Horizontal Gene Transfer: Transformation and Transduction

Plasmid Localization in F Plus and HFR Cells

• In an F plus cell, the plasmid resides independently in the cytoplasm.
• In an HFR cell, the plasmid integrates directly into the bacterial chromosome.
• The cellular location of the plasmid dictates how plasmid-borne genes transfer to other cells.

Mechanisms and Requirements of Transformation

• Mechanism: Competent recipient bacteria utilize specialized surface machinery to import naked linear DNA fragments from the external environment (typically released following donor cell death).
• Intracellular Fate of Linear DNA:
  • Approximately $90\%$ of internalized linear DNA is degraded by bacterial enzymes.
  • Approximately $10\%$ undergoes homologous recombination, mediated by recombinatory enzymes swapping homologous sequences into the bacterial chromosome.
• Prerequisites for Transferring Resistance:
  • The incoming linear DNA must harbor an antibiotic resistance gene.
  • Recombination must successfully integrate that gene into the recipient chromosome.
• Functional Outcome: Transformation replaces existing chromosomal sequences with incoming donor alleles, which can alter antibiotic targets or introduce metabolic enzymes (e.g., folic acid synthesis), potentially conferring sensitivity rather than resistance if a new drug target is introduced.

Mechanisms of Transduction

• Packaging Error: During bacteriophage replication, host bacterial chromosomal DNA is mistakenly packaged into viral capsids instead of the viral genome.
• Delivery: The bacteriophage binds to a specific recipient bacterium and injects the linear bacterial chromosomal DNA.
• Intracellular Fate:
  • Most linear DNA ($90\%$ to $95\%$) is destroyed by host antiviral defense enzymes targeting single-stranded and linear foreign DNA.
  • Homologous recombination must occur to integrate the injected DNA into the host chromosome; otherwise, no permanent genetic change occurs.

Comparison Between Transformation and Transduction

• Mode of Entry: Transformation relies on bacterial cell machinery pulling in naked DNA; transduction relies on a bacteriophage vector injecting host DNA.
• Intracellular Processing: Once linear DNA reaches the recipient cytoplasm, processing is identical: both pathways depend on a kinetic competition between DNA degradation enzymes and homologous recombination enzymes.

Questions & Discussion

• Question: What is the primary difference between an F plus cell and an HFR cell?
  • Response: The plasmid is located freely in the cytoplasm in an F plus cell, whereas in an HFR cell, the plasmid is integrated into the host bacterial chromosome.
• Question: Can a recipient cell only acquire DNA from a donor cell that has died?
  • Response: In natural environments, donor death and lysis are required to release naked chromosomal DNA; in a laboratory setting, competent bacteria can directly take up synthesized or purified naked DNA from solution.
• Question: If a recipient bacterium lacking a folic acid synthesis enzyme acquires that gene via transformation, does it gain antibiotic resistance against folic acid synthesis inhibitors?
  • Response: No; acquiring the gene produces the enzyme target, converting an intrinsically resistant cell (lacking the target) into an antibiotic-sensitive cell that the drug can now bind and inhibit.
• Question: What happens to an ineffective antibiotic inside the human body when treating a resistant bacterial infection?
  • Response: The drug undergoes standard pharmacokinetics according to its half-life, being processed by the liver and kidneys and excreted via urine or feces, while potentially affecting other susceptible non-pathogenic bacteria.
• Question: Does the bacteriophage physically enter the donor cell during infection and replication?
  • Response: No; the bacteriophage remains outside the cell and injects its nucleic acid. New virions are assembled inside using host machinery, packaging host DNA by accident before lysing the cell.
• Question: Are bacteriophages capable of infecting any bacterial species during transduction?
  • Response: No; bacteriophages exhibit host specificity and can only bind to and inject DNA into bacterial strains bearing compatible surface receptors.
• Question: Is the viral packaging of bacterial chromosomal DNA a selective or random process?
  • Response: It is a random physical and chemical process primarily constrained by the volumetric packaging capacity of the capsid head, rather than DNA sequence or function.