Temperature Adaptation of House Keeping and Heat Shock Gene Expression inNeurospora crassa

Abstract

Adaptation of house keeping and heat shock gene expression was determined inNeurospora crassaduring continuous exposure to different temperatures. Steady-state values of total protein synthesis differed little after incubation for 24 h at temperatures between 15 and 42°C. Adaptation kinetics at 42°C showed an initial, transient inhibition of total protein synthesis. Similar kinetics were observed with actin synthesis and tubulin mRNA. A priming 1-h heat shock of 42°C 2 h prior to a second continuous exposure to 42°C abolished the inhibitory effect of the second treatment and resulted in “acquired translational tolerance.” Steady-state values of HSP70 synthesis rates revealed increasing levels with increasing temperatures after incubation for 24 h at different temperatures. Adaptation kinetics of the synthesis rates of different HSPsin vivorevealed maximal rates after 2 h and then a decrease to the elevated steady-state levels. The total amount of the major constitutive and inducible HSP70 isoform as determined by Western blots reached a maximum 2 h after the beginning of 42°C exposure and only a slight decrease (25%) of the maximal value after 24 h. The inducible isoform of HSP70, in contrast, reached a maximum after 4–8 h and then decreased strongly after 24 h. HSP mRNAs reached maximal amounts 45–60 min after the beginning of 42°C exposure and then declined after 8 h as determined byin vitrotranslation. Northern blots revealed maximal mRNA amounts of the inducible HSP70 after 30 min and zero amounts after 4 h exposure to 42°C. After a shift to 42°C HSP70 isoforms were immediately translocated into the nucleus and reshuttled into the cytoplasm during the following 6 h. The nuclear content of HSP70 remained elevated during the adapted steady state at 24 h. It is concluded that the adapted state after 24 h is based on enhanced amounts of constitutive isoforms in the cytoplasm and in the nucleus, whereas the inducible isoforms of HSP70 show faster adaptation kinetics.