Known heat‐shock proteins are not responsible for stress‐induced rapid degradation of ribosomal protein mRNAs in yeast

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