Heat shock and recovery are mediated by different translational mechanisms

When Drosophila cells are shifted from 25 degrees C to 37 degrees C, protein synthesis is rapidly redirected from the complex pattern characteristic of normal growth to the simple pattern of heat shock proteins (HSPs). On return to 25 degrees C, synthesis of normal proteins is gradually reactivated and that of HSPs is repressed. In quantifying many different recovery experiments, we found that preexisting mRNAs always behaved as a cohort, with messages for different proteins returning to translation at the same rate. Heat shock mRNAs (HS mRNAs), on the other hand, never behaved as a cohort. Their repression was asynchronous, with translation of hsp70 always the first and translation of hsp82 always the last to be repressed. Although recovery times varied enormously (depending on the severity of the heat treatment), repression of hsp70 was always correlated with restoration of normal synthesis, suggesting a link between the two events, hsp70 repression was not simply due to competition with reactivated 25 degrees C mRNAs. A general decline in the translation efficiency of hsp70 mRNA was not observed. Instead, an increasing number of messages were translationally inactivated, while those remaining in the translational pool retained full ribosome loading. Unlike inactive 25 degrees C mRNAs, which are stable during heat shock, inactive HSP mRNAs are degraded during recovery.