Structural stability of ribosomes subjected to RNase treatment evidenced by dielectric spectroscopy and differential scanning microcalorimetry

Previous studies from our laboratory demonstrated the existence of at least two levels of structural complexity in E. coli 70S ribosomes. Ribosomal RN A seems to be principally involved in the overall stability of these struct ures. In this paper we present an investigation of ribosomes subjected to t reatment with RNase. The study is based on both differential scanning micro calorimetry and dielectric spectroscopy. In the thermograms obtained on tre ated ribosomes only the low temperature peak of the two typical denaturatio n events observed in native ribosomes, is promptly eliminated by the enzyme treatment. Dielectric spectroscopy measurements carried out on the same sa mples indicate an alteration of the dielectric behavior previously shown to consist of two subsequent relaxation processes. In fact, only the low freq uency relaxation is affected by the treatment. The second one, observed at higher frequency, remains unaltered. The same effect on the dielectric para meters is observed if the ribosome particles are heated and then cooled pri or to measurement. These results are consistent with the idea that two diff erent structures are present within the ribosome. One is very stable and wi thstands both temperature and RNase treatment while the second is promptly abolished by both treatments. Data presented here strongly suggest that the RNA domains exposed to the solvent play a fundamental role in the stabilit y of the 3-D structure of the ribosome particle. (C) 2000 Elsevier Science B.V. All rights reserved.