Abstract
Arrested Saccharomyces cerevisiae cells initiate the cell cycle in an asynchronous mode. The asynchronous manner of cycle initiation generates variability in cellβcycle times of individual cells. Limiting concentrations of adenine, methionine or histidine regulate the rate of cycle initiation in auxotrophs. A sigmoidal curve of rate vs. concentration is obtained for each of the three substances. Moreover, the three curves have similar Hill coefficients of 2.4, suggesting that a common intermediate requiring adenine, methionine and histidine regulates cellβcycle initiation in yeast.
Low concentrations of cycloheximide reduce the rate of cycle initiation of arrested cells that are released from the block in a similar way as limiting nutrients. It thus appears that the common intermediate that requires the limiting nutrients depends upon protein synthesis. The rate of cycle initiation is more sensitive to cycloheximide or nutrient limitation than is protein synthesis. It is also affected by limiting nutrients to a much greater extent than is the overall rate of protein accumulation (i.e., net protein synthesis). Hence the mechanism that controls cycle initiation does not depend on the overall synthesis or accumulation of proteins in the cell. It may depend on synthesis of particular proteins whose production or function requires the limiting nutrients.
The high sensitivity of cycle initiation to a decrease in the rate of protein synthesis could explain the ability of yeast cells to complete the cycle and arrest at stationary phase upon depletion of medium components. The cells cannot initiate the cycle although their protein synthesis capacity remains sufficiently high to allow traversal of the rest of the cell cycle.