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J. Cell Biol.,
Volume 143, Number 1, October 5, 1998 23-34
* Department of Biological Chemistry, University of California, Irvine, California 92697-1700; and 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.
Department of Anatomy and
Cell Biology, Health Science Center, University of Florida, Gainesville, Florida 32610-0235
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