Structural change of E. coli separated and complexed 30S and 50S ribosomal subunits due to Mg2+ ions: SANS experiments

Small-angle neutron-scattering experiments have been performed on E. Coli 70S ribosomes and on 50S and 30S separated subunits in the presence and absence of magnesium ions. In the 70S complex in presence of magnesium, the scattering intensity at Q = 0 (I(0)) is roughly two times higher than without magnesium, in apparent agreement with the general view of an association—dissociation of the subunits induced by magnesium. But a similar increment is observed in both separated subunits too. The probability distribution functions of the intra-particle distance p(r), obtained by Fourier transforming, the experimental data, indicate that, even at low temperature (5°C) and concentration (0.1 wt%), the 70S and the separated subunits form aggregates. In all samples, the absence of Mg2+ ions shifts and shrinks p(r) in the single-particle region, below 200 Å, and affects the shape of the curve in the aggregate region. Our results suggest that the presence of Mg2+ ions does not strongly affect the degree of complexation of the subunits: the 70S complex retains its individuality even in the absence of magnesium, but undergoes structural rearrangements similar to those in 30S and 50S