Specific release of ribosomal proteins by nucleic acid-intercalating agents

Increasing concentrations of ethidium bromide cause progressive inactivation of ribosomes, apparently by binding to double-stranded regions of the rRNA. At low drug concentrations (10(-4)M) the partial inhibition detected is due to specific release of proteins L7 and L12; activity can be restored by addition of an excess of these two proteins. At higher concentrations the inactivation is not reversed by supplementation with released proteins. The presence of ethanol affects the extent of ethidium binding and also the release of ribosomal proteins. In all tests the proteins most sensitive to the presence of the drug are L7 and L12, followed by L8/9, L11, L27, L28, L29 and L30. Despite the fact that L7 and L12 are the first two proteins released by ethidium they are never totally missing from drug-treated ribosomes, though the other proteins can be displaced completely. About 50% of proteins L7 and L12 remain on the ribosomes at the highest drug concentrations tested, possibly indicating heterogeneity in the binding sites for the several copies present in the ribosome.