The Saccharomyces cerevisiae TIF6 gene encoding translation initiation factor 6 is required for 60S ribosomal subunit biogenesis

Eukaryotic translation initiation factor 6 (eIF6), a monomeric protein of a bout 26 kDa, can bind to the 60S ribosomal subunit and prevent its associat ion with the 40S ribosomal subunit, In Saccharomyces cerevisiae, eIF6 is en coded by a single-copy essential gene, To understand the function of eIF6 i n yeast cells, we constructed a conditional mutant haploid yeast strain in which a functional but a rapidly degradable form of eIF6 fusion protein was synthesized from a repressible GAL10 promoter. Depletion of eIF6 from yeas t cells resulted in a selective reduction in the level of 60S ribosomal sub units, causing a stoichiometric imbalance in 60S-to-40S subunit ratio and i nhibition of the rate of in vivo protein synthesis. Further analysis indica ted that eIF6 is not required for the stability of 60S ribosomal subunits, Rather, eIF6-depleted cells showed defective pre-rRNA processing, resulting in accumulation of 35S pre-rRNA precursor, formation of a 23S aberrant pre -rRNA, decreased 20S pre-rRNA levels, and accumulation of 27SB pre-rRNA, Th e defect in the processing of 27S pre-rRNA resulted in the reduced formatio n of mature 25S and 5.8S rRNAs relative to 18S rRNA, which may ac count for the selective deficit of 60S ribosomal subunits in these cells. Cell fract ionation as well as indirect immunofluorescence studies showed that c-Myc o r hemagglutinin epitope-tagged eIF6 was distributed throughout the cytoplas m and the nuclei of yeast cells.