What are Cyt toxins in Bacillus thuringiensis?
Bacillus thuringiensis bacteria produce different insecticidal proteins known as Cry and Cyt toxins. Among them the Cyt toxins represent a special and interesting group of proteins.
What is Bacillus thuringiensis made of?
Bacillus thuringiensis(Bt) is a Gram positive, spore-forming bacterium that synthesizes parasporal crystalline inclusions containing Cry and Cyt proteins, some of which are toxic against a wide range of insect orders, nematodes and human-cancer cells.
What causes Bacillus thuringiensis-caused cell death?
A mechanism of cell death involving an adenylyl cyclase/PKA signaling pathway is induced by the Cry1Ab toxin of Bacillus thuringiensis. Proc. Natl. Acad. Sci.
Why is Bacillus thuringiensis used as an insecticide?
Abstract. Bacillus thuringiensis (Bt) is a unique bacterium in that it shares a common place with a number of chemical compounds which are used commercially to control insects important to agriculture and public health. Although other bacteria, including B. popilliae and B. sphaericus, are used as microbial insecticides,...
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Does Bacillus thuringiensis produce endotoxin?
Delta endotoxins (δ-endotoxins) are pore-forming toxins produced by Bacillus thuringiensis species of bacteria.
What is necessary for toxicity in Bacillus thuringiensis?
A Cry toxin exists as a toxic monomer capable of oligomerization. The parasporal crystals of Bt are oligomers composed of polypeptide protoxin subunits. The protoxin is the immediate atoxic precursor of a Cry toxin, i.e., upon activation of the protoxin, an insecticidal Cry toxin is generated.
What is the source of Bt toxin?
Bt is a bacterium that is found naturally in soils throughout the world. To reproduce, Bt makes spores that grow into new bacteria. Bt spores have proteins that are toxic to insect larvae when eaten. Because Bt comes from a natural source, it is called a biopesticide.
What are killed by Bacillus thuringiensis?
Bacillus thuringiensis kurstaki (Btk) is the best known and most widely used strain of Bt. It kills the larvae of many types of moths and butterflies. This means that you can control most caterpillars with Btk.
What is Bacillus thuringiensis for?
B.t. is a naturally occurring bacteria that is commonly found in soil and food. B.t. has been used safely for over 30 years to control insects in the United States, Canada, and other parts in the world. B.t. operates through a well-known protein mechanism that causes toxicity in caterpillars (i.e. insect larvae).
Is Bacillus thuringiensis harmful to humans?
Bt is a bacterium that is not toxic to humans or other mammals but is toxic to certain insects when ingested. Bt works as an insecticide by producing a crystal-shaped protein (Cry toxin) that specifically kills certain insects.
What is Bt cotton and Bt toxin?
1. All Bt cotton plants contain one or more foreign genes derived from the soil-dwelling bacterium, Bacillus thuringiensis; thus, they are transgenic plants. The insertion of the genes from B. thuringiensis causes cotton plant cells to produce crystal insecticidal proteins, often referred to as Cry- proteins.
How does Bt toxin cause the death of insects?
So, the correct answer is 'Creating pores in the midgut epithelial cells, leading to cell swelling and lysis.
What is true about Bt toxin?
Bt toxin kills certain insects such as lepidopterans (tobacco budworm, armyworm), coleopterans (beetles) and dipterans (flies, mosquitoes). But, actually, the Bt toxin protein exists as inactive protoxins.
What does Bt corn produce that kills insects?
In the case of Bt corn, the donor organism is a naturally occurring soil bacterium, Bacillus thuringiensis, and the gene of interest produces a protein that kills Lepidoptera larvae, in particular, European corn borer. This protein is called the Bt delta endotoxin.
What is Bt and how is it produced?
Bt Crops are transgenic crops that produce the same toxin as the bacterium Bacillus thuringiensis in the plant cell, thereby, protecting the crops from pests. The bacterium secretes specific proteins known as “cry proteins” that are toxic to insects. A few of the Bt crops include cotton, brinjal, corn, etc.
What is the name of that gene which is responsible for the coding of Bt toxin?
cryBt toxin is coded by a gene named cry.
How is Bacillus thuringiensis used as biocontrol agents?
Bacillus thuringiensis (B.t.) is a natural spore-forming bacterium that occurs in soils everywhere. As the spore matures, it produces a protein crystal that is toxic for lepidopteran insects like spruce budworm. When larvae ingest B.t., it destroys the lining in their gut, and they stop feeding and die within days.
How do Bacillus thuringiensis control the caterpillar larvae?
Bacillus thuringiensis produces an endotoxin which when ingested and released in the gut of the larvae of insect pest distrupts its gut lining and thus caterpillars are killed. By killing the caterpillars, it helps in pest control.
What is the mode of action of Bacillus Thuringensis once it is digest by the insect?
