RIBOSOME MUTANTS WITH ALTERED ACCURACY TRANSLATE WITH REDUCED PROCESSIVITY

We have determined whether or not there is a correlation between the accuracy of tRNA selection and the processivity of translation by mutant ribosomes. In our assay the estimation of processivity was made from a system in which transcription is dependent on T7 RNA polymerase in order to eliminate the influence of transcriptional polarity on our measurements. We found that the processivity of translation is reciprocally correlated to the degree with which mutant ribosomes restrict nonsense suppression events. This is observed when transcription is from both T7 RNA polymerase and Escherichia coli RNA polymerase. In other words, we have confirmed the expected correlation between increased accuracy of tRNA selection and decreased processivity of translation. No such monotonic correlation was obtained with ribosome ambiguity (ram) mutants, though all of these were less processive than wild-type ribosomes. In addition, streptomycin was found to simultaneously stimulate tRNA selection errors and processivity of translation for a mutant that is partially dependent on streptomycin. These data suggest that the growth rates of ribosome mutants are significantly decreased by the degree to which their processivity in translation is affected. Furthermore, we suggest that streptomycin dependence is a phenomenon reflecting the interplay of the restrictive mutation and antibiotic on the processivity of translation.