Characterization of factors that underlie transcriptional silencing in C. elegans oocytes

While it has been appreciated for decades that prophase-arrested oocytes are transcriptionally silenced on a global level, the molecular pathways that promote silencing have remained elusive. Previous work in C. elegans has shown that both topoisomerase II (TOP-2) and condensin II collaborate with the H3K9me heterochromatin pathway to silence gene expression in the germline during L1 starvation, and that the PIE-1 protein silences the genome in the P-lineage of early embryos. Here, we show that all three of these silencing systems, TOP-2/condensin II, H3K9me, and PIE-1, are required for transcriptional repression in oocytes. We find that H3K9me3 marks increase dramatically on chromatin during silencing, and that silencing is under cell cycle control. We also find that PIE-1 localizes to the nucleolus just prior to silencing, and that nucleolar dissolution during silencing is dependent on TOP-2/condensin II. Our data identify both the molecular components and the trigger for genome silencing in oocytes and establish a link between PIE-1 nucleolar residency and its ability to repress transcription. Author summaryIn most animals, oogenesis involves a genome-wide repression of transcription as oocytes approach maturation and fertilization. While transcriptional repression in fully grown oocytes was first documented in the early 1960s, the molecular players that promote genome silencing had yet to be identified. Here, we study the round worm C. elegans and we find that three previously defined genome silencing systems are all operational in pre-maturation oocytes and are all required for silencing. The first system, topoisomerase II acting together with the condensin II complex, promotes a hyper compaction of oocyte chromatin, which likely explains its role in genome silencing. The second, components of a system that modifies chromatin to produce heterochromatin, also drives compaction of the oocyte chromatin. Interestingly, the third system-the PIE-1 protein-is required for silencing but not for chromatin compaction. Our data show that while chromatin compaction is required for genome silencing in oocytes, it is not sufficient. Our studies open the door to more mechanistic analysis of how these pathways can silence gene expression on a global level.

Automated summary

The three genome silencing systems (TOP-2/condensin II, H3K9me, and PIE-1) are all involved in transcriptional repression in oocytes, with TOP-2/condensin II and H3K9me promoting H3K9me3 marks on chromatin, and PIE-1 localizing to the nucleolus prior to silencing. The dissolution of nucleolus during silencing is dependent on TOP-2/condensin II. These findings identify the molecular components and trigger for genome silencing in oocytes, and establish a link between PIE-1 nucleolar residency and transcriptional repression.