SUI1 P16 IS REQUIRED FOR THE ACTIVITY OF EUKARYOTIC TRANSLATION INITIATION-FACTOR-3 IN SACCHAROMYCES-CEREVISIAE/

A genetic reversion analysis at the HIS4 locus in Saccharomyces cerevi siae has identified SUI1 as a component of the translation initiation complex which plays an important role in ribosomal recognition of the initiator codon. SUI1 is an essential protein of 12.3 kDa that is requ ired in vivo for the initiation of protein synthesis. Here we present evidence that SUI1 is identical to the smallest subunit, p16, of eukar yotic translation initiation factor 3 (eIF3) in S. cerevisiae. SUI1 an d eIF3-p16 comigrate upon sodium dodecyl sulfate-polyacrylamide gel el ectrophoresis and cross-react with anti-SUI1 and anti-eIF3 antisera. A nti-SUI1 antisera immunoprecipitate all of the subunits of eIF3, where as antisera against the eIF3 complex and the individual PRT1 and GCD10 subunits of eIF3 immunoprecipitate SUI1. Finally, the N-terminal amin o acid sequence of a truncated form of eIF3-p16 matches the sequence o f SUI1. eIF3 isolated from a sui1(ts) strain at 37 degrees C lacks SUI 1 and fails to exhibit eIF3 activity in the in vitro assay for methion yl-puromycin synthesis. A free form of SUI1 separate from the eIF3 com plex is found in S. cerevisiae but lacks activity in the in vitro assa y. The results, together with prior genetic experiments, indicate that SUI1 is essential for eIF3 activity and functions as part of eIF3 and in concert with eIF2 to promote eIFZ-GTP-Met-tRNA(i) ternary complex recognition of the initiator codon.