Control of the Cell Cycle Start by Protein Kinase Genes in Saccharomyces Cerevisiae

Abstract

The cell division cycle of the budding yeast Saccharomyces cerevisiae is controlled during the G1 phase by a complex series of events designated as START. During START the yeast cell monitors its size and integrates nutritional and/or pheromonal signals in order to decide whether to stay in a resting state (G), to sporulate (in a diploid state), to conjugate or to start a new round of division. START is controlled by several cell division cycle (CDC) genes, and two groups of temperature-sensitive cdc mutants arrest upon shift to restrictive temperature (37°) either prior to the pheromone-sensitive step (group A, including cdc25 and cdc35) or after this step (group B, including cdc28) as stationary-like, round, unbudded and uninuclear cells (Pringle and Hartwell, 1981). Group A START mutants are unable to grow at 37°, do not form a spindle pole body satellite, and are unable to conjugate, and at least two mutants of this group (cdc25 and cdc35) can enter the meiotic pathway in a fully complemented medium, if in a diploid state, whereas start of mitosis is blocked (Shilo et al., 1978).