STRUCTURAL AND FUNCTIONAL-ANALYSIS OF THE NUCLEOLUS OF THE FISSION YEAST SCHIZOSACCHAROMYCES-POMBE

Yeasts are an attractive model for the study of ribosome synthesis. Ho wever, our understanding of the relationship between the structure and function of the yeast nucleolus, in which preribosomal particles are synthesized, requires further investigations using microscopic approac hes and in situ molecular biology. Combining cryofixation and cryosubs titution of Schizosaccharomyces pombe, we could identify morphological ly distinct substructures in the nucleolus similar to the components o f nucleoli of higher eukaryotes such as the fibrillar centers (FCs), t he dense fibrillar component (DFC) and the granular component (GC). We complemented this morphological study by performing in situ hybridiza tion and immunocytochemistry at the electron microscopy level. Using a probe complementary to the entire rRNA transcription unit of S. pombe , we detected rDNA at the periphery of the FCs, while immunocytochemis try with antibodies specific for the RNA polymerase I and the gar1 pro tein provided evidence that transcription and early steps of maturatio n take place in the DFC that extends throughout the nucleolus. We also present evidence that preribosomal subunits may be exported along tra cks to the cytoplasm through all of the pores of the nuclear envelope and not just those in the portion of the envelope close to the nucleol us.