Effect of Ribosomal Wash Factors on Inhibition by Chloramphenicol

Abstract

In vitro protein-synthesizing systems from Escherichia coli can be categorized as either chloramphenicol-sensitive or chloramphenicol-insensitive. The chloramphenicol-sensitive systems used in this study required the presence of factors removed from ribosomes with 1.0 M NH4Cl when chromatographically purified ribosomes were used for amino acid incorporation. These ribosomal wash factors inhibited but did not eliminate amino acid incorporation in chloramphenicol-insensitive systems. For both systems, addition of increasing amounts of the ribosomal wash factors increased the sensitivity to chloramphenicol inhibition.Previous studies on the mechanism of action of chloramphenicol have often utilized high concentrations (10-4 to 10-3 M) of the drug, and have shown inhibition of chain elongation. Such high concentrations are necessary to inhibit polyuridylic acid (polyU)-or polyadenylic acid (polyA)-directed in vitro amino acid incorporation (7, 9). However, much lower concentrations (10-5 M) of chloramphenicol are sufficient to give equivalent inhibition of bacterial cell growth or in vitro synthesis of peptides stimuLlated by polyC (6), some polynucleotide copolymers (8), or natural messenger (12).Elongation, in contrast to initiation or release, is thought to involve the same steps regardless of the messenger used. Such differential sensitivity is, therefore, difficult to account for if low con-.centrations of chloramphenicol affect sensitive systems in the same way as high levels affect insensitive systems.Protein-synthesizing systems using purified ribosomes normally utilized for polyA-and polyU-stimulated incorporation systems are deficient in ribosomal wash factors required for natural messenger translation. This report deals with the stimulating effect of these factors on polyC-directed incorporation of ['4C]proline, and with the effect of these factors on enhancement of inhibition by chloramphenicol of polyA-and polyU-stimulated incorporation.