MUTATIONAL ANALYSIS OF THE PRT1 PROTEIN SUBUNIT OF YEAST TRANSLATION INITIATION-FACTOR-3

The Saccharomyces cerevisiae PRT1 gene product Prt1p is a component of translation initiation factor eIF-3, and mutations in PRT1 inhibit tr anslation initiation. We have investigated structural and functional a spects of Prt1p and its gene. Transcript analysis and deletion of the PRT1 5' end revealed that translation of PRT1 mRNA is probably initiat ed at the second in-frame ATG in the open reading frame. The amino aci d changes encoded by six independent temperature sensitive prt1 mutant alleles were found to be distributed throughout the central and C-ter minal regions of Prt1p. The temperature sensitivity of each mutant all ele was due to a single missense mutation, except for the prt1-2 allel e, in which two missense mutations were required. In-frame deletion of an N-terminal region of Prt1p generated a novel, dominant-negative fo rm of Prt1p that inhibits translation initiation even in the presence of wild-type Prt1p. Subcellular fractionation suggested that the domin ant-negative Prt1p competes with wild-type Prt1p for association with a component of large Prt1p complexes and as a result inhibits the bind ing of wild-type Prt1p to the 40S ribosome.