Circular dichroism of Escherichia coli ribosomes: Effect of heating and metal ions

1. Ribosomes and rRNA from a low-nuclease strain of Escherichia coli were examined by means of circular dichroism (CD) in 0.001 M Tris-acetate buffer (pH 7.2). 2. The ribosome CD spectrum was altered when metal ions were chelated with EDTA. The following changes occurred: disappearance of the small negative RNA band at 297 mμ, a red shift of the positive RNA band at 264 mμ and a slight decrease of the protein band at 222 mμ. Addition of Mg²⁺ caused only partial reversal of these differences, and other metal ions including Ni²⁺ had even smaller effects. 3. Similar irreversible CD changes were noted after heating and recooling the ribosomes. Hysteresis was evident in the heating and cooling curves. EDTA caused a lowering of the melting temperature. 4. The CD spectrum of isolated 30-S and 50-S ribosomes showed similar temperature effects. The CD spectra of the subunits differ in the protein region. 5. The CD spectrum of rRNA is dependent only upon metal ion concentration and not upon prior EDTA or heat treatment.