Acetolactate synthase (ALS) catalyzes the first common step in the
biosynthesis of the essential amino acids isoleucine, leucine, and
Native ALS is normally inhibited by sulfonylurea herbicides.
However S4-HrA is a chimeric gene derived from two different
tobacco ALS genes that both encoded herbicide sensitive versions of
Two resistance mutations were introduced into one of the ALS genes
(Pro191Ala and Trp568Leu) via site directed mutagenesis. The DNA
fragment containing the resistance mutations was then moved into
the second ALS gene through a restriction enzyme fragment.