Among B-Type Cyclins Only CLB5 and CLB6 Promote Premeiotic S Phase in Saccharomyces cerevisiae

Department of Biochemistry, University of Alberta, Edmonton, Alberta, T6G 2H7, Canada.
Genetics (Impact Factor: 5.96). 12/2011; 190(3):1001-16. DOI: 10.1534/genetics.111.134684
Source: PubMed


The Saccharomyces cerevisiae cyclin Clb5 is required for premeiotic S phase, meiotic recombination, and successful progression through meiosis. Clb5 is not essential for mitotic proliferation because Clb1-Clb4 can support DNA replication in clb5 clb6 mutants. Clb1, Clb3, and Clb4 accumulate in clb5 clb6 cells during meiotic differentiation yet fail to promote premeiotic DNA replication. When expressed under the regulation of the CLB5 promoter, Clb1 and Clb3 accumulate and are active in the early stages of meiotic differentiation but cannot induce premeiotic DNA replication, suggesting that they do not target Cdk1 to the necessary substrates. The Clb5 hydrophobic patch (HP) residues are important for Clb5 function but this motif alone does not provide the specificity required for Clb5 to induce premeiotic S phase. Domain exchange experiments demonstrated that the amino terminus of Clb5 when fused to Clb3 confers upon Clb3 the ability to induce premeiotic S phase. Chimeric cyclins containing smaller regions of the Clb5 amino terminus displayed reduced ability to activate premeiotic DNA replication despite being more abundant and having greater associated histone H1 kinase activity than endogenous Clb5. These observations suggest that Clb5 has a unique ability to trigger premeiotic S phase and that the amino-terminal region of Clb5 contributes to its specificity and regulates the functions performed by the cyclin-Cdk complex.

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    • "Similar to Asf1 and CAF-1, two of the yeast six cyclin B homologs , Clb5 and Clb6, act redundantly in regulating the timing of DNA replication in mitosis and meiosis (DeCesare and Stuart, 2012; Donaldson et al., 1998); however, a clb5D clb6D double mutant is not particularly sick, suggesting that an additional pathway exists for this process. In both of these instances, we show genetic outcomes derived from the TMA approach that provide mechanistic insight into the pathways being interrogated that could not have been gleaned from the analysis of single mutants, illustrating the utility of this approach. "
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    ABSTRACT: Genetic interactions reveal the functional relationships between pairs of genes. In this study, we describe a method for the systematic generation and quantitation of triple mutants, termed triple-mutant analysis (TMA). We have used this approach to interrogate partially redundant pairs of genes in S. cerevisiae, including ASF1 and CAC1, two histone chaperones. After subjecting asf1Δ cac1Δ to TMA, we found that the Swi/Snf Rdh54 protein compensates for the absence of Asf1 and Cac1. Rdh54 more strongly associates with the chromatin apparatus and the pericentromeric region in the double mutant. Moreover, Asf1 is responsible for the synthetic lethality observed in cac1Δ strains lacking the HIRA-like proteins. A similar TMA was carried out after deleting both CLB5 and CLB6, cyclins that regulate DNA replication, revealing a strong functional connection to chromosome segregation. This approach can reveal functional redundancies that cannot be uncovered through traditional double-mutant analyses.
    Full-text · Article · Jun 2013 · Cell Reports