A Genetic and Molecular Analysis of the Control of the Start of the Cell Cycle in Saccharomyces cerevisiae

Progression through the eukaryotic cell cycle is controlled by the CDC28 protein kinase and its homologs. In the budding yeast Saccharomyces cerevisiae, this kinase is required for cell cycle START (commitment to cell cycle progression late in G1), and for entry into mitosis. Although constitutively present, CDC28 is only periodically activated. The activation of CDC28 involves its physical association with proteins called cyclins. Budding yeast have three CLN genes (CLNs 1-3), which have limited cyclin homology. At least one of the three is required for cell cycle START. Four B cyclins are known in yeast (CLB5 1-4); two have been shown to function in mitosis. This thesis reports the discovery of three genes which, when either mutated or overexpressed, relieve the requirement for CLN genes for the execution of START. The first gene was discovered as a mutation which bypasses the requirement for CLN genes, and which we have named the CLN bypass mutation (CBM). 15 distinct isolates of CBM were obtained; all were dominant, and 12 were shown to be linked or allelic with one another. Two of the isolates caused single division meiosis as an unselected, dominant phenotype. Two CEN plasmid clones were isolated which bypassed the CLN requirement. The first of these contained a novel B cyclin, which we have named CLB5. CLB5 transcript abundance peaks in G1, coincident with CLN2 transcript, but earlier than CLB2 transcript. CLB5 deletion does not cause lethality, either alone or in combination with other CLN or CLB deletions. However, strains deleted for CLB5 require more time to complete S phase, suggesting that CLB5 promotes some step in DNA synthesis. CLB5 is the only yeast cyclin whose deletion lengthens S phase. CLB5 may also have some role promoting the G1/S transition, since cln1 cln2 strains require both CLN3 and CLB5 for viability on glycerol media, and cln1, 2, 3- strains require CLB5 for rescue by the D. melanogaster cdc2 gene. The other CEN plasmid clone rescuing the cln1, 2, 3- genotype contained MPK1, already ...