Genetically modified potato and cotton crops that express insecticidal proteins from Bacillus thuringiensis (Bt) have recently been commercialized. These crops display autonomous resistance to specific insect pests, and thus offer major agricultural and environmental benefits. We have implemented a microbial screening program to discover new types of insecticidal proteins for use in transgenic crops. New proteins with diverse modes of action offer opportunities to control insect pests that are not susceptible to Bt insecticidal proteins and to delay or prevent the potential occurrence of resistance of insects to crops genetically modified with Bt genes. Cholesterol oxidase emerged from our screen as a new insecticidal protein with potent activity against the cotton boll weevil. Cholesterol oxidase was acutely toxic to boll weevil larvae, with an LC50 of 2–6 parts per million when ingested in artificial diet feeding assays, and caused marked reductions in fecundity when ingested by adult boll weevils. Cholesterol oxidase also exerted significant, though less severe, toxicity against several lepidopteran pests. The insecticidal action of cholesterol oxidase appears to be due to oxidation of midgut epithelial membrane cholesterol followed by membrane disruption. A cholesterol oxidase gene was cloned and expressed in transgenic tobacco plants to yield plant tissue that exerted potent activity against boll weevil. Expression of this cholesterol oxidase gene in cotton plants may offer significant protection against the cotton boll weevil and may also aid in the mitigation of resistance of cotton lepidopteran pests to Bt proteins.
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