RNA



Bms1p, a novel GTP-binding protein, and the related Tsr1p are required for distinct steps of 40S ribosome biogenesis in yeast


DANIEL  GELPERIN a1p1, LYNN  HORTON a2, JAKE  BECKMAN a1, JACK  HENSOLD a2a3 and SANDRA K.  LEMMON a1c1
a1 Department of Molecular Biology and Microbiology, Case Western Reserve University School of Medicine, Cleveland, Ohio 44106, USA
a2 Department of Medicine, Case Western Reserve University School of Medicine, Cleveland, Ohio 44106, USA
a3 Department of Medicine, Cleveland Veterans Affairs Medical Center, Cleveland, Ohio 44106, USA

Abstract

Bms1p and Tsr1p define a novel family of proteins required for synthesis of 40S ribosomal subunits in Saccharomyces cerevisiae. Both are essential and localize to the nucleolus. Tsr1p shares two extended regions of similarity with Bms1p, but the two proteins function at different steps in 40S ribosome maturation. Inactivation of Bms1p blocks at an early step, leading to disappearance of 20S and 18S rRNA precursors. Also, slight accumulation of an aberrant 23S product and significant 35S accumulation are observed, indicating that pre-rRNA processing at sites A0, A1, and A2 is inhibited. In contrast, depletion of Tsr1p results in accumulation of 20S rRNA. Because processing of 20S to 18S rRNA occurs in the cytoplasm, this suggests that Tsr1p is required for assembly of a transport- or maturation-competent particle or is specifically required for transport of 43S pre-ribosomal particles, but not 60S ribosome precursors, from the nucleus to the cytosol. Finally, Bms1p is a GTP-binding protein, the first found to function in ribosome assembly or rRNA processing.

(Received May 17 2001)
(Accepted May 23 2001)


Key Words: 18S rRNA; GTP-binding; nucleolus; P-loop; rRNA processing.

Correspondence:
c1 Reprint requests to: Sandra K. Lemmon, Department of Molecular Biology and Microbiology, Case Western Reserve University School of Medicine, Cleveland, Ohio 44106, USA; e-mail: skl@po.cwru.edu
p1 Current address: Department of Cellular and Developmental Biology, Yale University, New Haven, CT 06520-8103.