| Abstract: | Using an optical beating technique, the diffusion coefficients and relative scattered intensity of Escherichia coli 70S, 50S, and 30S ribosomes are measured as a function of temperature and Mg2+ concentration. For solutions at 10 mM Mg2+ and between 0°C and about 40°C, the values of D20,w obtained are 1.7, 1.9, and ≈2.1 × 10-7 cm2/s, respectively. Preparative procedures drastically affect these values and equivalent hydrodynamic ellipsoids of revolution models give large axial ratios indicating extensive hydration or a deviation from the assumed shape. Calculations also indicate that the subunits expand upon dissociation. Measurements of D20,w vs. temperature indicate that 70S particles undergo a conformational change prior to dissociation and can be heat dissociated at 30-32°C at low concentrations. Treatment of 70S ribosomes with EDTA causes a biphasic dissociation reaction. Addition of Mg2+ after dissociation with EDTA shows that longer waiting times yield fewer 70S particles and that even short waiting times may yield ribosomes differing from the native conformation. Addition of p-chloromercuribenzoic acid (PCMB) is shown to dissociate 70S particles, but to a lesser extent than ethylenediaminetetraacetic acid (EDTA). |
