| Abstract: | The 5-S RNA (A) and the proteins L 18 (B) and L25 (C) from Escherichia coli ribosomes form a ternary complex of the type ABC with a stepwise stability constant, log K111 approximately equal to 6.5. This is indicated from X-ray scattering titrations recorded at 21 degrees C in ribosomal reconstitutional buffer. When the ternary ABC complex forms there is only a limited change in the scattering curve compared to that of 5-S RNA, indicating that 5-S RNA does not undergo a major conformational change during the complex formation. The increase in the radius of gyration from 3.61 nm (5-S RNA) to 3.95 nm (ABC complex) as well as the experimental scattering curve can be explained by models where it is assumed that the elongated L 18 and L25 models are quite far from the electron density centre and where the protein molecules interact mainly with the minor arms of the supposed Y-shaped 5-S RNA molecule. |
