Abstract
The cruciferous plant Arabidopsis thaliana has two closely related, nonallelic tryptophan synthase beta genes (TSB1 and TSB2), each containing four introns and a chloroplast leader sequence. Both genes are transcribed, although TSB1 produces greater than 90% of tryptophan synthase beta mRNA in leaf tissue. A tryptophan-requiring mutant, trp2-1, has been identified that has about 10% of the wild-type tryptophan synthase beta activity. The trp2-1 mutation is complemented by the TSB1 transgene and is linked genetically to a polymorphism in the TSB1 gene, strongly suggesting that trp2-1 is a mutation in TSB1. The trp2-1 mutants are conditional: they require tryptophan for growth under standard illumination but not under very low light conditions. Presumably, under low light the poorly expressed gene, TSB2, is capable of supporting growth. Genetic redundancy may be common to many aromatic amino acid biosynthetic enzymes in plants because mutants defective in two other genes (TRP1 and TRP3) also exhibit a conditional tryptophan auxotrophy. The existence of two tryptophan pathways has important consequences for tissue-specific regulation of amino acid and secondary metabolite biosynthesis.