The yeast cell cycle: positive and negative controls

Cell cycle control in Saccharomyces cerevisiae is exerted mainly at the START point in the G1 phase of the cycle. At this point, yeast cells check physiological signals and external parameters such as nutrient availability or the presence of sexual pheromones, before proceeding to next stages of the cycle. G1 cyclins regulate START by activating Cdc28, the p34 protein kinase, involving physical association between the cyclin and the kinase catalytic subunit. As a consequence of the accumulation and subsequent degradation of G1 cyclins, a peak of p34 protein kinase activity is reached before START, which would regulate the pass through this point by phosphorylating specific substrates. Here we review the molecular aspects of START and the mechanisms, both positive and negative, that regulate this cell cycle point. We also describe a new gene that links nutrient availability to G1 cyclin expression and the possible connections with the Ras-cAMP pathway.