Physiological evidence for differently regulated tryptophan-dependent pathways for indole-3-acetic acid synthesis in Azospirillum brasilense

Disruption of ipdC, a gene involved in indole-3-acetic acid (IAA) productio n by the indole pyruvate pathway in Azospirillum brasilense Sp7, resulted i n a mutant strain that was not impaired in IAA production with lactate or. pyruvate as the carbon source. A tryptophan auxotroph that is unable to con vert indole to tryptophan produced IAA if tryptophan was present but did no t synthesise IAA fi-om indole. Similar results were obtained for a mutant s train with additional mutations in the genes idpC and trpD. This suggests t he existence of an alternative Trp-dependent route for IAA synthesis. On gl uconate as a carbon source, IAA production by the ipdC mutant was inhibited , suggesting that the alternative route is regulated by catabolite repressi on. Using permeabilised cells we observed the enzymatic conversion of trypt amine and indole-3-acetonitrile to IAA, both in the wild-type and in the ip dC mutant. IAA production from tryptamine was strongly decreased when gluco nate was the carbon source.