Ultrastructural changes in yeast following heat shock and recovery.

The effects of heat shock and heat stress on the ultrastructure of Saccharomyces cerevisiae is reported. Following a mild heat shock, referred to as an increase in temperature from 25 degrees C to 37 degrees C for 30 min, we observed contraction of the nucleolus, formation of electron-dense particles (90 nm) in mitochondria and heat-shock granules (30-40 nm) in the cytoplasm. The electron-dense particles in the mitochondria were similar in appearance to those previously reported in plant cells exposed to elevated temperatures. In a heat-sensitive yeast strain, the nucleolus was severely aggregated after a mild heat shock, a treatment which hardly affected relatively more heat-resistant strains. The nucleolus was aggregated in all strains after a more severe heat stress (50 degrees C for 2 or 4 min). When cells were observed during a recovery period after heat stress it was found that nucleolar ultrastructure was regained more rapidly in cells that were previously heat shocked compared to cells that were stressed directly with no prior heat shock.