Their primary action is to lyse midgut epithelial cells by inserting into the target membrane and forming pores. Among this group of proteins, members of the 3-Domain Cry family are used worldwide for insect control, and their mode of action has been characterized in some detail.
Is Bacillus thuringiensis genetically modified?
Bacillus thuringiensis (Bt) crops are plants genetically engineered (modified) to contain the endospore (or crystal) toxins of the bacterium, Bt to be resistant to certain insect pests.
What is Bt pesticide?
The first commercial insecticide based on Bt, Sporine, was produced in France in 1938 and used primarily to control flour moths. In the United States, Bt was first manufactured commercially in 1958 and, by 1961, Bt-based bioinsecticides were being registered by the US Environmental Protection Agency. Since 1996, insect-resistant transgenic crops, known as Bt crops, have expanded around the globe and are proving to be quite efficient and helpful in reducing the use of chemical insecticides.13,14Latest estimates indicate that more than 50% of the cotton and 40% of the corn planted in the US are genetically engineered to produce Bt insecticidal toxins. The current global market for pesticides (herbicides, insecticides, fungicides, nematicides and fumigants) is valued at $25.3 billion. Biopesticides represent only 2.5% of this market but their share is expected to increase to about 4.2%, or more than $1 billion, in 2010.
How many strains are in the BC group?
We have constructed a phylogenetic tree, based on sequence alignment of the 16S rRNA of 20 strains in the BC group using the Bootstrapped neighbor-joining method (Fig. 2). Bootstrap volumes are reported on the branches. The tree displays division of the 20 strains into two major groups. Group A includes all Ba, Bc and Bpm strains and Group B includes Bw strain KBAB4 and Bm strain SDANFMO448. There are two clusters in Group A, each marked with a Roman numeral. Cluster I includes Ba strains Sterne CDC684 and Ames Ancestor A0248; Bc strains AH187 and AH 820 and ATCC 10987 and 14579; Bpm strain CIP 5259; and Bt serovar konkukianand Bt Al-Hakm. Bc AH820 and serovar konkukianin Cluster 1, both alleged human pathogens, are more closely related whereas Bpm strain CIP 5259 and Bc ATCC 14579, neither of which are pathogenic, are the most divergent in the cluster. Cluster II includes Bt serovars tenebrionis, morrisoni, kurstaki, sotto, israelensisand berlinerand Bc G9842. We infer from the dendogram that both Bt serovar konkukianand the Al-Hakm strain group rather well with the Ba and Bc strains, but not with the Bt serovars, as previously suggested.39,40
What is the origin of Bt?
The era of Bt had its beginning when, in 1901, a Japanese scientist named Shigetane Ishiwata isolated a bacterium from dead silkworm larvae while he was investigating the cause of the socalled “sotto disease” (sudden-collapse disease). The disease was responsible for the loss of large numbers of silkworms in Japan and the surrounding region. Ishiwata named the bacterium Bacillus sotto.6A few years thereafter, in 1911, a German scientist Ernst Berliner isolated a related strain from dead Mediterranean flour moth larvae he found in a flour mill in the German state of Thuringia. He appropriately named the organism Bacillus thuringiensis. Berliner studied the bacterium and found inclusion bodies or “Restkorper” alongside the endospore.7,8The year was 1915. Mattes9in 1927 again observed the same inclusion bodies in Bt but it was not until much later (25 years) that insecticidal activity was attributed to these highly refractile bodies now referred to as “parasporal crystals,” a phrase coined by Christopher Hannay in 1953.10Once the significance of the parasporal crystals was realized by Thomas Angus, he promptly demonstrated in the same year the insecticidal activity of the inclusion bodies.11And, together with Philip Fitz-James, Hannay in 1955 discovered that the toxic parasporal crystals are composed of protein.12
What are the neighbor-joining strains of the BC group?
Bootstrapped neighbor-joining tree of 20 strains belonging to different species of the BC group. The tree was generated based on nucleotide sequence alignment of 16S rRNA. Bootstrap volumes are reported on the branches. The 20 strains are divided into two main groups. Group A includes all Ba, Bc and Bpm strains and Group B includes Bw strain KBAB4 and Bm strain SDANFMO448. There are two clusters within Group A, each marked with Roman numerals I and II. Cluster I includes Ba strains Sterne CDC684 and Ames Ancestor A0248; Bc strains AH187 and AH 820 and ATCC 10987 and 14579; Bpm strain CIP 5259; and Bt serovar konkukianand Bt Al-Hakm. Bpm strain CIP 5259 and Bc ATCC 14579 are the most divergent in this cluster. Cluster II includes Bt serovars tenebrionis, morrisoni, kurstaki, sotto, israelensisand berlinerand Bc G9842. The horizontal bar represents 0.02% differences in nucleotide similarities.
What is a Bt cell?
