The control of protein synthesis during heat shock in Drosophila cells involves altered polypeptide elongation rates

When Drosophila tissue culture cells are shifted from 25 to 36 degrees C (heat shocked) the pre-existing mRNAs (25 degrees C mRNAs) remain in the cytoplasm but their translation products are underrepresented relative to the induced heat shock proteins. Many of these undertranslated 25 degrees C mRNAs are found in association with polysomes of similar size in heat-shocked and control cells. Furthermore, the messages encoding alpha-tubulin, beta-tubulin, and actin are found associated with one-third to one-half as many total ribosomes in heat-shocked cells as in cells incubated at 25 degrees C. Increased temperature should lead to increased output of protein per ribosome. However, the 25 degrees C proteins are actually synthesized at less than 10% of 25 degrees C levels in heat-shocked cells. Thus, the rates of both elongation and initiation of translation are significantly (15- to 30-fold) slower on 25 degrees C mRNAs than they are on heat shock mRNAs in heat-shocked cells.