CORDYCEPIN BLOCKS RECOVERY OF NON-HEAT-SHOCK MESSENGER-RNA TRANSLATION FOLLOWING HEAT-SHOCK IN DROSOPHILA

Treatment of cells with cordycepin (3-deoxyadenosine), an inhibitor of cytoplasmic adenylation, blocks the restoration of normal translation following heat shock. Cordycepin also reduces heat-shock protein 70 (Hsp70) protein synthesis greater than 10-fold, while having little to no effect on mRNA accumulation. Parallel analysis of the poly(A)-binding protein detects no change in its abundance during heat shock or subsequent recovery. These results suggest that normal. non-heat-shock mRNA translational repression during heat shock may be caused by deadenylation, and that readenylation is required for restoration of activity. However, three independent analyses of the adenylation status of mRNAs during heat shock and recovery indicate that no significant changes in polyadenylation occur. (a) The total poly(A) content decreases by only about 10% during heat shock; (b) the size of the poly(A) tract decreases only marginally, from an average length of 75-90 nucleotides in non-heated cells to 45-60 nucleotides during heat shock; (c) virtually all mRNAs bind to oligo d(T)-cellulose, whether extracted from normal-temperature, heat-shock or recovered cells. Our results are most consistent with a model where the process of readenylation, rather than the specific poly(A) tail length, influences translational activation during recovery, paralleling a proposed model for the activation of translation during Xenopus oocyte maturation.