As indicated above, vegetative cells of Bt are characterized as large stout rods that are straight or slightly curved with rounded ends. They usually occur in pairs or short chains. Bt is Grampositive, non-capsulated and motile with peritrichous flagella. Classification of Bt strains has been accomplished by H serotyping, the immunological reaction to the bacterial flagellar antigen.20The haggene encodes flagellin, which is responsible for eliciting the immunological reaction in H serotyping. Specific flagellin amino acid sequences have been correlated to specific Bt H serotypes and at least 69 H serotypes and 82 serological varieties (serovars) of Bt have been characterized.21H serotyping, however, is limited in its capability to distinguish strains from the same H serotype or from the same serovar.22Due to its economic importance, it has become necessary to develop alternative tools for classification and grouping of Bt strains and isolates. Accordingly, several screening programs have been established to isolate novel Bt strains with unique insecticidal properties. As a result, numerous Bt strains with activity against lepidopteran, dipteran and coleopteran insects have been isolated. Additionally, Bt strains active against insects belonging to the orders Hymenoptera, Homoptera, Orthoptera and Mallophaga as well as nematodes, mites and protozoa have been isolated.
Why are crygenes so versatile?
This particular characteristic may contribute to the versatility of Cry toxins as it relates to their insect host range. The most likely explanation for such genetic plasticity is the presence of numerous transposons and insertion elements that flank the crygenes. Indeed, these transposable elements may facilitate gene multiplication and evolution of new toxins.79Furthermore, the fact that crygenes are carried on transmissible plasmids increases the likelihood of horizontal gene transfer among different Bt strains, which leads to the creation of new strains with different sets of Cry toxins.80,81
Is Bt a bacterial species?
The placement of Bt as a separate species within the genus Bacillus has been controversial since the publication of The Genus Bacillusin 1973,23and Bergey's Manual of Determinative Bacteriologyin 1974.24The genus Bacillus is one of the most diverse genera in the class Bacilli and includes aerobic and facultatively anaerobic, rod-shaped, Gram-positive spore-forming bacteria with G + C contents ranging from 32–69%.25Based on phylogenetic heterogeneity, eight genera in the class Bacilli have been proposed: Bacillus, Alicyclobacillus, Paenibacillus, Brevibacillus, Aneurinibacillus, Virgibacillus, Salibacillus and Gracilibacillus.26–30Many species of these genera are of practical importance because they produce antibiotics and peptides with anti-microbial, anti-viral and anti-tumor activities. They also synthesize thermostable enzymes and molecules that can suppress soil-borne phytopathogenic organisms.31–35
Mechanism of Action of Bt-toxin
Bacillus thuringiensis is used to kill certain insects such as lepidopterans (tobacco budworm), coleopterans (beetles) and also some flies and mosquitoes (dipterans).
Application
Bacillus Thuringiensis is widely used as a biocontrol agent and in genetic engineering to produce pest-resistant plants.
Biopesticide
Bacillus Thuringiensis is an example of a biocontrol agent. It is used to kill the caterpillars of butterflies.
Pest-resistant Plant
The Bt-toxin gene is cloned and inserted into the plant genome by recombinant DNA technology. These genetically modified (GM) plants express the Bt-toxin genes and become pest-resistant. These GM plants do not require the application of chemical insecticides. Some of the examples of crops are Bt cotton, Bt corn, soybean, potato, etc.
Abstract
Bacillus thuringiensis bacteria produce different insecticidal proteins known as Cry and Cyt toxins. Among them the Cyt toxins represent a special and interesting group of proteins. Cyt toxins are able to affect insect midgut cells but also are able to increase the insecticidal damage of certain Cry toxins.
1. Introduction
Bacillus thuringiensis ( Bt) bacteria produce different insecticidal toxins that have been used as an environmentally friendly alternative method to control insect pests [4]. Bt produce crystal inclusions composed of Cry or Cyt toxins during sporulation phase of growth [4].
2. Structure and mechanism of action of Cyt toxins
The structures of three Cyt proteins have been solved, Cyt1Aa, Cyt2Aa and Cyt2Ba showing similar topology displaying a single α-β domain composed of two outer layers of α-helix hairpins wrapped around a β-sheet [10], [11], [27]. The α-helices have an amphiphilic character, with the hydrophobic residues packed against the β-sheet ( Fig. 1 ).
3. Synergism between Cry and Cyt toxins
The synergism between Cyt and Cry toxins has been observed with the insecticidal proteins present in the mosquitocidal strain Bacillus thuringiensis subsp. israelensis ( Bti) [8], [56]. Bti shows high toxicity to Ae. aegypti, the vector of Dengue and Yellow fever, and Culex sp.
