Control of Cell Size at Bud Initiation in Saccharomyces cerevisiae

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Abstract
The cell volume at bud initiation for both haploid and diploid cells of S. cerevisiae depends on growth rate within the mass doubling time range 2.1-3.7 h. At slower growth rates, the volume at bud initiation is independent of growth rate. At all growth rates, the volume at bud initiation for diploids is 1.7-fold larger than that for haploids. When unbudded cells from synchronous cultures growing in poor media are shifted to rich medium, all except those very close to the size characteristic of bud production on poor medium go on to produce a bud at the larger size characteristic of the richer medium. Cells do not become committed to producing a bud at the size characteristic of bud initiation on ethanol medium (a poor medium) until they are within about 3 .mu.m3 of that size. When unbudded cells are shifted from rich medium to poor medium, cells that are smaller than the size of bud initiation on the poor medium produce a bud at the size characteristic of the poor medium. If they are larger than the commitment size for the poor medium at the time of the shift, they produce a bud without appreciable growth. These results are examined in relation to the unstable inhibitor model for the control of cell division.