Samuel, Margaret Jane; (2001) Novel regulators of the fission yeast cell cycle. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The aim of the work presented in this thesis was to further our understanding of how cyclin-dependent kinases (Cdks) control progression through the cell cycle in the fission yeast, Schizosaccharomyces pombe. Two different approaches were taken to address this question. The first was a genetic approach, to investigate the identity and function of a gene, mcs3, which had previously been implicated in the control of mitotic initiation. A functional complementation screen was performed to isolate mcs3 and multicopy suppressors of the mcs3 mutant. Seven distinct extragenic suppressors were isolated which included the Wis2 cyclophilin, Swol (an Hsp90 homologue), the Cek1 kinase and truncated forms of the Wee1 kinase and the Dis2 phosphatase, all of which have previously been implicated in the fission yeast cell cycle. Two novel genes were also identified as the S. pombe homologues of S. cerevisiae SPO12 and FKH2 genes. Initial characterisation of these two genes was performed. The transcript of the S. pombe spoil gene was found to be cell cycle regulated, with transcription peaking during mitosis, suggesting a role for Spol2 at this stage of the cell cycle. Loss of Spol2 had no discernible effect on the mitotic cell cycle and has no functionally overlapping role with Spol2p in budding yeast. The fkh2 gene encodes a non-essential putative transcription factor that is regulated by phosphorylation at the G2/Mail.ru transition. Fkh2 appears to regulate a novel wave of transcription of genes required for mitotic progression and cytokinesis. However, none of the genes identified in the screen proved to be allelic to mcs3, whose identity remains unknown. The second approach was biochemical. It was previously thought that a single Cdk-cyclin B kinase complex, Cdc2-Cdcl3, is sufficient to control progression through the cell cycle. This theory has been re-examined through the investigation of a second PSTAIRE-containing Cdk identified in the S. pombe databases. It was shown that this novel Cdk associates with the mitotic cyclin Cdcl3 and the G1 cyclin Cig2. Additionally this Cdk has a speculative role in G1 of the cell cycle. These results bring into question the theory that the Cdc2-Cdcl3 kinase is solely responsible for S-phase and mitotic regulation in fission yeast.

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