract piRNA pathway silencing. We are now using a variety of approaches to dissect the molecular mechanisms by which CSR-1 modulates chromatin at its targets to promote transcription. To identify chromatin factors that function in the CSR-1 pathway, we performed a candidate RNAi screen of chromatin-related factors previously implicated in RNAi-related pathways, searching for key phenotypes that are specific to loss of the CSR-1 pathway (RNAi deficiency, chromosome segregation defects, and P granule abnormalities). From this screen, we identified the essential gene nsbp-1/nap-1, which encodes a homolog of the human Nucleosome Assembly Protein, NAP-1, and is a member of the highly conserved NAP/SET histone chaperone family. Loss of nap-1 phenocopies loss of csr-1, and its expression pattern parallels that of CSR-1. We have identified the histone binding partners of NAP-1 and have determined that NAP-1 physically interacts with CSR-1. ChIP-seq, coupled with small RNA and gene-expression profiling by RNA-seq when nap-1 is compromised reveal insights into the role of NAP-1 in small RNA-mediated chromatin modulation in the CSR-1 pathway, as well as its more general roles in regulating chromatin and gene expression throughout development. Together, these studies reveal new insights into a novel role for this highly conserved histone chaperone in small RNA-mediated chromatin modulation, and demonstrate a key role for NAP-1 in the transmission of epigenetic information from parent to progeny. .
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