Increased rates of tRNA charging through modification of the enzyme-aminoacyl-adenylate complex of phenylalanyl-tRNA synthetase.

The transfer of amino acid to tRNA by Escherichia coli phenylalanyl-tRNA synthetase (PheRS) was studied using replacements of Ala294 in the alpha subunit previously shown to have modified amino acid specificity. Steady-state analysis of tRNA charging showed little difference between wild-type and mutants, whereas pre-steady-state analysis revealed higher rates of tRNA charging by both the A294S PheRS-phenylalanyl adenylate and the A294G PheRS-p-Cl-phenylalanyl adenylate. The decrease in energy required for the formation of the transition state of amino acid transfer in these mutants could be related to a weaker binding of the amino acid in the aminoacyl adenylate complex. Thus a compromise appears to exist between amino acid activation and tRNA charging, because slowing down the first step increases the rate of the second step, possibly as a result of decreased stability of the PheRS.amino acid-AMP complex.