Heat shock-induced translational control of HSP70 and globin synthesis in chicken reticulocytes.

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Abstract
Incubation of chicken reticulocytes at elevated temperatures (43 to 45 degrees C) resulted in a rapid change in the pattern of protein synthesis, characterized by the decreased synthesis of normal proteins, e.g., alpha and beta globin, and the preferential and increased synthesis of only one heat shock protein, HSP70. The repression of globin synthesis was not due to modifications of globin mRNA because the level of globin mRNA and its ability to be translated in vitro were unaffected. The HSP70 gene in reticulocytes was transcribed in non-heat-shocked cells, yet HSP70 was not efficiently translated until the cells had been heat shocked. In non-heat-shocked reticulocytes, HSP70 mRNA was a moderately abundant mRNA present at 1 to 2% of the level of globin mRNA. The rapid 20-fold increase in the synthesis of HSP70 after heat shock was not accompanied by a corresponding increase in the rate of transcription of the HSP70 gene or accumulation of HSP70 mRNA. These results suggest that the elevated synthesis of HSP70 is due to the preferential utilization of HSP70 mRNA in the heat-shocked reticulocyte. The heat shock-induced alterations in the reticulocyte protein-synthetic apparatus were not reversible. Upon return to control temperatures (37 degrees C), heat-shocked reticulocytes continued to synthesize HSP70 at elevated levels whereas globin synthesis continued to be repressed. Despite the presence of HSP70 mRNA in non-heat-shocked reticulocytes, we found that continued transcription was necessary for the preferential translation of HSP70 in heat-shocked cells. Preincubation of reticulocytes with the transcription inhibitor actinomycin D or 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole blocked the heat shock-induced synthesis of HSP70. Because the level of HSP70 mRNA was only slightly diminished in cells treated with actinomycin D, we suggest two possible mechanisms for the preferential translation of HSP70 mRNA: the translation of only newly transcribed HSP70 mRNA or the requirement of a newly transcribed RNA-containing factor.