SUPPRESSORS OF TRP1 FLUORESCENCE IDENTIFY A NEW ARABIDOPSIS GENE, TRP4, ENCODING THE ANTHRANILATE SYNTHASE BETA-SUBUNIT

Suppressors of the blue fluorescence phenotype of the Arabidopsis trp1 -100 mutant can be used to identify mutations in genes involved in pla nt tryptophan biosynthesis. Two recessive suppressor mutations define a new gene, TRP4. The trp4 mutant and the trp1-100 mutant are morpholo gically normal and grow without tryptophan, whereas the trp4; trp1-100 double mutant requires tryptophan for growth. The trp4; trp1-100 doub le mutant does not segregate at expected frequencies in genetic crosse s because of a female-specific defect in transmission of the double mu tant genotype, suggesting a role for the tryptophan pathway in female gametophyte development. Genetic and biochemical evidence shows that t rp4 mutants are defective in a gene encoding the beta subunit of anthr anilate synthase (AS). Arabidopsis AS beta subunit genes were isolated by complementation of an Escherichia coli anthranilate synthase mutat ion. The trp4 mutation co-segregates with one of the genes, ASB1, loca ted on chromosome 1. Sequence analysis of the ASB1 gene from trp4-1 an d trp4-2 plants revealed different single base pair substitutions rela tive to the wild type. Anthranilate synthase alpha and beta subunit ge nes are regulated coordinately in response to bacterial pathogen infil tration.