Molecular and Cellular Biology, December 1999, p. 8633-8645, Vol. 19, No. 12
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
and
Département de Biochimie
Médicale,
Received 17 June 1999/Returned for modification 4 August
1999/Accepted 2 September 1999
Dbp6p is an essential putative ATP-dependent RNA helicase that is
required for 60S-ribosomal-subunit assembly in the yeast Saccharomyces cerevisiae (D. Kressler, J. de la Cruz, M. Rojo, and P. Linder, Mol. Cell. Biol. 18:1855-1865, 1998). To identify factors that are functionally interacting with Dbp6p, we have performed
a synthetic lethal screen with conditional dbp6 mutants. Here, we describe the cloning and the phenotypic analysis of the previously uncharacterized open reading frame YPL193W, which we renamed
RSA1 (ribosome assembly 1). Rsa1p is not essential for cell
viability; however, rsa1 null mutant strains display a
slow-growth phenotype, which is exacerbated at elevated temperatures.
The rsa1 null allele synthetically enhances the mild growth
defect of weak dbp6 alleles and confers synthetic lethality
when combined with stronger dbp6 alleles. Polysome profile
analysis shows that the absence of Rsa1p results in the accumulation of
half-mer polysomes. However, the pool of free 60S ribosomal subunits is
only moderately decreased; this is reminiscent of polysome profiles
from mutants defective in 60S-to-40S subunit joining. Pulse-chase
labeling of pre-rRNA in the rsa1 null mutant strain
indicates that formation of the mature 25S rRNA is decreased at the
nonpermissive temperature. Interestingly, free 60S ribosomal subunits
of a rsa1 null mutant strain that was grown for two
generations at 37°C are practically devoid of the
60S-ribosomal-subunit protein Qsr1p/Rpl10p, which is required for
joining of 60S and 40S subunits (D. P. Eisinger, F. A. Dick,
and B. L. Trumpower, Mol. Cell. Biol. 17:5136-5145, 1997).
Moreover, the combination of the
rsa1 and
qsr1-1 mutations leads to a strong synthetic growth
inhibition. Finally, a hemagglutinin epitope-tagged Rsa1p localizes
predominantly to the nucleoplasm. Together, these results point towards
a function for Rsa1p in a late nucleoplasmic step of
60S-ribosomal-subunit assembly.
*
Corresponding author. Mailing address:
Département de Biochimie Médicale, Centre Médical
Universitaire, Université de Genève, 1 rue Michel-Servet,
CH-1211 Geneva 4, Switzerland. Phone: 41 22 702 54 84. Fax: 41 22 702 55 02. E-mail for Dieter Kressler: dieter.kressler{at}medecine.unige.ch. E-mail for Patrick
Linder: patrick.linder{at}medecine.unige.ch.
Present address: INSERM U523-Institut de Myologie, Groupe
Hospitalier Pitié-Salpêtrière, 75651 Paris Cedex 13, France.
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