Bt Toxin: Protoxin to Active Toxin — NEET Biology
✅ Asked in NEET 2025Some strains of Bacillus thuringiensis produce proteins that kill certain insects such as lepidopterans (tobacco budworm, armyworm), coleopterans (beetles) and dipterans (flies, mosquitoes). B. thuringiensis forms protein crystals during a particular phase of their growth. These crystals contain a toxic insecticidal protein. Why does this toxin not kill the Bacillus? Actually, the Bt toxin protein exist as inactive protoxins but once an insect ingest the inactive toxin, it is converted into an active form of toxin due to the alkaline pH of the gut which solubilise the crystals. The activated toxin binds to the surface of midgut epithelial cells and create pores that cause cell swelling and lysis and eventually cause death of the insect.
NTA focuses on why Bt toxin doesn't kill the bacterium itself but kills insects—the key is that it exists as inactive protoxins. Students often forget this crucial detail and think the toxin is already active, missing why it's harmless to B. thuringiensis. The trick: protoxins are activated only in the alkaline insect gut, where pH converts them to active toxins that bind midgut cells and cause lysis. Remember: inactive in the bacterium, active in the insect. This concept has appeared 4 times because it tests understanding of both bacterial genetics and practical biotech applications.
This paragraph was tested 4 times in NEET.
Bt toxin protein that can kill many insects doesn't kill the Bacillus because:
Given below are two statements: Statement I: Bt toxins are insect group specific and coded by a gene cry IAc. Statement II: Bt toxin exists as inactive protoxin in B. thuringiensis. However, after ingestion by the insect, the inactive protoxin gets converted into active form due to acidic pH of the insect gut.
Steps involved in Bt toxin action: A. Inactive toxin → Active form due to alkaline gut B. B. thuringiensis produces crystal proteins C. Alkaline pH solubilizes crystals D. Activated toxin binds to midgut cells E. Toxin exists as inactive protoxins. Choose the correct sequence:
In Bt cotton, the Bt toxin present in plant tissue as protoxin is converted into active toxin due to: (NEET 2015 / AIPMT 2015)
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