HEAT-SHOCK-INDUCED PROTEIN-SYNTHESIS IS RESPONSIBLE FOR THE SWITCH-OFF OF HSP70 TRANSCRIPTION IN TETRAHYMENA

We had previously described that new RNA synthesis is required for exp ression of the heat shock protein HSP70. Here, we find that the HSP70 mRNA decreases its levels under stress conditions, heat shock (HS) or arsenite (As), and that its levels start to decline at the same time a s maximal HSPs synthesis (including HSP70) occurs. This suggests that regulation of the hsp70 gene is mainly exerted at the transcriptional level. Accumulation of the HSP70 mRNA in cells stressed in presence of cycloheximide (CHX), indicates that (a) protein(s) non-existent befor e stress, possibly HSP70 itself (which is shown here to be relatively stable), is involved in negatively regulating hsp70 expression. Since degradation of the HSP70 mRNA is also shown to occur in cells heat-sho cked under CHX, as seen from decay of its levels upon addition of acti nomycin D (AMD), the protein(s) must repress hsp70 expression at the t ranscriptional level. Other conditions that affect normal protein synt hesis, namely the translation inhibitor puromycin and the arginine-ana log canavanine (shown here to be stress inducers in Tetrahymena pyrifo rmis), also cause a delay in transcription-arrest of the HSP70 mRNA. U nder severe stress conditions of HS (36-degrees-C) or As (350 muM), th e levels of HSP70 mRNA are higher than under mild stress conditions, h owever, no significant difference is seen in the pattern of HSP70 mRNA decay.