The Saccharomyces cerevisiae TIF3 gene
encodes a 436-amino acid (aa) protein that is the yeast
homologue of mammalian translation initiation factor eIF4B.
Tif3p can be divided into three parts, the N-terminal region
with an RNA recognition motif (RRM) (aa 1–182), followed
in the middle part by a sevenfold repeat of 26 amino acids
rich in basic and acidic residues (aa 183–350), and
a C-terminal region without homology to any known sequence
(aa 351–436). We have analyzed several Tif3 proteins
with deletions at their N and C termini for their ability
(1) to complement a tif3Δ strain in vivo, (2) to stimulate
Tif3-dependent translation extracts, (3) to bind to single-stranded
RNA, and (4) to catalyze RNA strand-exchange in vitro.
Here we report that yeast Tif3/eIF4B contains at least
two RNA binding domains able to bind to single-stranded
RNA. One is located in the N-terminal region of the protein
carrying the RRM, the other in the C-terminal two-thirds
region of Tif3p. The RRM-containing domain and three of
the seven repeat motifs are essential for RNA strand-exchange
activity of Tif3p and translation in vitro and for complementation
of a tif3Δ strain, suggesting an important role for
RNA strand-exchange activity in translation.