Genetics of commitment to cell division in S. cerevisiae
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In all eukaryotes, proliferation is regulated by cell cycle controls. Elucidating the intrinsic mechanism whereby these control mechanisms modulate proliferation is essential for understanding the role of the cell cycle in development, aging and cancer. Our laboratory uses budding yeast S. cerevisiae as a model system. In budding yeast, proliferation is dependent upon cell growth, cell size, and the expression of Gl phase cyclins (Clns). However, the relationship among these requirements is poorly understood. In this study, the relationship between cell growth, cell size, Cln expression, and proliferation was analyzed. It is found that rapidly growing cells express, and require, more Cln protein to divide than do slowly growing cells. To clarify the role of cell size, defined amounts of CLTN mRNA were expressed in cells of different sizes. It is found that a critical threshold of Cln protein was required for proliferation, and that Cln1 protein expression was strongly modulated by cell size. In addition, expression of high levels of CLNs promoted proliferation in a size-independent manner suggesting that Clns are rate-limiting. To examine the relationship between cell growth and the ability of cells to proliferate, a systematic genome-wide genetic screen was conducted to identify mutants that dramatically altered the proliferative capacity of cells. In so doing, 49 gene deletions that dramatically changed cell size were identified. Twenty of these made cells abnormally small (whi mutants), and 29 made cells abnormally large (uge mutants). Nearly all of these genes have putative human homologues. Interestingly, five uge gene products are components of Ccr4-Not transcriptional complexes. Furthermore, it is found that CCR4 positively regulates CLNl mRNA expression. In ccr4Ä strains, CLNl mRNA expression was decreased in asynchronous cultures and delayed in synchronized cultures, but restoration ofCCR4 expression induces CLNl mRNA expression and rescues the size phenotype ofccr4A. My results suggest that CCR4 modulates the ability of the Bck2 protein to induce CLNJ and CLN2 transcription. In summary, my research has identified new gene products involved in cell cycle control and has helped elucidate the mechanism whereby cells coordinate cell growth with proliferation.