microgen - Bacillus thuringiensis genome sequencing project

The sequencing of Bacillus thuringiensis subsp. kurstaki genome is a collaborative project between Dr. Lee Bulla and Dr. Mehmet Candas of the Department of Molecular and Cell Biology and Center for Biotechnology and Bioinformatics at the University of Texas at Dallas and Dr. David Dyer and Dr. Allison Gillaspy of the Laboratory for Microbial Genomics at the University of Oklahoma Health Sciences Center. The project is supported by National Science Foundation Award Number 0412257.

Bacillus thuringiensis (Bt) is a ubiquitous Gram-positive bacterium that has been isolated from a variety of ecological niches including soil, stored grain, insects and plant surfaces. It is a facultative aerobe and spore-former and is taxonomically classified in the Bacillus cereus group, which includes the closely related species Bacillus anthracis and Bacillus cereus as well as Bacillus mycoides, Bacillus megaterium, among others.

The most distinctive property of Bt is its entomopathogenicity (insect pathogenicity) and production of insecticidal Cry toxin proteins that accumulate in the mother cell as crystalline inclusions during sporulation of the bacterium. The production of Cry toxins in the form of parasporal inclusions is a unique genetically regulated biological phenomenon that, probably, is related to offsetting dehydration during spore formation and affords an additional survival advantage by exerting pathogenic action that kills host insects. In turn, killing its host provides sufficient nutrients to allow germination of the dormant bacterial spore and its return to vegetative growth.

The entomopathogenicity of Bt spans a number of insect species including moths, mosquitoes, blackflies, beetles, hoppers, aphids, wasps and bees as well as nematodes. Bt entomopathogenicity involves various virulence factors common to many bacteria however, Cry toxins can exert insert lethality without the presence of viable bacteria. The lethal action of Cry toxins is mediated by a specific cadherin receptor, BT-R1, which have been identified in the midgut of various lepidopteran larvae (Vadlamudi et al., 1995; Dorsch et al, 2002). Toxin binding to the cadherin receptor leads to cytotoxicity and cell death (Zhang et al, 2005). In host insect larva, toxin action destroys the midgut epithelium and brings sepsis, and, ultimately, death of the organism. The action of Cry toxin in the entomopathogenicity of Bt appears to share a common mechanism with other bacterial pathogens that target host cell adhesion molecules to overcome or evade epithelial barriers in their hosts (Dorsch et al, 2002).

Bt has been exploited for more than 40 years to control agriculturally and medically important pest and disease-vector insects. In fact, Bt and Cry toxins constitute one of the most important environmentally compatible biological pesticides. Formulations of Bt pesticides are not toxic to vertebrates and non-target organisms and are widely used by organic farmers to ward off insect pests. Furthermore, insecticidal Cry toxin genes have been incorporated into several major crops where they provide a model for genetic engineering in agriculture.

The Bacillus thuringiensis subsp. kurstaki sequencing project will reveal the complete genome of an entomopathogenic bacterium, which exhibits an interesting lifecycle by co-existing with plants and invertebrate hosts and occasionally becoming pathogenic. In this regard, Bt is an excellent model to address questions related to microbe-host interactions and understanding factors influencing symbiotic and pathogenic interactions. Deciphering the genetic constituents of Bt can help answer various questions related to bacterial disease processes as exemplified in insect and nematode systems and reveal agricultural, industrial as well as potential medical use of the bacterium.

There has been heightened interest in Bt in recent years not only because of its insecticidal and nematocidal properties but also because of its close relationship to B. anthracis, a human and animal pathogen that has gained notoriety as an agent for bioterrorism. Availability of genome sequences of B. anthracis Ames (TIGR) and B. anthracis Sterne (DOE) and partially-completed genome sequences of B. thuringiensis subsp. israelensis (Integrated Genomics) has allowed genome-scale comparison of sequences related to the physiology, sporulation and virulence of these bacteria (Anderson et al., 2005). Completion of the Bt kurstaki genome sequence will help determining the basis of unique gene occurrences, host-adaptation and cell metabolism variations not only within different Bt strains and isolates but within B. anthracis and other Group I bacilli. Thus, availability of the Bt kurstaki genome and delineation of differences and similarities in genomic organizations of the B. cereus group will provide an important paradigm for understanding genome evolution and will lay the groundwork for future investigations involving comparative genomic and proteomic applications.

Anderson et al. Comparative genome analysis of Bacillus cereus group genomes with Bacillus subtilis. FEMS Microbiol Lett. 2005 Sep 15;250(2):175-84.

Dorsch et al. Cry1A toxins of Bacillus thuringiensis bind specifically to a region adjacent to the membrane-proximal extracellular domain of BT-R(1) in Manduca sexta: involvement of a cadherin in the entomopathogenicity of Bacillus thuringiensis. Insect Biochem Mol Biol. 2002 Sep;32(9):1025-36.

Vadlamudi et al. Cloning and expression of a receptor for an insecticidal toxin of Bacillus thuringiensis. J Biol Chem. 1995 Mar 10;270(10):5490-4.

Zhang et al. Cytotoxicity of Bacillus thuringiensis Cry1Ab toxin depends on specific binding of the toxin to the cadherin receptor BT-R(1) expressed in insect cells. Cell Death Differ. 2005 May 27.

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