The Saccharomyces cerevisiae translation initiation factor Tif3 and its mammalian homologue, eIF-4B, have RNA annealing activity.

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Abstract
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.