Ribosomal Crystallography and Heteropolytungstates

Heteropolytungstates play a dual role in ribosomal crystallography. Beside generating phases, one of them, (NH4)6(P2W18O62)14H2O, was found to be extremely useful in inducing post crystallization rearrangements. These led to a significant increase in the internal order of crystals of the small ribosomal subunits from Thermus thermophilus, manifested in a dramatic extension of the resolution of their diffraction patterns, from the initial 7–9 A to 3 A. The current 3.3 A electron density map of this particle, constructed using phases obtained from this W cluster together with other metal compounds, shows the recognizable overall morphology of the small ribosomal subunit. Over 96% of the nucleotides were traced and the fold of all proteins was determined fully or partially. Specific sites were determined independently by covalently bound heavy atom clusters, among them the surface of two proteins and a functional center, the gate for mRNA binding. All tungsten-cluster sites detected in this map are located in close proximity to the proteins of the particle, in positions that may have an influence on the stability and the rigidity of this rather flexible ribosomal subunit.