Molecular Biotechnology: Principles and Applications - Glick B., Pasternak J. 2002

Molecular Biotechnology of Microbial Systems
Microbial Insecticides
Conclusion

Currently, microbial insecticides are becoming increasingly widespread because they do not have a harmful impact on the environment. Certain subspecies of the bacterium Bacillus thuringiensis produce a protoxin that, upon entering the insect gut, is converted into an active toxin under alkaline conditions and by the action of digestive proteases, leading to the insect's death. The lethal effect of the toxin is due to The formation of Ion Channels in the membranes of gut Cells, through which ATP leaks out of the cells. This leads to metabolic disruption, cessation of feeding, dehydration, etc. B. thuringiensis toxins are highly specific to a limited number of insect species and are non-toxic to all others; they degrade in the environment and therefore rarely exert a noticeable impact on it, which prevents the Selection of resistant insects. Due to all these properties, biological insecticides are promising candidates for agents to control the populations of agricultural pests and insect vectors of human diseases.

Genes encoding various B. thuringiensis toxins have been cloned and characterized. One such Gene was introduced into a non-sporulating Bacillus strain. The gene was expressed at all Stages of the microorganism's development, rather than only during sporulation when the parasporal crystal is formed.

To broaden the Specificity of the B. thuringiensis toxin, genes for different toxins were inserted into Plasmids and introduced into a host strain, either as part of broad-host-range plasmids or by integration into the host Cell's chromosomal DNA. Bacteria carrying two different toxin genes sometimes proved toxic to a third pest species, rather than just the two insect species targeted by the products of the original genes. Introduction/32.html">Genetic Engineering was used to create a recombinant protein consisting of two domains encoded by different B. thuringiensis genes, which exerted a dual toxic effect. In another experiment, the receptor-binding domain of one toxin was fused with the toxic domain of another. It is hoped that The Use of such hybrids will reduce the likelihood of insect resistance developing.

B. thuringiensis toxin genes have also been introduced into various aquatic microorganisms inhabiting the surface Water layer, which serve as food for mosquito larvae. This approach has proven highly effective for the direct delivery of B. thuringiensis toxins to the target insect. Genetically engineered rhizosphere-dwelling bacteria expressing B. thuringiensis toxin genes have also been developed, allowing for the control of insect pests that damage plant roots.

Baculoviruses are pathogenic to many insect species, but each strain is specific to only a small number of species. Typically, the death of an infected insect occurs only after a relatively long period, making baculoviruses less effective as a means of pest control. However, specific genes can be introduced into various baculovirus strains, allowing the virus to act as a delivery system for a gene that ensures insecticide synthesis throughout the viral life cycle. Preliminary Laboratory tests have yielded positive results. In addition, a gene encoding an insect-specific neurotoxin has been introduced into a baculovirus, and field trials have been conducted.

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Review Questions

1. What are the advantages of biological insecticides over chemical ones?

2. Why is the Bacillus thuringiensis toxin non-toxic to humans?

3. What approach would you use to identify the protoxin gene of Bacillus thuringiensis subsp. israelensis? What Structure/179.html">Practical Applications might this gene have?

4. How can you determine where a specific protoxin gene is localized: in a plasmid or in the chromosomal DNA of B. thuringiensis?

5. How can genetic engineering be used to improve the beneficial properties of a particular B. thuringiensis protoxin?

6. How can genetic engineering techniques be used to enhance the efficacy of baculoviruses as insecticidal agents?

7. How can the species specificity of toxins be expanded?

8. What information can you obtain by knowing which Class a particular Cry protein belongs to?

9. How would you modify the Cry protein to reduce the likelihood of The Emergence of toxin-resistant insects?

10. Why is the bacterium Asticcacaulis excentricus a highly attractive microorganism for expressing B. thuringiensis toxin genes?

11. How can the range of insects infected by this baculovirus be expanded?