ULTRASTRUCTURAL-CHANGES IN THE SCHIZOSACCHAROMYCES-POMBE NUCLEOLUS FOLLOWING THE DISRUPTION OF THE GAR2+ GENE, WHICH ENCODES A NUCLEOLAR PROTEIN STRUCTURALLY RELATED TO NUCLEOLIN

The nucleolar protein gar2, from the fission yeast Schizosaccharomyces pombe, is the functional homolog of NSR1 from Saccharomyces cerevisia e, and is structurally related to nucleolin from vertebrates. By immun ocytochemistry at the electron microscope level, we show that gar2 co- localizes with RNA polymerase I and the gar1 protein along the dense f ibrillar component of the nucleolus in a wild-type strain of S. pombe, suggesting that gar2 is involved in the transcription and/or in the e arly steps of maturation of the ribosomal RNAs. Since the effects of d isruption of the gar2+ gene might also shed light on the role of the g ad protein, we analyzed the ultrastructure of the nucleolus of a gar2- disruption mutant. The nucleolus of the gar2- mutant is dramatically r eorganized when compared with that of the wild-type gar2+ strain: a tr uncated protein containing the NH2-terminus of the gar2 protein is acc umulated in an unusual nucleolar ''dense body''. Our results also sugg est that the NH,(2)-terminus might be sufficient for nucleolar localiz ation via interaction with specific nucleolar components and support t he hypothesis that gar2 in wild-type S. pombe interacts with nascent p re-rRNA via its two RNA-binding domains in combination with the glycin e/arginine-rich domain. We also report that disruption of the gar2+ ge ne results in a mutant that is defective in cytokinesis and nuclear di vision.