The TIF3 gene of Saccharomyces cerevisiae was cloned and sequenced. The deduced amino acid sequence shows 26% identity with the sequence of mammalian translation initiation factor eIF‐4B. The TIF3 gene is not essential for growth; however, its disruption results in a slow growth and cold‐sensitive phenotype. In vitro translation of total yeast RNA in an extract from a TIF3 gene‐disrupted strain is reduced compared with a wild‐type extract. The translational defect is more pronounced at lower temperatures and can be corrected by the addition of wild‐type extract or mammalian eIF‐4B, but not by addition of mutant extract. In vivo translation of beta‐galactosidase reporter mRNA with varying degree of RNA secondary structure in the 5′ leader region in a TIF3 gene‐disrupted strain shows preferential inhibition of translation of mRNA with more stable secondary structure. This indicates that Tif3 protein is an RNA helicase or contributes to RNA helicase activity in vivo.

Translation initiation factor Prt1 was purified from a ribosomal salt wash fraction of Saccharomyces cerevisiae cells by ammonium sulfate precipitation, DEAE chromatography, phosphocellulose chromatography, sucrose density gradient centrifugation, and non-denaturing polyacrylamide gel electrophoresis. Prt1 protein cofractionates with four other polypeptides during all steps of purification suggesting that it is part of a protein complex containing polypeptide subunits with apparent molecular masses of 130, 80, 75 (Prt1), 40, and 32 kDa. Deletion of the first AUG codon in the published sequence of the PRT1 gene results in the synthesis of functional Prt1 protein indicating that the actual molecular mass of the Prt1 subunit is 82.7 kDa. This is in agreement with results from primer extension experiments reported earlier by Keierleber et al. (Keierleber, C., Wittekind, M., Qin, S., and McLaughlin, C. S. (1986) Mol. Cell. Biol. 6, 4419-4424). The Prt1-containing protein complex is an active translation factor as shown by its ability to restore translation in a cell-free system derived from a temperature-sensitive prt1 mutant strain in which endogenous Prt1 activity is inactivated by heating the extract to 37 degrees C. The question of whether the Prt1-containing protein complex represents the yeast homologue of mammalian translation initiation factor eIF-3 is discussed.

The Saccharomyces cerevisiae TIF3 gene encodes the yeast homologue of mammalian translation initiation factor eIF‐4B. We have added six histidine residues to the C‐terminus of Tif3 protein (Tif3‐His6p) and purified the tagged protein by affinity chromatography. Tif3‐His6p stimulates translation and mRNA binding to ribosomes in a Tif3‐dependent in vitro system. Furthermore, it binds to single‐stranded RNA and catalyses the annealing of partially complementary RNA strands in vitro. In parallel experiments, RNA annealing activity could also be demonstrated for mammalian eIF‐4B. A role for Tif3/eIF‐4B and RNA annealing activity in the scanning process is proposed.