The 2009 Nobel Prize in chemistry was awarded jointly to Ada Yonath (Institut Weizmann at Rehovot, Israël), Venki Ramakrishnan (Medical Research Council Molecular Biology Laboratory at Cambridge, GB) and Tom Steitz (University of Yale, USA) for discovering the 3D structure of ribosomes using X-ray crystallography (Figure 1). This news was widely reported by numerous synchrotron radiation centers for at least three reasons: (i) the results obtained are of exceptional (fundamental and applied) interest, (ii) they involve the elucidation of the largest biological structure without symmetry so far by X-ray diffraction and (iii), this work would have been impossible without synchrotron radiation.

Nano-factories

Ribosomes are non-membranous organelles with a diameter of about 20 nm present in the cells of all living organisms. A single cell can contain thousands of them. They act like nano-machines, or rather nano-factories, producing proteins, the large molecules that ensure nearly all the functions of the body. The DNA in cells contains the information required to produce each protein. Let us suppose that an organism requires insulin. The first stage in the manufacture of this specific protein occurs outside the ribosome: the amino acid sequence for insulin is transcribed onto a messenger RNA template (mRNA). The ribosome is a molecular motor that moves along the mRNA consuming energy produced by GTP hydrolysis. During its displacement, it reads the template and uses this information to link together, one by one, like the beads of a necklace, the amino acids in the specific sequence for insulin. Each amino acid is temporarily fixed to a specific carrier, a transfer RNA (tRNA). The tRNA charged with its specific amino acid is delivered to the ribosome by a protein called elongation factor Tu (EF-Tu). Once complete, the protein chain leaves the ribosome, which can then start producing another similar or different protein molecule. Protein production thus results from a complex orchestrated series of events involving, apart from ribosomes, many partner molecules, proteins and nucleic acids. Ribosomes form by self-assembly of (mostly) nucleic acids and various proteins. They are composed of two subunits bound together. The smallest reads the mRNA and the largest synthesizes the corresponding protein. A whole ribosome contains up to 300,000 atoms, and the precise three-dimensional position of each is now known, giving the structure. With this structure, it is now possible to understand in great detail the various stages in protein synthesis. The design or improvement of new generation antibiotics is also possible. These are molecules that block a particular stage in protein synthesis by attaching to the ribosome. Future research will involve determining and analyzing the 3D structure of the new antibiotic assemblies with bacterial ribosomes, to prevent the production of pathogenic proteins.

Synchrotron radiation is irreplaceable

Ribosome crystallography is faced with many difficulties and Ada Yonath and her collaborators have played a major role in solving these difficulties one by one: notably in making crystals (i.e. three-dimensional ribosome assemblies) and avoiding the problem of very rapid degradation of crystals when exposed to X-rays by rapid freezing in liquid nitrogen. To obtain usable diffraction data, a very bright X-ray beam was required (as the crystal diffraction was weak) that was also parallel, otherwise the diffraction spots would be so numerous as to merge with each other. This is where synchrotron radiation is irreplaceable. The diffraction measurements were carried out ESRF (Grenoble), APS, ALS and NSLS (USA), and SLS (Switzerland). SOLEIL, which was not open at that time, is now also involved. An IGBMC group at Strasburg (Marat Yusupov) uses the PROXIMA I beamline on a regular basis. This line is perfectly adapted for such studies (Figure 2), as it has a very intense, non-diverging and extremely stable X-ray beam and large CCD detector. The second biocrystallography line, PROXIMA II (headed by W. Shepard), designed for micro-crystals, will be another asset in very demanding studies such as this.

A. Thompson, P. Legrand, B. Guimaraez, W. Shepard & R. Fourme