The transcription of three specific genes has been examined in heat-shocked drosophila cells by hybridizing pulse-labeled nuclear RNA with cloned DNA sequences. Actin gene transcription is rapidly and profoundly suppressed upon heat shock but returns to near- normal levels after cells are placed back at their normal culture temperature (23 degrees C). Conversely, the transcription of genes coding from 70,000- and 26,000-dalton heat- shock proteins increases dramatically and with extraordinary rapidity (60 s) after heat shock. The temporal patterns of 70,000- and 26,000-dalton heat-shock gene transcription are nearly superimposable, indicating that, although they are closely linked cytologically, these genes are nevertheless tightly coregulated. The abundance of heat- shock gene transcripts reaches remarkable levels, e.g., 70,000-dalton heat-shock gene transcripts account for 2-3 percent of the nuclear RNA labeled during the first 30 min of heat shock. When heat-shocked cells are returned to 25 degrees C, the rates of transcription of the heat-shock genes fall back to the low levels characteristic of untreated cells. To confirm the low level of heat-shock gene transcription in normal cells, nuclear RNA was purified from unlabeled (and otherwise unhandled) 25 degrees C cells, end-labeled in vitro with (32)P, and hybridized to cloned heat-shock DNA sequences. These and other data establish that the genes for 70,000- and 26,000-dalton heat-shock proteins in culture drosophila cells are active at 25 degrees C, and that their rate of transcription is greatly accelerated upon heat shock rather than being activated from a true "off" state. The rapidity, magnitude, and reversibility of the shifts in actin and heat-shock gene transcription constitute compelling advantages for the use of cultured drosophila cells in studying the transcriptional regulation of eukaryotic genes, including one related to the cytoskeleton.

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