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A deletion in an indole synthase gene is responsible for the DIMBOA-deficient phenotype of bxbx maize

Melanson, D., Chilton, M.D., Masters-Moore, D., Chilton, W.S.
Proceedings of the National Academy of Sciences of the United States of America 1997 v.94 no.24 pp. 13345-13350
Zea mays, messenger RNA, biosynthesis, dimboa, structural genes, seedlings, indoles, gene expression, chromosome mapping, mutants, hydro-lyases
The biosynthesis of DIMBOA, a pesticidal secondary metabolite of maize; branches off the tryptophan pathway. We have previously demonstrated that indole is the last intermediate common to both the tryptophan and hydroxamic acid pathways. The earliest discovered mutant in the DIMBOA pathway, bxbx (benzoxazineless), is deficient in the production of DIMBOA and related compounds. This paper presents evidence that a gene identified by Kramer and Koziel [Kramer, V. C. & Koziel, M. G. (1995) Plant MoL Biol. 27, 1183-1188] as maize tryptophan synthase alpha (TSA) is the site of the genetic lesion in the DlMBOA-deficient mutant maize line bxbx. We demonstrate that the TS4 gene has sustained a 924 bp deletion in bxbx compared with its counterpart in wild-type maize. We report that the TSA gene maps to the same location as the bxbx mutation, on the short arm of chromosome 4. We present evidence that the very early and very high level of expression of TSA corresponds to the timing and level of DIMBOA biosynthesis but is strikingly different from the expression of the maize tryptophan synthase beta (TSB) genes. We show that feeding indole to bxbx seedlings restores their ability to synthesize DIMBOA. We conclude that the maize enzyme initially named tryptophan synthase alpha in fact is a DIMBOA biosynthetic enzyme, and we propose that it be renamed indole synthase. This work confirms and enlarges upon the findings of Frey et al. [Frey, M. Chomet, P., Glawischniq, E., Stettner, C., Grun, S., Winklmair, A., Eisenreich, W., Bacher, A. Meeley, R. B., Briggs, S. P., Simcox, K & Gierl, A. (1997) Science 277, 696-699], which appeared while the present paper was in review.