Tests of the ribosome editor hypothesis

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Abstract
Derivatives of isogenic stringent (relA +) and relaxed (relA) strains of Escherichia coli were compared in respect of rates of the dissociation of peptidyl-tRNA from ribosomes during protein synthesis. The derivatives both contained a mutant pth gene which rendered temperature-sensitive their peptidyl-tRNA hydrolase (E.C. 3.1.1.29) activities. After shifting from permissive 30° C to non-permissive 40° C, dissociated peptidyl-tRNA accumulated and was assayed chemically or by its cytotoxic effects. In unperturbed (except for the temperature shift) cultures the relA strain accumulated peptidyl-tRNA significantly more slowly than did its relA + isogenic cousin. Both strains responded approximately equally to non-lethal doses of erythromycin or to starvation for amino acids. Both these perturbations enhanced the dissociation and accumulation of peptidyl-tRNA. While growing at 30° C, both strains responded significantly to a nutritional downshift from growth in medium containing glucose plus amino acids to growth in medium containing only amino acids. Taken together the results suggested that different intracellular concentrations of ppGpp in unperturbed cells, attributable to the different relA alleles, could account for the differences in dissociation and accumulation of peptidyl-tRNA. Our observation of a lower rate of dissociation of peptidyl-tRNA in the relA strain, coupled with the reported lower intracellular ppGpp and lower accuracy of protein synthesis, is consistent with the idea that relA strains have less efficient ribosomal editing of erronous peptidyl-tRNA.