EFFICIENT TRANSLATION OF AN SSA1-DERIVED HEAT-SHOCK MESSENGER-RNA IN YEAST-CELLS LIMITED FOR CAP-BINDING PROTEIN AND EIF-4F

Eukaryotic mRNA molecules have a 5' cap structure that is recognized b y the cap-binding component of translation initiation factor eIF-4F du ring protein synthesis. In the budding yeast Saccharomyces cerevisiae this cap-binding protein is encoded by the CDC33 gene. We report here that decreased global translation initiation in cdc33 mutant cells has virtually no effect on the translation of mRNA from the SSA1-lacZ chi meric gene, comprised of yeast SSA1 hsp70 gene transcription and trans lation initiation sequences fused in-frame to the bacterial lacZ gene. When global translation initiation was limited in cdc33 mutant cells, Ssa1-LacZ polypeptide synthesis was increased relative to total prote in synthesis, and the beta-galactosidase activity of the Ssa1-LacZ fus ion protein was induced to wild-type levels. The normal rate of Ssa1-L acZ polypeptide synthesis in mutant cells was maintained by normal lev els of SSA1-lacZ mRNA. Furthermore, in cdc33 mutant cells, the size of polysomes containing SSA1-lacZ mRNA was unaffected, while polysomes c ontaining other specific mRNAs were smaller. Efficient Ssa1-LacZ polyp eptide synthesis was also seen during eIF-4F limitation produced by di sruption of the TIF4631 gene, encoding the large eIF-4F subunit. All o f these findings indicate efficient SSA1-lacZ mRNA usage under conditi ons of globally impaired translation initiation due to eIF-4F limitati on.