The Ribosome: A molecular machine with brains

Ribosomes are the universal cellular organelles catalyzing the sequential polymerization of amino acids according to the genetic blueprint, encoded in the mRNA. They are built of two subunits that associate for performing this task. The larger subunit creates the peptide bonds and provides the path along which the nascent protein chain emerges out of the ribosome. The smaller subunit has key roles in the initiation of the process; in decoding the genetic message; in discriminating against cognate, nonand near-cognate aminoacylated tRNA molecules; in controlling the fidelity of codon-anti-codon interactions and in mRNA/ tRNA translocation. The prokaryotic large ribosomal subunit (called 50S) has a molecular weight of 1.5x10 Dalton, and contains two RNA chains with a total of about 3000 nucleotides and around 35 proteins. The small ribosomal subunit (called 30S) has a molecular weight of 8.5x10 Dalton and contains one RNA chain of over 1500 nucleotides and around 20 proteins. Over two decade ago we initialized a long and demanding search for the determination of the threedimensional structure of the ribosome by X-ray crystallography. The key to high resolution data was to crystallize homogenous preparations under conditions similar to their in-situ environments or to induce a selected conformation after the crystals were formed. Relatively robust ribosomal particles were chosen, assuming that they would deteriorate less during preparation and therefore provide more homogenous starting materials for crystallization. The first crystals that yielded preliminary crystallographic information were grown from of the large subunit from Bacillus stearothermophilus (1). It took a few years until we identified an additional source, the large ribosomal subunit from Haloarcula marismortui (H50S) (2,3) that later yielded high resolution diffraction (4,5). A few additional years were needed for obtaining crystals of the small subunit from Thermus thermophilus (T30S) (6,7) and only recently we crystallized the large subunits from a mesophilic The Ribosome: A molecular machine with brains