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Molecular and Cellular Biology, March 2001, p. 1453-1462, Vol. 21, No. 5
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.5.1453-1462.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

The Saccharomyces cerevisiae TIF6 Gene Encoding Translation Initiation Factor 6 Is Required for 60S Ribosomal Subunit Biogenesis

Uttiya Basu,1 Kausik Si,1,dagger Jonathan R. Warner,2 and Umadas Maitra1,*

Department of Developmental and Molecular Biology1 and Department of Cell Biology,2 Albert Einstein College of Medicine, Bronx, New York 10461

Received 6 October 2000/Returned for modification 14 November 2000/Accepted 1 December 2000

Eukaryotic translation initiation factor 6 (eIF6), a monomeric protein of about 26 kDa, can bind to the 60S ribosomal subunit and prevent its association with the 40S ribosomal subunit. In Saccharomyces cerevisiae, eIF6 is encoded by a single-copy essential gene. To understand the function of eIF6 in 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 yeast cells resulted in a selective reduction in the level of 60S ribosomal subunits, causing a stoichiometric imbalance in 60S-to-40S subunit ratio and inhibition of the rate of in vivo protein synthesis. Further analysis indicated 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. The defect in the processing of 27S pre-rRNA resulted in the reduced formation of mature 25S and 5.8S rRNAs relative to 18S rRNA, which may account for the selective deficit of 60S ribosomal subunits in these cells. Cell fractionation as well as indirect immunofluorescence studies showed that c-Myc or hemagglutinin epitope-tagged eIF6 was distributed throughout the cytoplasm and the nuclei of yeast cells.

* Corresponding author. Mailing address: Department of Developmental and Molecular Biology, Albert Einstein College of Medicine, 1300 Morris Park Ave., Bronx, NY 10461. Phone: (718) 430-3505. Fax: (718) 430-8567. E-mail: maitra{at}aecom.yu.edu.

dagger Present address: Howard Hughes Medical Institute and Center for Neurobiology, Columbia University College of Physicians and Surgeons, New York, NY 10032.

Molecular and Cellular Biology, March 2001, p. 1453-1462, Vol. 21, No. 5
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.5.1453-1462.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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