YEAST PRT1 MUTATIONS ALTER HEAT-SHOCK GENE-EXPRESSION THROUGH TRANSCRIPT FRAGMENTATION

The inhibition of translation initiation by modification or mutation o f initiation factors can lead to disproportionate effects on gene expr ession. Here we report disproportionate decreases in gene expression i n cells with mutated Prt1 activity. The PRT1 gene product of the buddi ng yeast Saccharomyces cerevisiae is necessary for translation initiat ion and is thought to be a component of initiation factor 3. At a rest rictive temperature the prt1-1 mutation, in addition to decreasing glo bal protein synthesis, caused disproportionate decreases of the synthe sis of the Ssa1 and Ssa2 members of the hsp70 heat-shock gene family, and of the Hsp82 and Hsc82 heat-shock proteins. Quantification of puls e-labelled, immunoprecipitated lacZ fusion proteins showed that synthe sis 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 abun dance, as indicated by transcript analysis and in vitro translation. T hus the mRNAs of these hsp genes become degraded under the conditions of limited translation initiation that are imposed by the prt1-1 mutat ion. This untimely mRNA degradation accounts for the disproportionate decreases in polypeptide synthesis in prt1 mutant cells. We propose th at sequences at the translation initiation site of SSA2 mRNA bring abo ut the observed mRNA fragmentation.