5-FLUOROINDOLE RESISTANCE IDENTIFIES TRYPTOPHAN SYNTHASE BETA-SUBUNITMUTANTS IN ARABIDOPSIS-THALIANA

A study of the biochemical genetics of the Arabidopsis thaliana trypto phan synthase beta subunit was initiated by characterization of mutant s resistant to the inhibitor 5-fluoroindole. Thirteen recessive mutati ons were recovered that are allelic to trp2-1, a mutation in the more highly expressed of duplicate tryptophan synthase beta subunit genes ( TSB1). Ten of these mutations (trp2-2 through trp2-11) cause a tryptop han requirement (auxotrophs), whereas three (trp2-100 through trp2-102 ) remain tryptophan prototrophs. The mutations cause a variety of chan ges in tryptophan synthase beta expression. For example, two mutations (trp2-5 and trp2-8) cause dramatically reduced accumulation of TSB mR NA and immunologically detectable protein, whereas trp2-10 is associat ed with increased mRNA and protein. A correlation exists between the q uantity of mutant beta and wild-type alpha subunit levels in the trp2 mutant plants, suggesting that the synthesis of these proteins is coor dinated or that the quantity or structure of the beta subunit influenc es the stability of the alpha protein. The level of immunologically de tectable anthranilate synthase alpha subunit protein is increased in t he trp2 mutants, suggesting the possibility of regulation of anthranil ate synthase levels in response to tryptophan limitation.