Small‐Angle X‐Ray Titration Study on the Complex Formation between 5‐S RNA and the L18 Protein of the Escherichia coli 50‐S Ribosome Particle

Abstract

The 5-S RNA (A) and the L18 protein (B) from E. coli ribosomes form 1 single AB complex in the concentration ranges supposed to prevail in vivo; at concentrations of L18 higher than 40 mM there is some indication for a minor species, most probably an AB2 species. This is indicated from the X-ray scattering titration data of the 5-S RNA/L18 system recorded at 21.degree. C in ribosomal reconstitution buffer. As a result of the 1:1 complex formation, there is a relatively small but defined increase in the radius of gyration from 3.61 to 3.85 nm. This result and the experimental scattering curve can be explained by models where it is assumed that the elongated L18 model is quite far from the electron density center and where protein L18 interacts with 1 or both of the minor arms of the supposed Y-shaped 5-S RNA molecule.