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PLANT PHYSIOLOGY , Vol 110, Issue 4 1159-1165, Copyright © 1996 by American Society of Plant Biologists
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GENE REGULATION AND MOLECULAR GENETICS |
Molecular Basis of [alpha]-Methyltryptophan Resistance in amt-1, a Mutant of Arabidopsis thaliana with Altered Tryptophan Metabolism
J. A. Kreps, T. Ponappa, W. Dong and C. D. Town
Department of Biology, Case Western Reserve University, Cleveland, Ohio 44106
A mutant of Arabidopsis thaliana, amt-1, was previously selected for
resistance to growth inhibition by the tryptophan analog
[alpha]-methyltryptophan. This mutant had elevated tryptophan levels and
exhibited higher anthranilate synthase (AS) activity that showed increased
resistance to feedback inhibition by tryptophan. In this study, extracts of
the mutant callus exhibited higher AS activity than wild-type callus when
assayed with either glutamine or ammonium sulfate as amino donor, thus
suggesting that elevated AS activity in the mutant was due to an alteration
in the [alpha] subunit of the enzyme. The mutant also showed
cross-resistance to 5-methylanthranilate and 6-methylanthranilate and
mapped to chromosome V at or close to ASA1 (a gene encoding the AS [alpha]
subunit). ASA1 mRNA and protein levels were similar in mutant and wild-type
leaf extracts. Levels of ASA1 mRNA and protein were also similar in callus
cultures of mutant and wild type, although the levels in callus were higher
than in leaf tissue. Sequencing of the ASA1 gene from amt-1 revealed a G to
A transition relative to the wild-type gene that would result in the
substitution of an asparagine residue in place of aspartic acid at position
341 in the predicted amino acid sequence of the ASA1 protein. The mutant
allele in strain amt-1 has been renamed trp5-1.
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