Molecular Cell Biology: Organization and Molecular Evolution of rDNA, Nucleolar Dominance, and Nucleolus Structure

Abstract

Recently, the ribosomes — molecularly well-characterized cell organelles where the process of translation of messenger RNA into polypeptides occurs — have attracted new interest since they can be considered as a huge ribozyme-like complex consisting of different proteins and RNA — the RNAs mainly fulfilling functional tasks in translation, the proteins serving more structural functions (Moore and Steitz 2002). Within the eukaryotic cell, two (animals and fungi) or three (plants) sites of a translational machinery are present, the cytoplasm, mitochondria and chloroplasts, and, respectively, the components are mostly encoded in the cell nucleus or the mitochondrial or chloroplast genome. Since new insights are gained into the production and the assembly of the ribosome components, ribosomal RNA (rRNA) and ribosomal proteins (r-proteins), in this chapter, we focus on the nucleolus, the site for cytoplasmic ribosome biogenesis and assembly occurring in the light-microscopically visible structure of the cell nucleus. Similarly as in prokaryotes, for all higher organisms two ribosome subunits are preformed in the nucleolus, which are evolutionary conserved: the small 40S subunit (SSU) containing 18S rRNA, and the large 60S subunit containing 25/28S plus 5.8S and 5S rRNA. After export of the ribosome subunits (LSU) into the cytoplasm, the functionally active ribosome is associated with translatable mRNA to an 80S complex corresponding to the 70S structure in chloroplasts and prokaryotic cells.