CHARACTERIZATION OF TRYPTOPHAN SYNTHASE ALPHA-SUBUNIT MUTANTS OF ARABIDOPSIS-THALIANA

Three mutations in the Arabidopsis thaliana gene encoding the alpha su bunit of tryptophan synthase were isolated by selection for resistance to 5-methylanthranilate or 5-fluoroindole, toxic analogs of tryptopha n pathway intermediates. Plants homozygous for trp3-1 and trp3-2 are l ight-conditional tryptophan auxotrophs, while trp3-100 is a more leaky mutant. Genetic complementation crosses demonstrated that the three m utations are allelic to each other, and define a new complementation g roup. All three mutants have decreased steady-state levels of tryptoph an synthase alpha protein, and the trp3-100 polypeptide exhibits alter ed electrophoretic mobility. All three mutations were shown to be in t he TSA1 (tryptophan synthase alpha subunit) structural gene by several criteria. Firstly, the trp3-1 mutation is linked to TSA1 on the botto m of chromosome 3. Secondly, the trp3-1 mutation was complemented when transformed with the wild-type TSA1 gene. Finally, DNA sequence analy sis of the TSA1 gene revealed a single transition mutation in each trp 3 mutant.