Enzymological basis of reluctant auxotrophy for phenylalanine and tyrosine in Pseudomonas aeruginosa.

Dual biosynthetic pathways to L-phenylalanine and Ltyrosine exist in Pseudomonas aeruginosa (Patel, N., Pierson, D. L., and Jensen, R. A. (197’7) J. Biol. Chem. 252, 5839-5846). Tightly blocked phenylalanine or tyrosine auxotrophs are not obtained as the result of single mutations. Presumably the presence of a mutation that interrupts one pathway is masked by the presence of the alternative pathway. However, a leaky phenylalanine auxotroph (doubling time of 140 min in minimal glucose medium compared to 57 min with wild type) was isolated which completely lacked aminotransferase DE I. This is one of four aromatic aminotransferases of overlapping specificity, each capable of transamination with prephenate, phenylpyruvate, or 4-hydroxyph qylpyruvate. A suppressor mutation in the genetic background of the phenylalanine bradytroph was equated with the constitutive synthesis of aminotransferase HA I, normally a catabolic enzyme induced in wild type in the presence of either L-tyrosine or L-phenylalanine. The synthesis of aminotransferase HA II is repressed in the presence of phenylalanine and tyrosine, a result which suggests its probable role in aromatic amino acid biosynthesis. Aminotransferase HA III is unregulated by aromatic metabolites and is thought to function primarily in branched-chain amino acid metabolism. Although the suppressor mutation restores the wild type growth rate in minimal glucose medium, aromatic biosynthesis is highly stressed in this strain as revealed by its hypersensitivity to antimetabolite analogues of phenylalanine and tyrosine. In fact, the DE I aminotransferase deficiency is no longer suppressed when fructose, a carbon source previously shown in wild type to render aromatic biosynthesis limiting to growth, is used. It is likely that the phenylalanine bradytroph can be utilized as the genetic background for the isolation of otherwise silent mutations that inactivate the various biosynthetic enzymes of tyrosine and phenylalanine biosynthesis.