L-O-Methylthreonine-Resistant Mutant of Arabidopsis Defective in Isoleucine Feedback Regulation.

Threonine dehydratase/deaminase (TD), the first enzyme in the isoleucine biosynthetic pathway, is feedback inhibited by isoleucine. By screening M2 populations of ethyl methane sulfonate-treated Arabidopsis thaliana Columbia wild-type seeds, we isolated five independent mutants that were resistant to L-O-methylthreonine, an isoleucine structural analog. Growth in the mutants was 50- to 600-fold more resistant to L-O-methylthreonine than in the wild type. The resistance was due to a single, dominant nuclear gene that was denoted omr1 and was mapped to chromosome 3 in GM11b, the mutant line exhibiting the highest level of resistance. Biochemical characteristics (specific activities, Km, Vmax, and pH optimum) of TD in extracts from the wild type and GM11b were similar except for the inhibition constant of isoleucine, which was 50-fold higher in GM11b than in the wild type. Levels of free isoleucine were 20-fold higher in extracts from GM11b than in extracts from wild type. Therefore, isoleucine feedback insensitivity in GM11b is due to a mutant form of the TD enzyme encoded by omr1. The mutant allele omr1 of the line GM11b could provide a new selectable marker for plant genetic transformation.