Gametogenesis, or sporulation in Saccharomyces cerevisiae, is an essential survival and reproductive strategy during starvation. During this process, diploid cells undergo DNA replication and two cellular divisions called meiosis, before forming four starvation-resistant haploid spores. The decision to enter meiosis is governed by a regulatory network, which activates the transcription of early meiotic genes (EMGs). It consists of Ime1, a transcriptional activator, Ume6, a transcriptional repressor which turns into an activator by providing a genomic platform to Ime1, and Rim11, a GSK-3β kinase homolog which phosphorylates Ime1 and Ume6, required for the activating Ime1-Ume6 complex to form. While some aspects of the Rim11-Ime1-Ume6 network are well characterised, our understanding on how this regulatory network is activated is incomplete.
By using various experimental approaches, first, I found that the Rim11-Ime1-Ume6 regulon not only activates EMGs transcription, but also increases the expression of IME1, UME6 and RIM11 through a feedback loop regulation. I showed that meiotic cells express higher Ime1, Rim11 and Ume6 levels, compared to non-meiotic cells, pointing to a functional role of the feedback loop regulation in meiotic entry. Second, I found that Rim11 localisation and expression is regulated in a TORC1- and PKA-dependent manner. In the presence of nutrients, Rim11 localises preferably to the cytoplasm, whereas during starvation it accumulates in the nucleus. I showed that the nuclear localisation of Rim11 is critical for Ume6 phosphorylation, correct EMGs induction and for meiosis to occur. Third, I found that forcing Rim11 to the nucleus is not sufficient to phosphorylate Ume6, but also requires Ime1. Moreover, Ime1 and Rim11 are both required to disassemble the histone deacetylase complex Sin3-Rpd3L at Ume6-bound EMGs allowing meiotic entry.
My analyses demonstrate that feedback loop regulations and the role of Rim11 as a central signal integrator, are key for controlling meiotic entry in yeast.