Effects of aminoacridines and related compounds on the conformation of rat-liver ribosomes

Abstract

(1) A temperature-dependent, conformational reaction was induced in ratliver ribosomes by aminoacridines and a number of other positively charged, planar, heterocyclic compounds with established or presumptive intercalating activity. The reaction involved a specific alteration in the pattern of structural shielding of the ribosomal proteins. Most strikingly, a previously resistant protein (“protein 10”) in the larger subunit became accessible to molecular probes (chymotrypsin, thermolysin, procion blue). The unmasking could be semi-quantitatively assayed by discelectrophoresis. The usefulness of this system for analyzing the intercalating activity of compounds without visible absorption is illustrated by experiments with harmine. (2) The concentration-temperature curve for the conformational reaction showed a marked inflection at 25° (experiments with atebrin). The corresponding concentration (0.8 m M) may be just sufficient for effective saturation of available, intercalative binding sites. The data suggest that the activation energy of the conformational reaction was progressively reduced by the intercalating agents up to this saturation limit. (3) The selective unmasking of protein 10 was enhanced by increased ionic strength. At KC1 concentrations above 0.7 M intercalating agents were no longer needed for unmasking at 35°. Bivalent cations above a certain level had a fairly moderate influence on the reaction. (4) The selective unmasking was basically reversible. In practice, reversibility was limited by the different strength of binding of the active compounds to the ribosomes. With phenazonium dyes reversibility was readily achieved after reduction. The inhibition of the amino acid incorporating activity was reversed under comparable conditions. (5) At high concentrations (3–5 m M) the conformational reaction lost much of its selectivity. At the same time the difference between planar and non-planar compounds became less striking. The experiments suggest that the selective unmas-king was related to an intercalative deformation of RNA-helices, while the unspecific reaction was due to a more general interference of positively charged aromatic compounds with the tertiary RNA structure.