Expression of a Drosophila heat-shock protein in Xenopus oocytes: conserved and divergent regulatory signals.

On injection of cloned Drosophila hsp70 heat-shock genes into Xenopus oocytes, heat-inducible expression is observed: the level of correctly initiated transcripts is increased 20- to 100-fold on heat shock at 34 degrees C. We show that this induction is due to activation of the heat-shock gene promoter, and that the DNA sequences required for induction lie between 10 and 66 bases upstream from the transcription start site. Most heat-induced transcripts have a correct 3' end, and hsp70 mRNA activity is detectable after extraction of the RNA from oocytes and subsequent in vitro translation. Drosophila heat-shock protein (hsp70) is synthesised in the injected oocytes after heat-shock, but only at low temperature. Under heat-shock conditions, Drosophila hsp70 mRNA translation is reduced 10-fold as is translation of the normal (25 degrees C) mRNAs, whereas translation of the endogenous Xenopus hsp70 mRNA is strongly induced. Translation of Drosophila heat-shock mRNAs extracted from flies and injected into the oocytes is also reduced by heat-shock. This suggests that Xenopus oocytes do not recognise the translational regulatory signals of Drosophila heat-shock mRNAs. In contrast, the signals for heat-induced transcription must be strongly conserved between Xenopus and Drosophila.