Question

The inactive form of Bt toxin is converted to the active form in the insect gut

• A. by nucleases
• B. due to alkaline pH
• C. due to acidic pH
• D. by proteases

Hint:

• Bt toxin does not affect mammals (including humans) because the mammalian stomach is highly acidic, preventing the solubilization and activation of the protoxin crystals.

• Transgenic plants (such as Bt Cotton and Bt Brinjal) express these bacterial cry genes to provide targeted resistance against specific pests without chemical pesticides.

Answer 1

The Correct Option is B

Concept:

• Bacillus thuringiensis (Bt) is a soil bacterium that synthesizes insecticidal crystalline proteins (Cry proteins) during its sporulation phase.

• These proteins are initially synthesized as inactive

protoxins inside the bacterium, preventing damage to the bacterial host.

• Activation of the protoxin requires specific physical and chemical conditions inside the gastrointestinal tract of target insect larvae.

Step 1: Identify the biochemical changes occurring inside the insect gut
Upon ingestion by a susceptible insect, the insoluble crystalline protoxins enter the midgut. The midgut environment in lepidopteran, dipteran, and coleopteran insects has a highly alkaline pH.

Step 2: Trace the activation mechanism of the Bt toxin

• The

alkaline pH of the insect gut facilitates the solubilization of the toxic protein crystals.

• Proteolytic enzymes (proteases) present in the gut then cleave the solubilized protoxin, converting it into its active toxic form.

• Although proteases assist in cleavage, the primary physical-chemical requirement for activating the inactive crystal lattice is its solubilization, which is driven directly by the

alkaline pH.

• The active toxin then binds to epithelial cells of the midgut, creating pores that disrupt osmotic balance, leading to swelling, lysis, and eventually starvation and death of the insect.
Therefore, the transition from the inactive form to the active form relies on the alkaline pH, which matches option (B).