Overview of Transduction
The general mechanism of transduction is tied to the viral replication cycle inside a bacterial cell. When a bacteriophage infects a bacterium, it hijacks cellular machinery to make copies of itself. During viral assembly, an error occasionally occurs: a fragment of the host bacterium's chromosome is accidentally packaged inside the viral protein coat.
Upon infecting a new recipient bacterium, this phage injects the bacterial DNA instead of viral DNA. This represents a key mechanism of horizontal gene transfer in microbiology. There are two main classes of transduction: generalized (nonspecific) and specialized transduction.
Generalized Transduction
The defining feature of generalized transduction is that the phage can transfer any random fragment of the bacterial chromosome. A classic example includes P-phages.
The process occurs in several steps:
- Fragmentation: The DNA of the infected bacterium is degraded into pieces.
- Erroneous Packaging: Pieces of bacterial DNA that match the size of normal phage DNA are mistakenly packaged into the viral capsid.
- Defective Particle Formation: A defective phage particle is formed, lacking viral genes while carrying donor bacterial DNA. The error frequency is about 1 in 1000 normal phages.
- Recipient Infection: The defective phage attaches to a new cell and injects the carried bacterial DNA.
- Recombination: Inside the recipient, homologous recombination takes place—the introduced fragment replaces the homologous region of the recipient's chromosome, creating a stable recombinant.
Specialized Transduction
Specialized transduction follows a different pathway and requires a prerequisite: the phage DNA must be previously integrated into the bacterial chromosome (existing as a prophage). Temperate phages integrate only into specific attachment sites on the bacterial chromosome.
Transfer logic:
- Excision Error: The process begins when the viral DNA reactivates and excises from the bacterial chromosome. An enzymatic error occurs: alongside the viral DNA, flanking bacterial DNA adjacent to the integration site is excised.
- Resulting Phage: A defective phage is produced in which a portion of its own genome is permanently replaced by bacterial genes.
- Specificity: Because the phage was bound to a specific site, it can transfer only those genes located immediately adjacent to the integration site. A well-known example is the transfer of the gal gene (responsible for galactose fermentation) in E. coli.
- Recipient Process: Upon entering a new cell, the defective phage DNA undergoes site-specific recombination with the recipient's DNA. As a result, the recipient cell becomes a merodiploid (partial diploid) because it now contains both its own gene and the exact donor gene introduced.
Comparison of Mechanisms
| Feature | Generalized Transduction | Specialized Transduction |
|---|---|---|
| Transferred Material | Any fragment of the donor chromosome | Only genes adjacent to the prophage integration site |
| Initiation Trigger | Degradation of donor DNA | Error during prophage excision from the chromosome |
| Recombination Type | Homologous recombination | Site-specific recombination |
| Cellular Outcome | Formation of a stable recombinant | Cell becomes a merodiploid |
| Example | Typical of P-phages | Transfer of the gal gene in E. coli |