| Abstract: | Mutants of Escherichia coli K-12 that require L-tryptophan (trp) are normally unable to utilize D-tryptophan to fulfill their requirement. However, secondary mutations (dadR) that confer this ability can be isolated. In such strains two distinct enzymes are found to be produced at high levels: D-amino acid oxidase (EC 1.4.3.3) and D-tryptophan oxidase. A convenient assay procedure for D-tryptophan oxidase is described. The two enzymes could be distinguished on the basis of their sensitivity to inhibition by L-phenylalanine and L-tyrosine. Strains that were trp dadR could not grow with D-tryptophan in the presence of L-phenylalanine, but further mutations, Fyo, could be isolated that allowed growth under these conditions. Some of them were characterized by further increases in the level of D-tryptophan oxidase activity and a sharp decrease in D-amino acid oxidase. These kinds of Fyo mutations lay in or near the dadR gene. The substrate specificity of the two enzymes toward a large number of compounds was examined. The transamination of aromatic keto acids was investigated. In the wild-type strain only a single enzyme, transaminase A (EC 2.6.1.5), was found, and it was irreversibly activated when subjected to elevated temperatures. The present state of our knowledge on D-amino acid utilization in E. coli is summarized. |
