Academic journal article Genetics

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

Academic journal article Genetics

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

Article excerpt

ABSTRACT 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.

CYCLIN-dependent kinases (Cdks) in conjunction with their activating subunits, the cyclins, provide the impetus for transitions between distinct phases of the eukaryotic cell cycle (Morgan 1997). All organisms from yeast to humans express multiple cyclins that are regulated at the levels of transcription, protein stability, and subcellular localization (Pines and Hunter 1991; Lew and Reed 1992; Koepp et al. 1999). The oscillation of cyclin abundance throughout the cell cycle allows Cdk subunits to form distinct cyclin- Cdk complexes that act in a specific temporal order. The budding yeast Saccharomyces cerevisiae expresses a single Cdk (Cdk1) whose activity is required for progression through G1, S phase, and mitosis (Reed and Wittenberg 1990). Cdk1 accomplishes these functions by associating sequentially with G1 cyclins, Cln1, Cln2, and Cln3, the Sphase cyclins, Clb5 and Clb6, and finally the mitotic cyclins, Clb1-Clb4 (Nasmyth 1996).

Cyclin-Cdk activity is also required for cellular differentiation and development (Shuster and Byers 1989; Edgar and Lehner 1996; Chellappan et al. 1998). Gametogenesis is a differentiation program that employs a modified cell cycle to direct parental diploid cells through meiosis to the formation of haploid gametes (Kupiec et al. 1997; Albertini and Carabatsos 1998). S. cerevisiae initiates meiotic differentiation in response to nutrient deprivation, and the program leads to the formation of haploid spore gametes (Kupiec et al. 1997). Much of the machinery required for the mitotic cell cycle is employed to promote progression through meiotic differentiation. However, the meiotic program uses some distinct factors to achieve specific developmental aims (Simchen 1974; Orr-Weaver 1994). One example of meiosis-specific regulation is the control of premeiotic DNA replication. Clb5 is the major S-phase cyclin in S. cerevisiae but is not essential during mitotic proliferation as other cyclins can promote DNA replication in its absence (Schwob et al. 1994; Donaldson et al. 1998). Although Clb5 is not required for mitotic proliferation, it is essential for premeiotic DNA replication, the induction of meiotic recombination, some aspects of synaptonemal complex formation, and activation of the S-M checkpoint in meiosis (Dirick et al. 1998; Stuart and Wittenberg 1998; Smith et al. 2001; Henderson et al. 2006; Wan et al. 2008; Zhu et al. 2010).

A growing body of evidence supports the contention that cyclins target Cdk activity to the correct substrates and hence the cyclin provides specificity to the Cdk activity (Cross et al. …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.