Question

Currently the crystalline toxins obtained from Bacillus thuringiensis are classified on the basis of

• A. Serology
• B. Transgenic capabilities
• C. Aminoacid sequences
• D. Specificity

Hint:

The modern Crickmore nomenclature system discarded target insect host specificity in favor of objective, reproducible amino acid sequence homology.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
As genomic and molecular biology technologies have advanced, the nomenclature and classification systems of microbial toxins have shifted from phenotypic traits to sequence-based genetic relationships.

Step 2: Detailed Explanation:
Historically, the delta-endotoxins (Cry and Cyt proteins) of Bacillus thuringiensis were classified based on their insecticidal target range (host specificity) into four major groups (CryI to CryIV).
However, this phenotypic system became inadequate due to the discovery of new toxins with novel specificities and overlapping host ranges.
In 1998, a new, systematic nomenclature system (the Crickmore et al. system) was adopted.
Under this modern system, Bt crystalline toxins are classified strictly on the basis of their amino acid sequence homology.
Toxins sharing more than \(95\%\) amino acid sequence identity are grouped together under a common quaternary rank (e.g., Cry1Aa1), those with \(45\text{--}95\%\) identity share the same tertiary rank, and those with less than \(45\%\) identity are classified into different primary groups.
This sequence-based classification provides a clear, evolutionary, and highly objective nomenclature for biotechnology.

Step 3: Final Answer:
Thus, Bt crystalline toxins are currently classified on the basis of Aminoacid sequences.