Effects of heat shock and cycloheximide on growth and division of the fission yeast,Schizosaccharomyces pombe

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Abstract
Effects of superoptimal temperature shock (HS) and the protein synthesis inhibitor, cycloheximide (CH), on growth and division of the fission yeast, Schizosaccharomyces pombe, were studied. Experiments on asynchronous cultures have shown that a 15-min HS of 41 °C inhibits RNA and protein synthesis and growth in cell length while delaying mitosis, division, and DNA synthesis. A 10-min CH pulse (100 μg/ml) inhibits protein and RNA synthesis briefly while delaying mitosis, division, and DNA synthesis. These single heat or CH pulses partially synchronize mitosis, division, and DNA synthesis.Experiments in which either 15-min HS or 10-min CH pulses were applied at different times in selection-synchronized cultures have demonstrated several kinds and periods of sensitivity to these agents. During roughly the first two-thirds of the cell cycle (measured between divisions) mitosis, division, and DNA synthesis are delayed equally, delay increasing as a pulse is applied progressively later in the cycle. The magnitude of the delay from a heat shock is always greater than that from a CH pulse, but for both agents there is a period during which delay is greater in magnitude than pulse length. The pattern of delay from cultures synchronized by an induction method suggests that the period of increasing delay lies obligately within G2.At 0 ·65 in the cycle the nature of the sensitivity to heat and CH changes. Between this transition point and the formation of a cell plate, CH has no effects on the timing of mitosis or cell plate stage. However, CH can block the final splitting of the cell plate, leading to a permanent cell plate and the formation of transient 4-celled pseudo-filaments upon resumed growth. HS following the transition point allows mitosis to be completed with normal timing to a stage in which daughter nuclei occupy terminal positions in the cell, but formation of the cell plate is delayed by about 30 min. Cells pulsed in the last third of the cycle may develop several morphological aberrations. The cell plate is sometimes oblique or positioned at one end of the cell, giving rise to daughter cells with 2 or no nuclei. Thus, it appears that some functions related to normal positioning of the cell plate occur during the last third of the cycle.Recovery from a heat shock applied prior to 0 ·65 in the cycle includes a period in which there is increasing delay when a CH pulse is applied progressively later following the heat shock. However, applied together, CH plus HS produce roughly the amount of delay due to the HS alone. These facts are considered evidence for a common effect of both agents during the first two-thirds of the cycle.The results are discussed with reference to possible controls over several events of the cell cycle.