Yeast prt1 mutations alter heat-shock gene expression through transcript fragmentation

The inhibition of translation initiation by modification or mutation of initiation factors can lead to disproportionate effects on gene expression. Here we report disproportionate decreases in gene expression in cells with mutated Prt1 activity. The PRT1 gene product of the budding yeast Saccharomyces cerevisiae is necessary for translation initiation and is thought to be a component of initiation factor 3. At a restrictive temperature the prt1-1 mutation, in addition to decreasing global protein synthesis, caused disproportionate decreases of the synthesis of the Ssa1 and Ssa2 members of the hsp70 heatshock gene family, and of the Hsp82 and Hsc82 heatshock proteins. Quantification of pulse-labelled, immunoprecipitated lacZ fusion proteins showed that synthesis of each of these proteins was disproportionately decreased in prt1-1 mutant cells. Although the mRNAs of affected genes were shown to be polysomal in mutant cells, they were fragmented and of decreased abundance, as indicated by transcript analysis and in vitro translation. Thus the mRNAs of these hsp genes become degraded under the conditions of limited translation initiation that are imposed by the prt1-1 mutation. This untimely mRNA degradation accounts for the disproportionate decreases in polypeptide synthesis in prt1 mutant cells. We propose that sequences at the translation initiation site of SSA2 mRNA bring about the observed mRNA fragmentation.