In maize the orange pericarp mutant is a seedling lethal mutant caused by mutations in the duplicate unlinked recessive loci orp1 and orp2; duplicate genes for tryptophan synthase B. Mutant seedlings accumulate two tryptophan precursors, anthranilate and indole (Wright et al, 1992).

Mutants of Chlamydomonas reinhardtii selected for resistance to 5-fluoroindole have altered tryptophan synthase B (TSB) activity and fall into 3 complementation groups; one locus corresponds to the structural gene for TSB, and the other 2 loci may act to regulate TSB (Palombella and Dutcher, 1998). The gene enoding TSB exists as a single copy in the Chlamydomonas reinhardtii genome (Palombella and Dutcher, 1998).

The deduced amino acid sequence of the carboxy-terminal moiety of O-acetylserine sulfydrylase A [EC 4.2.99.8] shows significant similarity to the amino acid sequence of tryptophan synthase beta chain [EC 4.2.1.20] from several organisms (Levy and Danchin, 1988). This sequence similarity reflects the structural homologies of substrates shared by both enzymes. These proteins, although catalyzing different reactions in distinct metabolic pathways, may have evolved from a common ancestral gene (Levy and Danchin, 1988). This sequence similarity was confirmed and extended to other pyridoxal-phosphate-dependent enzymes by Bork and Rohde (1990). Tryptophan synthase beta subunit consensus motifs share significant similarities with O-acetylserine sulfhydrolases [EC 4.2.99.8], threonine synthases [EC 4.2.99.2], L- and D-serine dehydratases [EC 4.2.1.13/EC 4.2.1.14] and threonine dehydratases [EC 4.2.1.16] (Bork and Rohde, 1990).

Bork P, Rohde K 1990 Sequence similarities between tryptophan synthase beta subunit and other pyridoxal-phosphate-dependent enzymes. Biochem. Biophys. Res. Commun. 171: 1319-1325.

Levy S, Danchin A 1988 Phylogeny of metabolic pathways: O-acetylserine sulphydrylase A is homologous to the tryptophan synthase beta subunit. Mol. Microbiol. 2: 777-783.

Palombella AL, Dutcher SK 1998 Identification of the gene encoding the tryptophan synthase B-subunit from Chlamydomonas reinhardtii. Plant Physiol. 117: 455-464.

Wright AD, Moehlenkamp CA, Perrot GH, Neuffer MG, Cone KC 1992 The maize auxotrophic mutant orange pericarp is defective in duplicate genes for tryptophan synthase B. Plant Cell 4: 711-719.