The condensin II/TOP-2 axis silences transcription during germline specification inC. elegans

Abstract

In many organisms, mRNA transcription must be globally repressed in germline precursor cells during specification for them to maintain a germline fate. In C. elegans, germline precursor cells are known as the P lineage, and it is the PIE-1 protein that is responsible for silencing transcription in these cells. Loss of PIE-1 results in a fate switch, whereby the P2 cell loses germline fate and instead adopts a somatic fate. Previous work had also defined a global chromatin compaction (GCC) pathway that silences transcription in oocyte, spermatocytes, and primordial germ cells in C. elegans. GCC silencing requires the topoisomerase II (TOP-2)/condensin II chromatin compaction pathway together with components of H3K9me pathway. These factors come together to promote a global compaction of chromatin, thereby silencing gene expression. The goal of this study was to ask if the GCC pathway is also important for genome silencing in the P lineage. We report that GCC per se is not required, as we find that the H3K9me pathway is dispensable for transcriptional repression in P cells. However, we do uncover a critical role for TOP-2/condensin II in P-cell silencing. Loss of TOP-2 and/or the CAPG-2 subunit of condensin II leads to active, elongating RNA polymerase II in P cells, leading to expression of both somatic and germline genes. These data shed new light on how transcription is regulated during germline specification.