Heat-Shock Induction of Ionizing Radiation Resistance in Saccharomyces cerevisiae, and Correlation with Stationary Growth Phase

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Abstract
Radiation resistance and thermal resistance vary as a function of culture temperature in logarithmically growing S. cerevisiae and are related to the optimum temperature for growth. Radiation resistance and thermal resistance were also induced when cells grown at low temperatures were subjected to a heat shock at or above the optimum growth temperature. Exposure to ionizing radiation followed by a short incubation at low temperature also induced resistance to killing by heat. Heat-shocked cells are induced to a level of thermal and radioresistance much greater than the characteristic resistance level of cells grown continuously at the shock temperature. This high level of resistance, which resembles that of stationary-phase cells, decays to the characteristic log-phase level within one doubling of cell number after the heat shock. Both induction of resistance and decay of that induction require protein synthesis. Induction of resistance by heat shock of oinizing radiation may be a response of the cells to stress and represents a preparation to enter stationary phase.