Eukaryotic translation initiation factor 5 (eIF-5) catalyzes hydrolysis of GTP bound to a 40 S ribosomal initiation complex with the subsequent joining of a 60 S ribosomal subunit to form an 80 S initiation complex. The yeast gene that encodes eIF-5, designated TIF5, has been isolated and expressed in Escherichia coli to yield a catalytically active eIF-5 protein. TIF5 is a single-copy gene that maps on yeast chromosome XVI and is essential for cell viability. The gene contains an intron-free open reading frame that encodes a protein of calculated M(r) 45,346 in close agreement with the apparent molecular weight of eIF-5 isolated from yeast cells. Sequence analysis of the gene reveals several interesting features. First, the presence of two in-frame translational start sites located 51 base pairs apart suggests the possibility that two proteins, differing by an amino-terminal extension of 17 amino acids, could be generated from the TIF5 gene via differential translational starts. This would explain the presence, in yeast cell lysates, of two forms of eIF-5 differing in molecular weight by about 2,000. Second, the predicted amino acid sequence of eIF-5 contains sequence motifs characteristic of proteins of the GTPase superfamily.