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© The Rockefeller University Press, 0021-9525/1998//23 $5.00
The Journal of Cell Biology, Volume 143, Number 1, , 1998 23-34

Regular Articles

Mutational Analysis of the Structure and Localization of the Nucleolus in the Yeast Saccharomyces cerevisiae



M. Oakes*, J.P. Aris{ddagger}, J.S. Brockenbrough{ddagger}, H. Wai*, L. Vu*, and M. Nomura*

* Department of Biological Chemistry, University of California, Irvine, California 92697-1700; and {ddagger} Department of Anatomy and Cell Biology, Health Science Center, University of Florida, Gainesville, Florida 32610-0235

The nucleolus in Saccharomyces cerevisiae is a crescent-shaped structure that makes extensive contact with the nuclear envelope. In different chromosomal rDNA deletion mutants that we have analyzed, the nucleolus is not organized into a crescent structure, as determined by immunofluorescence microscopy, fluorescence in situ hybridization, and electron microscopy. A strain carrying a plasmid with a single rDNA repeat transcribed by RNA polymerase I (Pol I) contained a fragmented nucleolus distributed throughout the nucleus, primarily localized at the nuclear periphery. A strain carrying a plasmid with the 35S rRNA coding region fused to the GAL7 promoter and transcribed by Pol II contained a rounded nucleolus that often lacked extensive contact with the nuclear envelope. Ultrastructurally distinct domains were observed within the round nucleolus. A similar rounded nucleolar morphology was also observed in strains carrying the Pol I plasmid in combination with mutations that affect Pol I function. In a Pol I–defective mutant strain that carried copies of the GAL7-35S rDNA fusion gene integrated into the chromosomal rDNA locus, the nucleolus exhibited a round morphology, but was more closely associated with the nuclear envelope in the form of a bulge. Thus, both the organization of the rDNA genes and the type of polymerase involved in rDNA expression strongly influence the organization and localization of the nucleolus.

Key Words: nucleus • nucleolus • nuclear envelope • ribosomal DNA (rDNA) • RNA polymerases I and II

Abbreviations used in this paper: CF, core factor; FISH, fluorescence in situ hybridization; IFM, immunofluorescence microscopy; Pol I and Pol II, polymerase I and II; rDNA, ribosomal DNA; snoRNA, small nucleolar RNA; UAF, upstream activation factor.

Address all correspondence to Masayasu Nomura, University of California, Irvine, Department of Biological Chemistry, Irvine, CA 92697-1700. Tel.: (949) 824-4564. Fax: (949) 824-3201. E-mail: mnomura{at}uci.edu


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