Insecticidal 6-endotoxin

Thuringiensin produced by strains of B. thuringiensis insecticidal endotoxin. ... 

Bacillus thuringiensis Berliner (commonly known as Bt) is an insecticidal bacterium discovered in the early 20th century. The commercial Bt products are powders containing a mixture of dried spores and crystalline 8-endotoxin, but some contain only the toxin component. Both spores and toxin crystals are produced within the bacterial vegetative cell by Bt (Figure 4.12). Currently, there are six strains that possess specific activity against different insect species as follows ... 

Bacillus thuringiensis (Bt) Bacteria produce endotoxins that kill insects and larvae (insecticide)... 

Li JD, Carroll J, Ellar DJ (1991) Crystal structure of insecticidal delta-endotoxin from Bacillus thuringiensis at 2.5 A resolution. In Nature 353 815-821. 

Commercial preparations of B.t.. obtained through conventional fermentation techniques, have been used for more than two decades as biological insecticides (I). They exhibit desirable properties such as high insect toxicity and environmental safety. B.t. does not affect non-target insects and is completely nontoxic to vertebrates. Nevertheless its use has been limited due to high production costs, limited stability in field conditions and a too narrow insecticidal spectrum. The insecticidal activity of B.t. relies in the crystalline inclusions which are produced upon sporulation. The crystals contain insecticidal proteins, delta-endotoxins, which affect the midgut epithelium of sensitive insects. The exact mechanism of their toxic activity is still unknown. 

Bt2 is an example of a type 1 delta-endotoxin. It has a relatively broad spectrum of insecticidal activity, but is nearly nontoxic to S. littoralis. A type 2 toxin is found in pure form in the crystals of B.t. thuringiensis strain 4412 and is toxic to Pieris brassicae but not to M. sexta and S. littoralis. An example of a type 3 toxin is present together with a type 1 toxin in B.t. aizawai and is characterized by high toxicity to S. littoralis. 

Mode of Action. Despite the wide spectrum of insect host range activities displayed by Bt varieties, the mode of action of the insecticidal delta endotoxins is similar. When susceptible insects ingest Bt protein crystals, the first gross symptom observed is feeding inhibition due to paralysis of the digestive tract, including mouth parts. This usually occurs within one hour of ingestion ( ). 

Figure 1. Foliar persistence of microbial insecticides on cabbage. Arlington, Wisconsin 1988. Dipel and Javelin are commercial products based on Bt var. kurstaki. Persistence was measured by bioassay of field treated foliage, removed at various times post-application against 3rd instar DBM larvae. Rates of product applied are expressed in terms of grams of delta endotoxin (as measured by polyacrilamide gel electrophoresis) applied per acre.

Unique Mode of Action. As described above, the Bt delta endotoxin is a stomach poison which acts on the midgut epithelial cells of susceptible insects. This is in contrast to the typical mode of action for synthetic chemical insecticides, which act as nerve poisons. Because the Bt site of action is so different from that of synthetic insecticides, Bt has been successfully utilized for control of otherwise resistant insects such as mosquitoes, the diamondback moth and the Colorado potato beetle. 

Flexibility of Use. Products based on Bt have been successfully incorporated into pest management programs in agriculture, forestry and vector control. This is largely due to the fact that the Bt delta endotoxin and spores are relatively stable when tank mixed with conventionally used fungicides, insecticides and spray adjuvants. In addition, most commercial Bt formulations have been designed to optimize shelf life, as well as physical compatibility with other agricultural chemicals and application systems. 

As mentioned, the 8-endotoxin (Cry protein) that makes up most of the conspicuous crystals is the main insecticidal component of B. thuringiensis. At sporulation, the majority of Bt strains produce crystalline inclusions that contain this insecticidal 8-endotoxin. The crystals account for 20% or more of the total bacterial protein at sporulation and may contain one or several endotoxins, which differ in activity. Many Bt toxin genes and genes for some... 

The 6-endotoxin of B. . israelensis (H-14) is not as clearly understood as that of BTk. The parasporal crystal of BTi contains several proteins with molecular weights ranging from 26,000 to over 100,000d (9). Several authors have reported that the 26,000d protein has insecticidal activity (j , 11, 12). However, other reports suggest that the toxin is 66,000d, (13, 1 ). Because BTi has hemolytic and cytolytic activities in addition to insecticidal activities (15) the possibility exists that multiple toxin proteins exist in the crystal. However, evidence from genetics... 

The 6-endotoxin gene from BTk was first cloned by recombinant WJA techniques in 1961 ( 6). Since then, the toxin genes have been cloned from a variety of B.t. strains and from B. stAiaericus. A list of these cloned genes is in Table I. The isolation of these genes enables both the analysis and manipulation necessary to alter or inprove the insecticidal proteins. 

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