RNA Pol I activity maintains chromatin condensation and the H3K4me3 gradient essential for oogenesis, independent of ribosome production

Oogenesis requires extensive and dynamic chromatin remodeling that primes gene promoters for later transcriptional activation during embryonic development. Here, we uncover a pivotal, non-canonical role for RNA Polymerase I (Pol I) in driving these chromatin state transitions during Caenorhabditis elegans oogenesis. Using the auxin-inducible degron system to selectively deplete either Pol I catalytic subunits or ribosome assembly factors, we disentangle the consequences of impaired nucleolar integrity from reductions in ribosome biogenesis. Strikingly, although disrupting ribosome assembly caused minimal effects on oocyte production, loss of Pol I activity led to widespread changes in chromatin accessibility, a dampening of the distal-proximal H3K4me3 gradient required for oogenesis, reduced synapsis, and elevated ATM/ATR phosphorylation, resulting in fewer but significantly larger oocytes. Despite their promoters becoming more accessible, oogenesis genes did not show large changes in steady-state mRNA, consistent with transcriptional repression prior to fertilization. Instead, Pol I depletion prematurely remodeled oogenic chromatin, through a misdirection of H3K4me3 deposition towards promoters normally primed for zygotic genome activation. These findings reveal an epigenetic gating function for nucleolar integrity in oocyte maturation: Pol I preserves three-dimensional chromatin organization and maintains proper spatiotemporal regulation of histone modifications, independent of ribosome production. Given the evolutionary conservation of nucleolar dynamics and histone modifications during gametogenesis, our work suggests that nucleolar stress, whether from environmental factors, aging, or genetic disorders, could broadly compromise fertility by disrupting oogenic chromatin priming. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=111 SRC="FIGDIR/small/652530v2_ufig1.gif" ALT="Figure 1"> View larger version (24K): org.highwire.dtl.DTLVardef@9b95bforg.highwire.dtl.DTLVardef@844eb5org.highwire.dtl.DTLVardef@4b7a79org.highwire.dtl.DTLVardef@155ca31_HPS_FORMAT_FIGEXP M_FIG C_FIG