Expression of heat shock genes of Neurospora crassa: effect of hyperthermia and other stresses on mRNA levels.

Neurospora crassa mycelium was heat shocked for intervals varying from 15-180 min. Heat shock mRNA was monitored by hybridization of Northern blots with the Drosophila hsp-70 gene probe and an inducible member of the yeast hsp-70 gene family, YG100. A 2.7 kilobase (kb) transcript, with homology to these two probes, was detected in cultures shocked for 15 min; its levels increased up to 60-90 min and declined thereafter. Sodium arsenite, too, induced the synthesis of this transcript. An additional, constitutively synthesized 2.4-kb transcript was revealed by hybridization with the yeast probe. The synthesis of this message was terminated during heat shock. Hybridization of Northern blots with the Drosophila actin gene probe demonstrated two size classes, 1.85 and 1.63 kb; the former decreased dramatically following heat shock. Recovery, as assessed by the disappearance of the 2.7-kb hsp-70-mRNA and restoration of the 1.85-kb actin message to the prestress levels, was essentially complete within 60 min of transfer to 28 degrees C. In vitro translations of RNA from stressed cells showed the heat shock messages to be stable and readily translatable. RNA of cells subjected to heat shock plus CdCl2 showed a higher content of messages for heat shock proteins of 70, 80, and 90 kilodaltons.