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This review provides an overview of the current knowledge of ribosome biogenesis, nucleolus structure and function and protein traffic into and out of the nucleus, with emphasis on the potential of yeast Saccharomyces cerevisiae as a model organism.
The role of the yeast vacuole, a functional analogue of the mammalian lysosome, in the turnover of proteins and organelles has been well documented. This review pro­vides an overview of the current knowledge of vesicle mediated vacuolar transport in the yeast Saccharomyces cerevisiae cells. Due to the conservation of the molecular transport machinery S. cerevisiae has become an important model system of vacuolar trafficking because of the facile application of genetics, molecular biology and bio­chemistry.
The newly discovered Saccharomyces cerevisiae gene KRR1 (YCL059c) encodes a protein essential for cell viability. Krr1p contains a motif of clustered basic amino acids highly conserved in the evolutionarly distant species from yeast to human. We demonstrate that Krr1p is localized in the nucleolus. The KRR1 gene is highly expressed in dividing cells and its expression ceases almost completely when cells enter the stationary phase. In vivo depletion of Krr1p leads to drastic reduction of 40S ribosomal subunits due to defective 18S rRNA synthesis. We propose that Krr1p is required for proper processing of pre-rRNA and the assembly of preribosomal 40S subunits.
Sporulation of the budding yeast Saccharomyces cerevisiae - equivalent to gametogenesis in higher organisms, is a complex differentiation program induced by starvation of cells for nitrogen and carbon. Such environmental conditions activate coordinated, sequential changes in gene expression leading to production of haploid, stress-resistant spores. Sporulation comprises two rounds of meiosis coupled with spore morphogenesis and is tightly controlled to ensure viable progeny. This review concerns the regulation of differentiation process by nutritional and transcriptional signals.
The Krr1 protein of Saccharomyces cerevisiae is involved in processing of pre-rRNA and assembly of pre-ribosomal 40S subunits. To further investigate the function of Krr1p we constructed a conditional cold sensitive mutant krrl-21, and isolated seven genes from Schizosaccharomyces pombe whose products suppressed the cold sensitive phenotype of krrl-21 cells. Among the multicopy suppressors we found genes coding for translation elongation factor EF-1a, a putative ribose methyltransferase and five genes encoding ribosomal proteins. Using the tandem af­finity purification (TAP) method we identified thirteen S. cerevisiae ribosomal pro­teins interacting with Krr1p. Taken together, these results indicate that Krr1p inter­acts functionally as well as physically with ribosomal proteins. Northern blot analy­sis revealed that changes in the level of krrl-21 mRNA were accompanied by similar changes in the level of mRNAs of genes encoding ribosomal proteins. Thus, Krr1p and the genes encoding ribosomal proteins it interacts with seem to be coordinately regulated at the level of transcription.
Saccharomyces cerevisiae homologues of phosphotyrosyl phosphatase activator (PTPA) are encoded by RRD1 and RRD2, genes whose combined deletion is synthetic lethal. Previously we have shown that the lethality of rrd 1,2Δ can be suppressed by in­creasing the osmolarity of the medium. Here we show that the lethality of rrd1,2Δ is also suppressed under oxygen-limited conditions. The absence of respiration perse is not responsible for the suppression since elimination of the mitochondrial genome or a block in heme biosynthesis fail to rescue the rrd1,2Δ double mutation.
The genome of the yeast Saccharomyces cerevisiae was sequenced by an international consortium of laboratories from Europe, Canada, the U.S.A. and Japan. This project is now finished and the complete sequence of the first eukaryotic genome was released to the public data bases in April 1996. An overview and preliminary analysis of the entire genome sequence was presented in a special issue of Nature in May 1997, entitled "The yeast genome directory". At its origin the Yeast Genome Sequencing Project provoked much debate and controversy; however, the final results obtained and the insights this has given us into the organisation and content of a eukaryotic genome have more than justified the expectations of the supporters of the project. The importance of genomic sequencing and analysis, especially of model organisms, is now widely accepted and this has resulted in the birth of the new science of genomics (Botstein & Cherry, Proc. Natl. Acad. Sci. U.S.A. 94, 5506, 1997). The information from gene and protein sequences ultimately lead to functional description of all genes. The main strategies describing possible ways to analyse the function of new genes that have been identified by systematic sequencing of Saccharomyces cerevisiae genome are described.
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