Target-site mutation

The challenge here is to find suitable inhibitors at the enzyme level that retain this activity at the whole-plant level. In addition, this approach is only feasible when resistance is conferred by target site mutations if resistance were conferred also by metabolic detoxification an additional set of screens against detoxifying enzymes would have to be included. 

Cases analyzed to date show that herbicide resistance is very frequently based on a target-site mutation. Within the past 35 years weed species have developed target-site resistance to most known herbicide chemistries. Those of major importance are discussed below. 

Gene in pla.smid with mutation target. site X... 

An example of miR-dependent deadenylation of mRNA measured by this method is shown in Fig. 6.3C. In this case, HeLa cells (which express let-7) were transfected with a pDNA R-luc construct encoding three let-7 binding sites in the 3 UTR (3 X bulge), or with control constructs encoding either no sites (plasmid) or three mutated let-7 sites (3 x bulge mut) (constructs described in Pillai et al, 2005). Cells were harvested 24 h after transfection, RNA was purified for the PAT assay, and luciferase activity was measured from cell lysates. As reported previously, the presence of functional let-7 target sites results in specific repression of luciferase expression with very minor effects on mRNA stability (Pillai et al., 2005). The experiment in Fig. 6.3C demonstrates that the let-7 targeted reporter mRNA is selectively deadenylated. 

Antibiotic resistance in bacteria is not a fixed property, and the degree of resistance detectable in the laboratory probably bears litde relationship to the resistance of the organism when growing in the intestinal tract of animals. The types of resistance that bacteria may develop to the action of antibiotics involve two distinct mechanisms mutation and inheritance. The former mechanism affects DNA sequence and results in the synthesis of a protein or macromolecule by the bacterial chromosome that differs from the original chemical entity, with the ability to interfere with the antibiotic activity. Because an antibiotic hinders a bacterium only after it has entered or crossed the cell wall and has bound to a target site, resistance can develop directly if the mutation has so altered the characteristics of the protein or macromolecule that the cell wall, receptor site, or transport mechanism is no longer friendly to the antibiotic. 

Gene in plasmid with target site ( ) for mutation... 

Observed differences between strains of rats and mice, as described below, may be the result of gene polymorphisms. In cases involving insecticide selection pressure, resistant populations may arise as a result of direct mutations of insecticide-metabolizing enzymes and/or insecticide target sites that are passed on to succeeding generations. 

In summary, triazine resistance in weeds is most commonly due to a target site alteration that confers a very high level of resistance to. y-triazinc herbicides. Although a Ser264 to Gly mutation in the D1 protein is most common, additional alterations have been identified that confer resistance to triazines and other classes of PS II inhibitors. Enhanced herbicide metabolism plays a major role in conferring resistance in only a few weed biotypes. In these biotypes, the pattern of resistance may be broader, with some cross-resistance to av-trazinones, uracils, heterocyclic ureas and phenyl ureas. The level and pattern of resistance to various herbicides in these biotypes depend, presumably, on the activity and specificity of the enzyme(s) responsible for the enhanced herbicide metabolism. 

The ALS inhibitors are at the highest risk for the selection of resistance in weeds because they have a single target site, are effective against a wide spectrum of weeds, are now used extensively on many crops, and are relatively persistent - often providing season-long control of germinating weed seeds (Brown et al., 1995). Also, the various sites of mutations for resistance are not near the active site of the enzyme. As a result, there is no fitness loss due to a lower affinity for the normal substrates (Christoffoleti et al., 1997). 

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