Properties of Tyrosyl Transfer Ribonucleic Acid Synthetase from Two tyrS Mutants of Escherichia coli K-12

Tyrosyl transfer ribonucleic acid synthetase was purified from two tyrS mutants of Escherichia coli. A comparison of the properties of the mutant and wild type enzymes demonstrated a striking difference in the apparent Km for tyrosine; it had been increased by these mutations several hundred-fold in both the ATP-pyrophosphate exchange and the tRNA acylation reactions. The affinity for the tyrosine analogue, tyramine, was not as drastically affected. There was little or no change in the apparent Km for tRNA in the acylation reaction, in its Ki in ATP-PPi exchange and in the apparent Km for tyrosyl-tRNA in deacylation. The apparent Km for ATP was also probably not altered by these mutations. The change in the Km for tyrosine and the heat lability of the mutationally altered enzymes establish that tyrS is the structural gene for tyrosyl-tRNA synthetase. The growth of tyrS mutants that were phenotypically Tyr- was inhibited by phenylalanine. Phenylalanine inhibited the uptake of tyrosine into the cell which prevented protein synthesis in these mutants but not in the wild type strain.