4. Similarities of Cyt toxins with other proteins
Sequence searches in the databases using Cyt1Aa amino acid sequence as query, revealed several proteins with significant homology. In Fig. 2 we show a MUSCLE alignment [17] of hits obtained by PSI-BLAST search [1] of proteins in the NCBI database showing similarities with Cyt1Aa toxin, using a maximum E -value of 0.01.
5. Final remarks
Cyt toxins from Bt are pore-forming toxins that in vivo are specific to their target insect, despite the fact that in vitro they show toxicity against different cell types. Two different models of Cyt toxin mode of action have been proposed, a pore formation model and a detergent mode of action.
Why do plants have two types of delta endotoxins?
Plants with two kinds of delta endotoxins tend to make resistance happen slower, as the insects have to evolve to overcome both toxins at once. Planting non-Bt plants with the resistant plants will reduce the selection pressure for developing the toxin.
What is Delta Endotoxins?
structure summary. Delta endotoxins ( δ-endotoxins) are pore-forming toxins produced by Bacillus thuringiensis species of bacteria. They are useful for their insecticidal action and are the primary toxin produced by Bt corn.
What is the name of the gene that encodes the delta endotoxin?
B. thuringiensis encodes many proteins of the delta endotoxin family ( InterPro : IPR038979 ), with some strains encoding multiple types simultaneously. A gene mostly found on plasmids, delta-entotoxins sometimes show up in genomes of other species, albeit at a lower proportion than those found in B. thuringiensis. The gene names looks like Cry3Bb, which in this case indicates a Cry toxin of superfamily 3 family B subfamily b.
What is the name of the protein that is produced during spore formation?
During spore formation the bacteria produce crystals of such proteins (hence the name Cry toxins) that are also known as parasporal bodies, next to the endospores; as a result some members are known as a parasporin. The Cyt (cytolytic) toxin group is a group of delta-endotoxins different from the Cry group.
How do insects activate endotoxins?
Once activated, the endotoxin binds to the gut epithelium and causes cell lysis by the formation of cation-selective channels, which leads to death.
Is cry1ac a toxins?
Other common toxins include Cry2Ab and Cry1F in cotton and corn. In addition, Cry1Ac is effective as a vaccine adjuvant in humans. Some insects populations have started to develop resistance towards delta endotoxin, with five resistant species found as of 2013.
Overview
Use of Bt genes in genetic engineering of plants for pest control
The Belgian company Plant Genetic Systems (now part of Bayer CropScience) was the first company (in 1985) to develop genetically modified crops (tobacco) with insect tolerance by expressing cry genes from B. thuringiensis; the resulting crops contain delta endotoxin. The Bt tobacco was never commercialized; tobacco plants are used to test genetic modifications since they are easy to m…
Taxonomy and discovery
In 1902, B. thuringiensis was first discovered in silkworms by Japanese sericultural engineer Ishiwatari Shigetane (石渡 繁胤). He named it B. sotto, using the Japanese word sottō (卒倒, 'collapse'), here referring to bacillary paralysis. In 1911, German microbiologist Ernst Berliner rediscovered it when he isolated it as the cause of a disease called Schlaffsucht in flour moth caterpillars in Thuringia (hence the specific name thuringiensis, "Thuringian"). B. sotto would later …
Genetics
Some strains are known to carry the same genes that produce enterotoxins in B. cereus, and so it is possible that the entire B. cereus sensu lato group may have the potential to be enteropathogens.
The proteins that B. thuringiensis is most known for are encoded by cry genes. In most strains of B. thuringiensis, these genes are located on a plasmid (in other words cry is not a chromosomal g…
Proteome
Has similar proteome diversity to close relative B. cereus.
Mechanism of insecticidal action
Upon sporulation, B. thuringiensis forms crystals of two types of proteinaceous insecticidal delta endotoxins (δ-endotoxins) called crystal proteins or Cry proteins, which are encoded by cry genes, and Cyt proteins.
Cry toxins have specific activities against insect species of the orders Lepidoptera (moths and butterflies), Diptera (flies and mosquitoes), Coleoptera (beetles) and Hymenoptera (wasps, bees, ants
Use of spores and proteins in pest control
Spores and crystalline insecticidal proteins produced by B. thuringiensis have been used to control insect pests since the 1920s and are often applied as liquid sprays. They are now used as specific insecticides under trade names such as DiPel and Thuricide. Because of their specificity, these pesticides are regarded as environmentally friendly, with little or no effect on humans, wildlife, pollinators, and most other beneficial insects, and are used in organic farming; however, the man…
Beta-exotoxins
Some isolates of B. thuringiensis produce a class of insecticidal small molecules called beta-exotoxin, the common name for which is thuringiensin. A consensus document produced by the OECD says: "Beta-exotoxins are known to be toxic to humans and almost all other forms of life and its presence is prohibited in B. thuringiensis microbial products". Thuringiensins are nucleoside analogues. They inhibit RNA polymerase activity, a process common to all forms of lif…