Acetolactate synthase (ALS) is the first common enzyme in the biosynthetic pathways to valune, isoleucine, and leucine. It is the target of two stuturally unrelated classes of herbicides, the sulfonylureas and the imidazolinones. Genomic clones encoding ALS have been isolated from the higher plants Arabidopsis thaliana and Nicotiana tabacum, using a yeast ALS gene as a heterologous hybridization probe. Clones were positively identified by the homology of their deduced amino acid sequences with those of yeast and bacterial ALS isozymes. The tobacco and Arabidopsis ALS genes have approximately 70% nucleotide homology, and encode mature proteins which are approximately 85% homologous. Little homology is seen between the amino acid sequences of the presumptive N-terminal chloroplast transit peptides. Both plant genes lack introns. The tobacco ALS gene was isolated from a line of tobacco which is resistant to the sulfonylurea herbicides due to an alteration in AILS. The tobacco gene which was isolated codes for an ALS that is sensitive to the herbicides, as assayed by transformation of the gene into sensitive tobacco cells.ALS' (EC 4.1.3.18) catalyzes the first common step in the biosynthesis of the essential amino acids isoleucine, leucine, and valine. ALS has attracted enhanced interest recently in that it has been demonstrated to be the target of two new and structurally unrelated classes of herbicides, the sulfonylureas (1,4,12,24) and the imidazolinones (29). Both classes of compounds are notable for their high herbicidal potencies, their low mammalian toxicities, and, for some analogs, their selective toxicity to weed species as compared to crop species (16, 29). These compounds inhibit plant growth by inactivating an enzyme in an essential amino acid biosynthetic pathway, rather than by the common alternate herbicidal mode of inactivating a component in a photosynthetic pathway. Because essential amino acids are not produced by mammals, they lack the target enzyme for these compounds, which presumably contributes to the low mammalian toxicities of these herbicides. The selective toxicity to weeds ofcertain ofthese compounds is due to their metabolism by crop plants; analogs have been synthesized which are metabolically inactivated by particular crop species, but not by most weed species (32,28).Tobacco mutants resistant to the sulfonylurea herbicides chlorsulfuron (Glean; Du Pont) and sulfometuron methyl (Oust; Du X Abbreviations: ALS, acetolactate synthase; bp, base pairs. Pont) have been isolated by selection for herbicide resistant haploid cell lines in tissue culture. Diploid plants were regenerated from these lines, and the resistance trait was shown to be expressed at the whole plant level. Through genetic crosses it was then established that the resistance was due to single semidominant nuclear mutations (2). The mode of action of the sulfonylurea herbicides was deduced in bacteria (12), when it was found that some bacteria are sensitive to sulfonylurea herbicides when grown on minimal medium, b...

The enzyme acetolactate synthase (ALS) is the target enzyme for the sulfonylurea and imidazolinone herbicides. We describe the isolation and characterization of the ALS genes from two herbicide‐resistant mutants, C3 and S4‐Hra, of Nicotiana tabacum. There are two distinct ALS genes in tobacco which are 0.7% divergent at the amino acid sequence level. The C3 mutant has a single Pro–Gln replacement at amino acid 196 in one ALS gene. This gene is termed the class I gene and is equivalent to the SuRA locus. The S4‐Hra mutant has two amino acid changes in the other ALS gene. This gene is termed the class II gene or the SuRB locus. The S4‐Hra mutant includes a Pro–Ala substitution at amino acid 196 and a Trp–Leu substitution at amino acid 573. Gene reintroduction experiments have confirmed that these amino acid substitutions are responsible for the herbicide resistance phenotypes. Transgenic plants carrying these genes are highly resistant to sulfonylurea herbicide applications.

No abstract