An Arabidopsis mutant (trp5) has been identified (by selection for resistance to 6-methylanthranilate) which has a feedback-resistant anthranilate synthase [EC 4.1.3.27] due to a single amino acid substitution in the A subunit structural gene (ASA1). The enzyme is 3-fold less sensitive to tryptophan inhibition than wild-type enzyme, and consequently the mutant plants accumulate 3-fold greater amounts of tryptophan (Li and Last, 1996). It is proposed that 6-methylanthranilate is toxic because it is metabolized to 4-methyltryptophan by the tryptophan biosynthetic enzymes. A naturally occurring cell line of tobacco that is resistant to 5-methyl-tryptophan also has an anthranilate synthase A subunit sequence that confers feedback insensitivity (Song et al, 1998).

The mutant allele amt-1 in Arabidopsis, conferring resistance to alpha-methyltryptophan, 5-methylanthranilate and 6-methylanthranilate, and exhibiting altered feedback inhibition of anthranilate synthase by tryptophan, has been renamed trp5-1 (Kreps et al, 1996).

Plant anthranilate synthase (AS), like its microbial counterparts, has two subunits (A and B). The A subunit binds the substrate chorismate and catalyzes its aromatization, and the B subunit supplies the glutamine amidotransferase activity. In Arabidopsis two separate genes encode the A subunit (ASA1 and ASA2), and two separate genes encode the B subunit (ASB1 and ASB2). The trp4 mutants of Arabidopsis are defective in the B subunit of AS and these mutations map to the same locus as ASB1; the predominantly expressed ASB gene in Arabidopsis (Niyogi and Fink, 1992). The ASB1 genes from trp4 mutants exhibit single base pair substitutions relative to wild-type. The ASB1 predicted protein product has an amino-terminal extension with characteristics of chloroplast transit peptides.

Bohlmann J, DeLuca V, Eilert U, Martin W 1995 Purification and cDNA cloning of anthranilate synthase from Ruta graveolens: modes of expression and properties of native and recombinant enzymes. Plant J. 7: 491-501.

Bohlmann J, Lins T, Martin W, Eilert U 1996 Anthranilate synthase from Ruta graveolens: duplicated ASa genes encode tryptophan-sensitive and tryptophan-insensitive isoenzymes specific to amino acid and alkaloid biosynthesis. Plant Physiol. 111: 507-514.

Kreps JA, Ponappa T, Dong W, Town CD 1996 Molecular basis of alpha-methyltryptophan resistance in amt-1, a mutant of Arabidopsis thaliana with altered tryptophan metabolism. Plant Physiol. 110: 1159-1165.

Li J, Last RL 1996 The Arabidopsis thaliana trp5 mutant has a feedback-resistant anthranilate synthase and elevated soluble tryptophan. Plant Physiol. 110: 51-59.

Niyogi KK, Fink GR 1992 Two anthranilate synthase genes in Arabidopsis: defense-related regulation of the tryptophan pathway. Plant Cell 4: 721-733.

Song HS, Brotherton JE, Gonzales RA, Widholm JA 1998 Tissue culture-specific expression of a naturally occurring feedback-insensitive anthranilate synthase. Plant Physiol. 117: 533-543.