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 produ ce ppGpp during amino acid starvation. One consequence of this deficiency i s a tenfold increase in misincorporation at starved codons compared to the wild-type. Previous work had shown that the charging levels of tRNAs were t he same in Rel(+) and Rel(-) strains and reduced, at most, two- to fivefold in both strains during starvation. The present reinvestigation of the char ging 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 re duced charging levels tenfold to 40-fold. This reduction corresponds much b etter with the decreased rate of protein synthesis during starvation than t hat reported earlier. The determination of the charging levels of tRNA(2)(A rg) and tRNA(1)(Thr) during starvation were accurate enough to demonstrate that charging levels were at least fivefold lower in the Rel(+) strain comp ared to the Rel(+) strain. Together with other data from the literature, th ese new data suggest a simple model in which mis-incorporation increases as the substrate availability decreases and that ppGpp has no direct effect o n enhancing translational accuracy at the ribosome. (C) 2001 Academic Press .