Hyman Hartman (MIT-Dept. of Earth, Atmospheric & Planetary Sciences)
Messenger RNA is translated into Protein by the translational machinery of the cell, which includes twenty enzymes that attach the twenty amino acids to the transfer RNAs (tRNAs) with the correct anticodon. These enzymes are aptly called the Aminoacyl-tRNA synthetases. The machinery also includes the initiation and elongation factors and then there is the Ribosome, which is composed of a large subunit (LSU) and a small subunit (SSU). The ribosome has been crystallized and its three dimensional structure of the Large and Small subunit has been revealed by X-Ray diffraction. We can now reconstruct its evolutionary History. The most conserved proteins in the Proteome (all the proteins) of the Biosphere are those proteins involved in the translational apparatus of the cell. The most conserved of these translational proteins are the ribosomal proteins. There are 34 Universal Ribosomal proteins found in the three cellular domains (Bacteria, Archaea and Eukarya) and it is among these highly conserved proteins and others in the translational apparatus that the evolution of the Genetic Code is to be found. These Universal Ribosomal Proteins contain the record of the evolution of the Evolution of the Ribosome and the Genetic Code. The conclusions from a study of these proteins are: 1) These proteins were formed from Peptides 2) The globular form of these proteins have a different amino acid composition from their extensions. 3) The Large Subunit of the ribosome (LSU) is older than the Small Subunit (SSU) of the ribosome. 4) The protein extensions into the Peptide Transfer Center (PTC) of the LSU are the oldest and most informative peptides and it is these extensions, which reveal the early Genetic Code. The evolution of the Universal Ribosomal Proteins of the PTC and the evolution of the RNA of the PTC point to an early stage in which there were peptides and small RNAs. The relevance to the Origin of Life of this stage in the evolution of the Genetic Code and the Ribosome will be discussed.