The Reactivity of Escherichia coli Ribosomal Sulphydryl Groups with 5,5′‐Dithiobis(2‐nitrobenzoic Acid)

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Abstract
A study has been made of the reactivity of the sulphydryl groups of Escherichia coli ribosomes with 5,5′‐dithiobis(2‐nitrobenzoic acid). Our results show that all the sulphydryl groups of the ribosomal proteins will react with this sulphydryl‐specific reagent.Measurements of the rate of reaction provide evidence for conformational changes, independent of the association –dissociation reaction. The presence of 100 mM K+ or Na+ will markedly reduce the reactivity of the sulphydryl groups of 70‐S ribosomes or the ribosomal subunits. This conformational change has been interpreted in terms of a “tightening” of the ribosomal structure by the binding of monovalent cations.A similar conformational change was observed on decreasing the temperature of the reaction mixture. The difference in the reaction rate was more than can be explained by the temperature dependence of the kinetics of the simple reaction, investigated using a model sulphydryl compound, 2‐mercaptoethanol.