Charging levels of four tRNA species in Escherichia coli Rel(+) and Rel(-) strains during amino acid starvation: a simple model for the effect of ppGpp on translational accuracy.

Escherichia coli strains mutated in the relA gene lack the ability to produce ppGpp during amino acid starvation. One consequence of this deficiency is a tenfold increase in misincorporation at starved codons compared to the wild-type. Previous work had shown that the charging levels of tRNAs were the same in Rel(+) and Rel(-) strains and reduced, at most, two- to fivefold in both strains during starvation. The present reinvestigation of the charging levels of tRNA(2)(Arg), tRNA(1)(Thr), tRNA(1)(Leu) and tRNA(His) during starvation of isogenic Rel(+) and Rel(-) strains showed that starvation reduced charging levels tenfold to 40-fold. This reduction corresponds much better with the decreased rate of protein synthesis during starvation than that reported earlier. The determination of the charging levels of tRNA(2)(Arg) and tRNA(1)(Thr) during starvation were accurate enough to demonstrate that charging levels were at least fivefold lower in the Rel(-) strain compared to the Rel(+) strain. Together with other data from the literature, these new data suggest a simple model in which mis-incorporation increases as the substrate availability decreases and that ppGpp has no direct effect on enhancing translational accuracy at the ribosome.