Induction of Acquired Thermotolerance in Tetrahymena thermophila: Effects of Protein Synthesis Inhibitors

Abstract

When Tetrahymena thermophila cells growing at 30 degrees C are shifted to either 40 or 43 degrees C, the kinetics and extent of induction of heat shock mRNAs in both cases are virtually indistinguishable. However, the cells shifted to 40 degrees C show a typical induction of heat shock protein (HSP) synthesis and survive indefinitely (100% after 24 h), whereas those at 43 degrees C show an abortive synthesis of HSPs and die (less than 0.01% survivors) within 1 h. Cells treated at 30 degrees C with the drugs cycloheximide or emetine, at concentrations which are initially inhibitory to protein synthesis and cell growth but from which cells can eventually recover and resume growth, are after this recovery able to survive a direct shift from 30 to 43 degrees C (ca. 70% survival after 1 h). This induction of thermotolerance by these drugs is as efficient in providing thermoprotection to cells as is a prior sublethal heat treatment which elicits the synthesis of HSPs. However, during the period when drug-treated cells recover their protein synthesis ability and simultaneously acquire the ability to subsequently survive a shift to 43 degrees C, none of the major HSPs are synthesized. The ability to survive a 1-h, 43 degrees C heat treatment, therefore, does not absolutely require the prior synthesis of HSPs. But, as extended survival at 43 degrees Celsius depends absolutely on the ability of cells to continually synthesize HSPs, it appears that a prior heat shock as well as the recovery from protein synthesis inhibition elicits a change in the protein synthetic machinery which allows the translation of HSP mRNAs at what would otherwise be a nonpermissive temperature for protein synthesis.

No Heat Shock Protein Synthesis Is Required for Induced Thermostabilization of Translational Machinery

“…By contrast, naive (i.e., unstressed) cells are rapidly killed by the same shift from 30 to 43°C (<0.01% survivors after 1 h). This druginduced tolerance to a 43°C exposure is as effective as a prior 60-min, 40°C treatment which is itself nonlethal but which induces hsp synthesis (7). The acquisition of the ability of drug-treated cells to survive at 43°C correlates with the recovery of the protein synthetic capacity of the cells.…”

“…The acquisition of the ability of drug-treated cells to survive at 43°C correlates with the recovery of the protein synthetic capacity of the cells. However, during the time that cells recover protein synthesis capacity, no hsp synthesis occurs (7). After this recovery the protein synthetic machinery of these cells, unlike that of unstressed cells, is no longer thermolabile at 43°C.…”

“…After this recovery the protein synthetic machinery of these cells, unlike that of unstressed cells, is no longer thermolabile at 43°C. Whereas a 30 to 43°C shift induces the rapid and quantitatively identical accumulation of hsp mRNAs in both unstressed and drug-adapted cells, only in the latter are they efficiently translated into hsps (7), which is presumably what allows the drug-adapted cells to survive.…”

“…Effects of two different concentrations of cycloheximide on the induction of tolerance to a 43°C heat shock. T. thermophila growing at 30°C were treated with cycloheximide at 0.5 ,ug/ml (A, *) or 15 ,ug/ml (A, E) (the details of growth conditions have been described previously [6,7]). At the lower concentration of the drug, cells recovered protein synthetic activity within 2.5 h (7), while at the higher concentration, protein synthesis was completely abolished and was never restored (6,7,18).…”

“…T. thermophila growing at 30°C were treated with cycloheximide at 0.5 ,ug/ml (A, *) or 15 ,ug/ml (A, E) (the details of growth conditions have been described previously [6,7]). At the lower concentration of the drug, cells recovered protein synthetic activity within 2.5 h (7), while at the higher concentration, protein synthesis was completely abolished and was never restored (6,7,18). At 1.75 h (O, *) and 2.5 h (A, A) after administration of the drug, cells were collected by centrifugation and washed twice in fresh growth medium, allowed to recover at 30°C for 5 min, and then transferred directly to 43°C.…”