A SACCHAROMYCES-CEREVISIAE HOMOLOG OF MAMMALIAN TRANSLATION INITIATION FACTOR-4B CONTRIBUTES TO RNA HELICASE ACTIVITY

The TIF3 gene of Saccharomyces cerevisiae was cloned and sequenced. Th e deduced amino acid sequence shows 26% identity with the sequence of mammalian translation initiation factor eIF-4B. The TIF3 gene is not e ssential 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 wi th 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 deg ree 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.