Transduction (genetics)

Transduction is the process by which bacterial DNA is moved from one bacterium to another by a virus.

When bacteriophages (viruses that infect bacteria) infect a bacterial cell, their normal mode of reproduction is to harness the DNA replication machinery of the host bacterial cell and make numerous copies of their own DNA or RNA. These copies of bacteriophage DNA or RNA are then packaged into newly synthesized copies of bacteriophage virions.

Lytic and lysogenic cycles
Transduction happens through either the lytic cycle or the lysogenic cycle.

If the lysogenic cycle is adopted, the phage chromosome is integrated into the bacterial chromosome, where it can remain dormant for thousands of generations. If the lysogen (eg. UV light) is induced, the phage genome is excised from the bacterial chromosome and initiates the lytic cycle, which culminates in lysis of the cell and the release of phage particles.

The lytic cycle leads to the production of new phage particles which are released by lysis of the host.

Mistakes
However, the packaging of bacteriophage DNA is not fool-proof and at some low frequency, small pieces of bacterial DNA, rather than the bacteriophage genome, will be packaged into the bacteriophage genome. At the same time, some phage genes are left behind in the bacterial chromosome.

There are generally two types of mistakes that can lead to this incorporation of bacterial DNA into the viral DNA, leading to two modes of recombination.

Generalized transduction
If bacteriophages undertake the lytic cycle of infection upon entering a bacteria, the virus will take control of the cell’s machinery for use in replicating its own viral DNA. If by chance bacterial chromosomal DNA (instead of viral DNA) is inserted into the viral capsid used to contain the viral DNA, while this lytic pathway is proceeding, the mistake will lead to generalized transduction. The new virus capsule now loaded with part bacterial DNA is no longer infectious, but will still attempt to infect another bacterial cell.

When the new DNA is inserted into this recipient cell it can fall to one of three fates:


 * 1. The DNA will be absorbed by the cell and be recycled for spare parts.


 * 2. If the DNA was originally a plasmid, it will re-circularize inside the new cell and become a plasmid again.


 * 3. If the new DNA matches with a homologous region of the recipient cell’s chromosome, it will exchange DNA material similar to the actions in conjugation.

This type of recombination is random and the amount recombined depends on the size of the virus being used.

Specialized transduction
The second type of mistake is called specialized transduction. If a virus removes itself from the chromosome incorrectly, it can leave part of the viral DNA in the chromosome. Some of the bacterial DNA can be packaged into the virion. Mistakes in this process of viral DNA going from the lysogenic to the lytic cycle lead to specialized transduction. There are three possible results from specialized transduction:


 * 1. DNA can be absorbed and recycled for spare parts.


 * 2. The bacterial DNA can match up with a homologous DNA in the recipient cell and exchange it. The recipient cell now has DNA from both itself and the other bacterial cell.


 * 3. DNA can insert itself into the genome of the recipient cell as if still acting like a virus resulting in a double copy of the bacterial genes.

This type of recombination is not random and only small portions of genes are recombined.

Example of specialized transduction is λ phages in Escherichia coli.

RNA, DNA
Viruses with RNA genomes are not able to package DNA and so do not usually make this mistake.

Upon lysis of the host cell, the mispackaged virions containing bacterial DNA can attach to other bacterial cells and inject the DNA they have packaged, thus transferring bacterial DNA from one cell to another. This DNA can become part of the new bacterium's genome and thus be stably inherited.

More general uses of the term
More generally, transduction is the process by which genetic material, e.g. DNA or siRNA, is inserted into a cell. Common techniques in molecular biology are the use of viral vectors (including bacteriophages), electroporation, or chemical reagents that increase cell permeability. Transfection and transformation are more common terms, although these sometimes imply expression of the genetic material as well.

Alternatively, transduction can be shorthand for Signal transduction or longhand for trans.

Discovery
Transduction was discovered by Norton Zinder and Joshua Lederberg at the University of Wisconsin-Madison in 1952.