How do combinations of rpsL- and miaA- generate streptomycin dependence?

Petrullo et al. (1983) have studied the consequences of combining a mutation (rpsL-) that normally generates streptomycin resistant (Smr) ribosomes with a mutation (miaA-) that leads to loss of a tRNA hypermodification. They found surprisingly that such doubly mutant bacteria become streptomycin dependent (Smd). Here, we show in vitro that ribosomes purified from an Smr mutant behave very like Smd ribosomes when they are combined with tRNA from an miaA- mutant. Our analysis suggests that proofreading becomes excessively intense when the mutant components are combined, and that this reduces the efficiency of translation to the very low levels characteristic of Smd ribosomes. We show that Sm increases the efficiency of translation in vitro by suppressing the proofreading flows. We suggest that this will explain the growth stimulatory effect of Sm on the rpsL-, miaA- double mutants.