Genetic control of cell division in yeast cultured at different growth rates

Abstract

HARTWELL et al.1,2 have isolated a number of temperature-sensitive mutants blocked at various stages of the cell division cycle of Saccharomyces cerevisiae. A characteristic feature of any temperature-sensitive cell cycle mutant is its execution point in the cell cycle, that is, the point in the cycle after which cells when shifted to the restrictive temperature undergo one further cell division. If cells are shifted to the restrictive temperature before the execution point, they are unable at the elevated temperature to complete any cell division. The execution point for a particular mutant is taken to be the stage in the cycle that the defective gene completes its function. The execution point of mutant strain cdc28 is at an earlier stage in the cycle than other cell cycle mutants3. Detailed investigations of a number of cell cycle mutants have suggested that the events in early G1 that culminate in the execution of the cdc28-mediated step represent ‘start’. There is evidence that passing ‘start’ in the cycle represents a point of commitment to division. Before the execution of the cdc28-mediated step, ‘start’, yeast is capable of several developmental programmes but after this step it is committed to the mitotic cell cycle2. We report here that at generation times varying from 2.1–6.18 h ‘start’ occurs approximately 2h before division. At slower growth rates the length of time from ‘start’ to division increases somewhat. These results are similar to the finding in the bacterium Escherichia coli that the time from the initiation of DNA synthesis to cell division is independent of growth rate except at slow growth rates4.