Growth and translation elongation rate are sensitive to the concentration of EF-Tu

We have used quantitative immunoblotting to estimate the amount of EF-Tu in a variety of S. typhimurium strains with wild-type, mutant, insertionally inactivated or plasmid-borne tuf genes. In the same strains we have measured translation elongation rate, exponential growth rate and the level of nonsense codon readthrough. In the wild-type strain, at moderate to fast growth rates, our data show that EF-Tu makes up 8-9% of total cell protein. Strains with either of the tuf genes insertionally inactivated have 65% of the wild-type EF-Tu level, irrespective of which tuf gene remains active, or whether that gene is wild-type or a kirromycin-resistant mutant. Strains with only one active tuf gene have reduced growth and translation elongation rates. From the magnitude of the reduction in elongation rate relative to the level of EF-Tu we calculate that in glucose minimal medium the in vivo saturation level of wild-type ribosomes by ternary complexes is only 63%. Strains with a ribosome mutation causing a poor interaction with ternary complex are non-viable on minimal medium when the level of EF-Tu is reduced.