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.
Stability of maternal mRNA in Xenopus embryos: role of transcription and translation.