Two different methods were employed to effect changes in the timing of cell cycle events, alteration of growth rate, and alteration of the length of S phase. Saccharomyces cerevisiae S288C/1 was grown in a glucose- limited chemostat at population doubling times of 80 to 736 minutes. D, P and B all showed biphasic linear relationships to t. quantitatively different from estimates for the same strain in batch culture. Median cell volume and dry weight per cell increased at the faster growth rates, but the average cell density reached a minimum at t= 150 minutes. The contiguous array of bud scars on parent cells became increasingly irregular as t increased from 140 minutes. At the fastest growth rates filamentous forms constituted a small percentage of the total cell number and were presumed to arise from the failure of cells to undergo cell separation. The phenomonen was distinct from chain formation, dimorphism and pseudo-mycelial growth and showed extensive analogies with the duplication cycle of filamentous fungi. By reducing the concentration of 5'-dTMP offered to the Saccharomyces cerevisiae tmp mutant MB1093~1D it was possible to increase the length of S phase, without altering t. The percentage of the cell cycle S occupied could be increased from 30% at nonlimiting concentrations, to over 60% at limiting concentrations. The increase was compensated for by a reduction in G1 and G2. M and G1* remained constant. At limiting concentrations P increased while D decreased. A novel method of autoradiography using the mutant was developed. 5'-dTMP limitation also affected growth and morphology. At limiting concentrations, where t was unaltered, up to 10% of cells had abnormal morphologies. They were of three major classes, (l) cells showing secondary bud formation, (2) cells with their bud in the centre of a "bean-shaped" cell, and (3) cells that were elongated. The length of B and volume at bud emergence increased with decreasing 5'-dTMP concentration.
|Date of Award||1982|