Aminotransferases of Agrobacterium tumefaciens. Transamination between tryptophan and phenylpyruvate.

Tryptophan is recognized to be the precursor of the plant hormone indolylacetic acid. The role of tryptophan in the biosynthesis of the auxin and also the probable intermediates involved have been discussed by Fawcett (1961). Indolylacetic acid has also been implicated in tumour formation in plants and this aspect has been reviewed by Braun (1962). The hyperauxinic condition of tomato crown-gall tissues has been demonstrated by Link & Eggers (1941) and Dye, Clarke & Wain (1962). It was therefore decided to study the metabolism of tryptophan by a virulent strain of the crown-gall organism, Agrobacterium tumefaciens. The conversion of tryptophan into the plant hormone may involve either the initial transamination or deamination of the amino acid followed by decarboxylation (Thimann, 1935; Wildman, Ferri & Bonner, 1947), or initial decarboxylation followed by deamination (Clarke & Mann, 1957; Crady & Wolf, 1959; Dannenburg & Liverman, 1957). The available evidence (Stowe, 1955, 1959; Dannenburg & Liverman, 1957; Liverman & Dannenburg, 1957) seems to support the view that the first step involves the formation of the oxo acid which in turn forms indolylacetic acid via indolylacetaldehyde. Kaper & Veldstra (1958) have reported the accumulation of indolylpyruvic acid in culture filtrates ofA. tumefaciens grown in media containing tryptophan, which further suggests that the conversion of tryptophan into indolylacetic acid involves the formation of indolylpyruvic acid as an intermediate, probably by transamination. Preliminary studies in our Laboratory showed that crude dialysed extracts of A. tumefaciens contained transaminase activity with various amino acids as amino donors and a number of oxo acids as acceptors. Of the oxo acids tested as acceptors with tryptophan as amino donor, phenylpyruvate was the most effective, the next in order being dimethylpyruvate, p-hydroxyphenylpyruvate and oi-oxoglutarate. Pyruvate, glyoxylate and oxaloacetate were completely inactive as amino acceptors with tryptophan. The reverse reaction with indolylpyruvic acid as amino acceptor and various amino acids as donors was also highest when phenylalanine was used as the amino donor. However, the tryptophan-oa-oxoglutarate-transaminase activity could be separated from the other activities by the method of purification reported below.