How are composite transposons formed in bacterial genomes?
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ABy a small deletion in the transposase gene of a single IS element
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BBy a deletion in an IS transposase gene followed by plasmid integration
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CBy an IS element integrating into another IS element via a plasmid intermediate
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DBy two IS elements inserting near each other in a chromosome and bracketing intervening genes
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EBy homologous recombination between rRNA operons
Answer
Correct Answer: By two IS elements inserting near each other in a chromosome and bracketing intervening genes
Explanation
Introduction / Context:Composite transposons are larger mobile elements that capture and mobilize antibiotic resistance or metabolic genes. They form a potent mechanism for rapid genetic innovation and spread of resistance clusters.
Given Data / Assumptions:
- Insertion sequences (IS) are short transposable elements encoding transposase, flanked by inverted repeats.
- Two IS elements can land near one another in direct or inverted orientation.
- The chromosomal DNA between them can be mobilized as a single unit.
Concept / Approach:When two IS elements flank a region, the pair functions as the ends of a larger transposon. The captured central genes (for example, antibiotic resistance) move with the composite element during subsequent transposition events, facilitating horizontal spread.
Step-by-Step Solution:Define composite transposon architecture: IS–[cargo genes]–IS.Establish formation mechanism: independent IS insertions bracketing a locus.Conclude option describing two IS elements flanking intervening DNA is correct.
Verification / Alternative check:Classic examples include Tn9 and Tn10 derivatives where resistance genes lie between IS elements acting as transposition termini.
Why Other Options Are Wrong:
- Single IS deletions do not create composites.
- Plasmid integration is not required for the composite structure.
- “IS into IS” is not the defining mechanism for bracketing chromosomal DNA.
Common Pitfalls:Confusing simple IS transposition with composite element formation that captures adjacent genes.
Final Answer:By two IS elements inserting near each other in a chromosome and bracketing intervening genes