Induction of heat-shock proteins at permissive growth temperatures in the plasmodium of the myxomycete Physarum polycephalum.

When synchronous plasmodia of the myxomycete Physarum polycephalum were submitted to temperature shifts from 22 degrees C to 32 degrees C, the highest physiological temperature, protein synthesis was increased during at least 10 h. Moreover during 2 h, four proteins (69, 74, 82 and 105 kDa) showed a transient increase of their synthesis, independently of the period of the temperature shift during the cell cycle. The stability of these proteins and the susceptibility of their synthesis to actinomycin suggested that they corresponded to four different proteins. Temperature shifts from 22 degrees C or 29 degrees C to 37 degrees C, a non-physiological temperature, demonstrated that the 69-kDa, 74-kDa, 82-kDa and 105-kDa proteins were identical to the four heat-shock proteins which could be detected in Physarum. Although the physiological significance of these heat-shock proteins remained unclear, comparison between the extent of their synthesis and the length of the mitotic delays induced by various temperature shifts ruled out a direct relationship between mitotic delays and synthesis of the 74-kDa, 82-kDa and 105-kDa proteins.