Archaeal Ribosomes

Ribosomes, the essential cellular organelles carrying out protein synthesis, have a basic design that is fundamentally conserved in all three kingdoms of life (Archaea, Bacteria and Eukarya). Nevertheless, there are ribosomal features specific of each kingdom. Archaeal ribosomes have a size and composition similar to those of their bacterial counterparts: they contain three ribonucleic acid (RNA) molecules, 16S, 23S and 5S RNA and 50–70 proteins depending on the species. However, the primary structures of both archaeal ribosomal RNA and r‐proteins are closer to those of eukaryotes. As many Archaea have adapted to function under conditions of extreme salt or temperature, their ribosomal components are highly resistant to such adverse conditions, and the overall ribosome structure often has a higher rigidity than that of mesophilic microorganisms. This makes archaeal ribosomes optimally suited for crystallographic studies, and in fact, high‐resolution three‐dimensional structures have been obtained with ribosomal crystals from halophilic and thermophilic Archaea. These studies pioneered the resolution at the atomic level of ribosome architecture, a feat that won the 2009 Nobel Prize in Chemistry to Ada Yonath, Thomas Steitz and Venki Ramakrishnan. Key Concepts Ribosome structure is very well conserved in all cells. However, specific structural differences exist among the ribosomes in the three domains of life (Archaea, Bacteria and Eukarya). Archaeal ribosomes are composed of 30S and 50S subunits that join to make a 70S particle. They contain three rRNA molecules (16S, 23S and 5S) and up to 68 ribosomal proteins. The primary sequences of both archaeal rRNA and r‐proteins are closer to those of eukaryotes than to those of bacteria. All of the archaeal r‐proteins are represented in eukaryotes, whereas no r‐proteins are shared by Archaea and Bacteria only. Archaeal ribosomes have a heterogeneous protein composition. Early branching Archaea (Crenarchaeota) tend to have protein‐richer ribosomes, whereas late‐branching species (halobacteriales, Thermoplasmatales) tend to have protein‐poorer ribosomes. The ribosomes of the extremophilic Archaea show specific adaptations to harsh environmental conditions. In general, they have a more rigid structure than mesophilic ribosomes. Halophilic ribosomes increase their hydration capacity by having acidic instead of basic ribosomal proteins.