MUTATIONAL ANALYSIS OF THE STRUCTURE AND LOCALIZATION OF THE NUCLEOLUS IN THE YEAST SACCHAROMYCES-CEREVISIAE

The nucleolus in Saccharomyces cerevisiae is a crescent-shaped structu re that makes extensive contact with the nuclear envelope. In differen t chromosomal rDNA deletion mutants that we have analyzed, the nucleol us is not organized into a crescent structure, as determined by immuno fluorescence microscopy, fluorescence in situ hybridization, and elect ron microscopy. A strain carrying a plasmid with a single rDNA repeat transcribed by RNA polymerase I (Pol I) contained a fragmented nucleol us distributed throughout the nucleus, primarily localized at the nucl ear periphery. A strain carrying a plasmid with the 35S rRNA coding re gion 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 obse rved in strains carrying the Pol I plasmid in combination with mutatio ns 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 c hromosomal 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 organiz ation and localization of the nucleolus.