RESUMEN
Adaptation of house keeping and heat shock gene expression was determined in Neurospora crassa during 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 degreesC. Adaptation kinetics at 42 degreesC 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 degreesC 2 h prior to a second continuous exposure to 42 degreesC 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 HSPs in vivo revealed 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 degreesC 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 degreesC exposure and then declined after 8 h as determined by in vitro translation. Northern blots revealed maximal mRNA amounts of the inducible HSP70 after 30 min and zero amounts after 4 h exposure to 42 degreesC. After a shift to 42 degreesC 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.