Tryptophan mutants in Arabidopsis: the consequences of duplicated tryptophan synthase beta genes.

Abstract

The cruciferous plant Arabidopsis thaliana has two closely related, nonallelic tryptophan synthase , f 3 genes (7SBl and 7SB2), each containing four introns and a chloroplast leader sequence. Both genes are transcribed, although 7SBl produces >90% of tryptophan synthase ,f3 mRNA in leaf tissue. A tryptophan-requiring mutant, trp2-1, has been identified that has about 10% of the wild-type tryptophan synthase B activity. The trp2-1 mutation is complemented by the 7SBl transgene and is linked genetically to a polymorphism in the 7SBl gene, strongly suggesting that trp2-1 is a mutation in 7SB1. 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, 7SB2, 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 (7RP1 and 7RP3) 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.

Tryptophan-dependent auxin biosynthesis is required for HD-ZIP III-mediated xylem patterning

“…The auxin precursor Trp is synthesized from chorismate through several catalytic steps (Radwanski and Last, 1995). The final step, the conversion of indole-3-glycerol phosphate to Trp, is catalyzed by an enzyme complex consisting of TRYPTOPHAN SYNTHASE α-and β-subunits (Barczak et al, 1995;Last et al, 1991). In Arabidopsis, two homologous genes encode α-subunits and four genes encode β-subunits (TSB1, TSB2, TSB3 and TSBt2) (Yin et al, 2010;Zhang et al, 2008).…”

Tryptophan-independent auxin biosynthesis contributes to early embryogenesis in Arabidopsis

“…We therefore investigated whether INS participates in IAA biosynthesis through the Trp-independent pathway. Based on the findings that both trp3-1 and trp2-1 mutants, defective in TSA and TSB1, respectively, are Trp auxotrophic (15,16), we show here that they have lower contents of Trp, whereas the INS null mutant shows little Trp auxotrophic phenotype and has a normal Trp content ( Fig. 1 B and C and Fig.…”

“…The Trp biosynthetic mutant trp1, defective in phosphoribosylanthranilate transferase (PAT), and indole-3-glycerol phosphate synthase (IGS) antisense plants are deficient in steps earlier than indole-3-glycerol phosphate (IGP) formation and display decreased levels of both IAA and Trp (10,14). However, the trp3 and trp2 mutants, defective in tryptophan synthase α (TSA) and tryptophan synthase β (TSB) subunits, respectively, accumulate higher levels of IAA than the wild type despite containing lower Trp levels (10,11,15,16). These data strongly suggested the existence of the Trp-independent IAA biosynthetic pathway that might branch from IGP and/or indole (10).…”

Description of 31 YAC contigs spanning the majority of Arabidopsis thaliana chromosome 5

“…Carland Last et al, 1991;TSL, Roe et al, 1993;ve026, ve027, ve029, ve033, D. Bouchez, unpublished results. All clones identified in the colony hybridization experiments were subjected to a Southern blot analysis. The Southern blots that contained YAC clone DNA digested with EcoRI/BamHI were hybridized to the markers to confirm the results of the colony hybridization experiments.…”