Utilization of Indole Analogs by Carrot and Tobacco Cell Tryptophan Synthase in Vivo and in Vitro

Twenty-three indole analogs were used to inhibit the growth of carrot and tobacco suspension cultures. The addition of tryptophan or indole partially reversed the inhibition of both cell lines only for 4-fluoroindole, 5-fluoroindole, and 6-fluoroindole. Inhibition of tobacco cell growth by 5-aminoindole, 5-methoxyindole, 6-methoxyindole, or 7-methoxyindole was also partially reversed. Previously selected carrot and tobacco lines, which have high free tryptophan levels, grew in the presence of the analogs for which reversal was noted in all cases except 5-aminoindole and also in some other cases. Growth inhibition caused by all 10 tryptophan analogs studied was partially reversible by tryptophan or indole and the high tryptophan lines were also able to grow in the presence of concentrations inhibitory to the wild type lines.Tryptophan synthase activity from both species could utilize most of the 2 or 3 position unsubstituted indole analogs to produce the corresponding tryptophan analogs. Using carrot cell extracts, the identity of the correct product was confirmed by automated amino acid analysis for 4-fluoroindole, 5-fluoroindole, and 5-hydroxyindole. The analogs most rapidly utilized by carrot tryptophan synthase were 6-methoxyindole and the 4-, 5-, or 6-fluoroindoles. When 4-fluoroindole, 5-hydroxyindole, 5-methoxyindole, or 5-methylindole were incubated with carrot cells the corresponding tryptophan analog was formed in all cases as identified by amino acid analysis.These results indicate that cultured carrot and tobacco cells can convert certain nontoxic indole analogs into toxic tryptophan analogs. Using 5-fluoroindole and 6-fluoroindole, attempts were made to isolate tryptophan auxotrophs lacking tryptophan synthase from diploid carrot and tobacco lines. However, only 5-methyltryptophan-resistant lines, which presumably accumulate high levels of free tryptophan, were recovered.