PLANT PHYSIOLOGY , Vol 107, Issue 1 43-52, Copyright © 1995 by American Society of Plant Biologists
|
GENE REGULATION AND MOLECULAR GENETICS |
L-O-Methylthreonine-Resistant Mutant of Arabidopsis Defective in Isoleucine Feedback Regulation
G. Mourad and J. King
Department of Biology, Indiana-Purdue University, Fort Wayne, Indiana 46805-1499 (G.M.)
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.
