Pseudomonas aeruginosa tryptophan synthase originates from an unusual two-gene biosynthetic operon regulated by substrate induction rather than end product repression. Expression of the adjacent structural genes for the two enzyme subunits depends on a closely linked regulatory gene producing a diffusible activator molecule augmenting expression in the presence of effector molecules. There is suggestive evidence that this regulatory gene product may also repress expression somewhat in the absence of the inducer. The primary aim of this project is to clarify the molecular events that are responsible for this regulation, which appears to be unrelated to the regulation of tryptophan synthase in other bacteria and to that of other enzymes of the tryptophan pathway in Pseudomonas. A second aim is to elucidate the evolutionary changes that have affected subunit interaction between the highly homologous Alpha2Beta2 tryptophan synthase molecules of the fluorescent pseudomonads and the enteric bacteria. Conditions will be sought under which mutations improving heterologous subunit cooperation can be selected. A generous sampling of such mutations should help to identify a subset of amino acids that: a) increase heterologous subunit affinity, or b) enhance cooperativity once the subunits have associated. When these changes are identified in the Pseudomonas Beta chains the molecules containing them will be screened by ELISA tests against a battery of monoclonal antibodies directed to the native E. coli Beta2 subunit.