Activation of the major drosophila heat-shock genes in vitro.

Inactive Drosophila heat-shock genes of isolated diploid nuclei can be induced to a transcriptionally active state by exposure to cytoplasmic extracts from heat-shocked Drosophila cells. No effect was observed on histone gene transcription, and extracts from non-heat-shocked cells were ineffective. The factor in the cytoplasmic extract has been partially purified and characterized. It is protease-sensitive and heat-labile. A striking change accompanies in vitro activation that permits transcription by E. coli RNA polymerase of the chromatin 5' -distal to the structural genes at the 87A and 87C heat-shock gene loci; we have previously observed a similar change after in vivo heat-shock induction. That this change occurred in the absence of endogenous RNA polymerase II activity suggests that these changes may reflect the earliest event in gene activation. Inasmuch as activation also took place after histone H1 depletion, this histone does not appear to be essential for this step of gene activation.