KNOWN HEAT-SHOCK PROTEINS ARE NOT RESPONSIBLE FOR STRESS-INDUCED RAPID DEGRADATION OF RIBOSOMAL-PROTEIN MESSENGER-RNAS IN YEAST

We have previously shown that the heat-induced enhanced decay of yeast mRNAs encoding ribosomal proteins (rp-mRNAs)requires ongoing transcri ption during the heat treatment [Herruer et al. (1988) Nucl. Acids Res . 16,7917]. In order to determine whether this requirement reflects th e need for heat-shock protein (hsp), we analysed the effect of heat sh ock on rp-mRNA levels in several yeast strains in which each of the he at-shock genes encoding hsp26, hsp35 or hsp83 had been individually di srupted. In all three strains we still observed increased degradation of rp-mRNAs immediately after the temperature shift, demonstrating tha t hsp26, hsp35 and hsp83 are not required for this effect. Accelerated turnover of rp-mRNA was also found to occur upon raising the growth t emperature of a mutant strain that contains a disruption of the gene s pecifying the heat-shock transcription factor and in wild-type yeast c ells treated with canavanine, an arginine analogue that will be incorp orated into all known hsps and that is known to cause misfolding of th e polypeptide chain. Latter observation suggests that enhanced rp-mRNA decay is a more general stress-related phenomenon. Taken together, th ese data strongly indicate that the trans-acting factor required for t he increase in the rate of degradation of rp-mRNAs upon stress is not one of the known yeast hsps.