The Escherichia coli argU10 (Ts) Phenotype Is Caused by a Reduction in the Cellular Level of the argU tRNA for the Rare Codons AGA and AGG

Abstract

The Escherichia coli argU10 (Ts) mutation in the argU gene, encoding the minor tRNA ^Arg species for the rare codons AGA and AGG, causes pleiotropic defects, including growth inhibition at high temperatures, as well as the Pin phenotype at 30°C. In the present study, we first showed that the codon selectivity and the arginine-accepting activity of the argU tRNA are both essential for complementing the temperature-sensitive growth, indicating that this defect is caused at the level of translation. An in vitro analysis of the effects of the argU10 (Ts) mutation on tRNA functions revealed that the affinity with elongation factor Tu-GTP of the argU10 (Ts) mutant tRNA is impaired at 30 and 43°C, and this defect is more serious at the higher temperature. The arginine acceptance is also impaired significantly but to similar extents at the two temperatures. An in vivo analysis of aminoacylation levels showed that 30% of the argU10 (Ts) tRNA molecules in the mutant cells are actually deacylated at 30°C, while most of the argU tRNA molecules in the wild-type cells are aminoacylated. Furthermore, the cellular level of this mutant tRNA is one-tenth that of the wild-type argU tRNA. At 43°C, the cellular level of the argU10 (Ts) tRNA is further reduced to a trace amount, while neither the cellular abundance nor the aminoacylation level of the wild-type argU tRNA changes. We concluded that the phenotypic properties of the argU10 (Ts) mutant result from these reduced intracellular levels of the tRNA, which are probably caused by the defective interactions with elongation factor Tu and arginyl-tRNA synthetase.