|Ph.D Thesis||Department of Biology|
|Supervisor:||Prof. Kassir Yona|
|Full Thesis text - in Hebrew|
Progression through the cell cycle depends on sequential activation of CDKs. In the budding-yeast Saccharomyces cerevisiae a single CDK, Cdc28, suffices for controlling progression through the cell cycle. In mammals, three interphase Cdks (Cdk4, Cdk6 and Cdk2) and one metaphase kinase (Cdk1) are responsible for controlling the cell cycle. Knock-out mice for all interphase Cdks revealed that Cdk1 suffices to drive the mammalian cell cycle. This result contradicts the classical view assuming that Cdk2 is essential for the mitotic cell cycle, indicating redundant functions between the Cdks However, these Cdks may have specific functions, as knock-out mice for Cdk2, for instance, are viable but sterile, as a result of pachytene arrest.
Yeast meiosis is regulated by both Cdc28 (yCdk1) and Ime2 - a meiosis-specific Cdk-like kinase, which is homologous to hCdk2. Ime2 is required for multiple events, the correct time cells enter pre-meiotic S and nuclear divisions, the transient transcription of IME1, the maximal level of transcription of the early meiosis-specific genes (MSG), the transcription of the middle and late genes, and for spore formation. Cdc28, is required for pre-meiotic S and nuclear divisions.
In the first part of the research we used budding-yeast meiosis as a tool to determine the specific functions of mammalian Cdks. We showed that Cdk2 is a functional homolog of Ime2. Cdk2 could advance pre-meiotic S and MI entry of ime2Δ cells. Moreover, it suppressed ime2Δ for the transcription of IME1, the early and middle MSGs. This suppression was specific for Cdk2, as neither Cdk1 nor Cdk4 could do it. We showed that Cdk1 is a functional homolog of Cdc28 in both the meiotic and mitotic pathways and Cdk4 functions as a meiosis-specific transcriptional activator.
In the second part we examined the effect of mammalian Cdks on the mitotic cycle. In contrast to previous reports, overexpression of Cdk2 did not suppress the DNA replication defect of cdc28, but moderate expression did. Apparently, overexpression of Cdk2 has a dominant negative effect on the activity of Cdc28 in both the meiotic and mitotic cycles. We showed that overexpression of either Ime2 or Cdk2 in wild type cells is toxic. Moreover, overexpression of either Ime2 or Cdk2 delayed the DNA damage response mediated by the kinase Rad53. We further showed that ectopic overexpression of Ime2 (but not Cdk2) is genotoxic, resulting in an increase in the frequency of intragenic recombination and the frequency of chromosomes loss.