RNA Polymerase I, the Nucleolus and Synthesis of 35S rRNA in the Yeast Saccharomyces Cerevisiae

Abstract

The synthesis of ribosomes is regulated in response to environmental changes, and this regulation is important in connection with the regulation of growth. This is true for both prokaryotic and eukaryotic organisms. In E. coil, the cellular concentration of ribosomes is roughly proportional to the growth rate except under slow growth conditions; this regulatory feature is called growth-rate-dependent control of ribosome synthesis. Extensive studies of this regulation in E. coli have shown that the regulation acts primarily on the synthesis of rRNA, and that growth-rate-dependent control of ribosomal protein (r-protein) synthesis is achieved indirectly through various autogenous (feedback) repression systems. In eukaryotes, although the synthesis of r-proteins may be regulated directly, and independently of rRNA synthesis, there is no doubt that the synthesis of rRNAs in the nucleolus is of primary importance in overall regulation of ribosome biosynthesis (see reviews, Warner, 1982; Sollner-Webb and Tower, 1986; Sollner-Webb and Mougey, 1991). A few years ago, our laboratory initiated studies on the synthesis of rRNA in the yeast Saccharomyces cerevislae as a model eukaryotic organism, with the eventual goal of understanding molecular events involved in the regulation of ribosome biosynthesis.