INVOLVEMENT OF L-TRYPTOPHAN AMINOTRANSFERASE IN INDOLE-3-ACETIC-ACID BIOSYNTHESIS IN ENTEROBACTER-CLOACAE

L-Tryptophan aminotransferase (L-tryptophan:2-oxoglutarate aminotransf erase; EC 2.6.1.27) from Enterobacter cloacae was purified 62-fold and characterized to determine its role in indole-3-acetic acid biosynthe sis. The enzyme reversibly catalyzed the transamination of L-tryptopha n with 2-oxoglutarate as the amino acceptor to yield indole-3-pyruvic acid and L-glutamate, and the K-m values for L-tryptophan and indole-3 -pyruvic acid were 3.3 mM and 24 mu M, respectively. In the indole-3-a cetaldehyde synthesis experiments in vitro, 94% of L-tryptophan was ef ficiently converted to indole-3-acetaldehyde by the purified L-tryptop han aminotransferase plus indolepyruvate decarboxylase. Furthermore, t he amounts of L-tryptophan decreased with increases in the indolepyruv ate decarboxylase activity, while the amounts of indole-3-acetaldehyde increased with increases in this activity. In genetic experiments, th e amounts of L-tryptophan produced by Enterobacter and Pseudomonas str ains harboring the gene for indolepyruvate decarboxylase were lower th an those produced by these same strains without the gene, while the am ounts of indole-3-acetic acid produced by Enterobacter and Pseudomonas strains harboring the gene for indolepyruvate decarboxylase were high er than those produced by these same strains without the gene. These r esults clearly show that L-tryptophan aminotransferase is involved in the indole-3-acetic acid biosynthesis and that indolepyruvate decarbox ylase is the rate-limiting step in this pathway.