Transduction is the genetic transfer mechanism by which a virus transfers genetic material from one bacterium to another. Other types of genetic transfer mechanisms include conjugation and transformation. Transduction is mediated by viruses. Viruses called bacteriophages are able to infect bacterial cells and use them as hosts to make more viruses. After multiplying, these viruses assemble and occasionally remove a portion of the host cell’s bacterial DNA. Later, when one of these bacteriophages infects a new host cell, this piece of bacterial DNA may be incorporated into the genome of the new host. Transduction is the process by which foreign DNA is introduced into a bacterial cell by a virus or viral vector. An example is the viral transfer of DNA from one bacterium to another and hence an example of horizontal gene transfers. In transduction, DNA is accidentally moved from one bacterium to another by a virus or phage.
There are usually two types of transduction: generalized transduction and specialized transduction. In generalized transduction, the bacteriophages can pick up any portion of the host’s genome. In contrast, with specialized transduction, the bacteriophages pick up only specific portions of the host’s DNA. Scientists have taken advantage of the transduction process to stably introduce genes of interest into various host cells using viruses.
MECHANISM OF TRANSDUCTION
In transduction, viruses that infect bacteria move short pieces of chromosomal DNA from one bacterium to another “by accident.” The viruses that infect bacteria are called bacteriophages. Bacteriophages, like other viruses, are the pirates of the biological world, and this is because bacteriophages commandeer a cell’s resources and use them to make more bacteriophages.
Virus infects cell by injecting its DNA into the target bacterial cell (Figure 1). Bacterial DNA is fragmented and viral DNA is replicated. New viral particles are made and exit the cell. One contains host DNA instead of viral DNA. When this virus infects a new host, it injects the bacterial DNA, which can recombine with the chromosome of the new host. Bacteria are infected by bacteriophages. Archaea are not infected by bacteriophages but have their own viruses that move genetic material from one individual to another.
Figure 1. Illustration of Transduction
TYPES OF TRANSDUCTION
Generalized transduction is the process by which any bacterial DNA may be transferred to another bacterium through a bacteriophage. It is a rare event; a very small percentage of phage particles happen to carry a donor bacterium’s DNA, on the order of 1 phage in 10,000. In essence, this is the packaging of bacterial DNA into a viral envelope. This may occur in two main ways, either recombination or heedful packaging.
If bacteriophages undertake the lytic cycle of infection upon entering a bacterium, the virus will take control of the cell’s machinery for use in replicating its own viral DNA. If by chance bacterial chromosomal DNA is inserted into the viral capsid which is usually used to encapsulate the viral DNA, the mistake will lead to generalized transduction.
If the virus replicates using ‘heedful packaging’, it attempts to fill the nucleocapsid with genetic material. If the viral genome results in spare capacity, viral packaging mechanisms may incorporate bacterial genetic material into the new virion.
The new virus capsule now loaded with part bacterial DNA then infects another bacterial cell. This bacterial material may become recombined into another bacterium upon infection.
When the new DNA is inserted into this recipient cell it can fall to one of three fates as follows:
- The DNA will be absorbed by the cell and be recycled for spare parts.
- If the DNA was originally a plasmid, it will re-circularize inside the new cell and become a plasmid again.
- 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 bacterial recombination.
Specialized transduction is the process by which a restricted set of bacterial genes is transferred to another bacterium. The genes that get transferred (donor genes) depend on where the phage genome is located on the chromosome. Specialized transduction occurs when the prophage excises imprecisely from the chromosome so that bacterial genes lying adjacent to the prophage are included in the excised DNA. The excised DNA is then packaged into a new virus particle, which then delivers the DNA to a new bacterium, where the donor genes can be inserted into the recipient chromosome or remain in the cytoplasm, depending on the nature of the bacteriophage.
When the partially encapsulated phage material infects another cell and becomes a “prophage” (is covalently bonded into the infected cell’s chromosome), the partially coded prophage DNA is called a “heterogenote”.
Lateral transduction is the process by which very long fragments of bacterial DNA are transferred to another bacterium. So far, this form of transduction has been only described in Staphylococcus aureus, but it can transfer more genes and at higher frequencies than generalized and specialized transduction. In lateral transduction, the prophage starts its replication before excision in a process that leads to replication of the adjacent bacterial DNA. When the replicated DNA excises from the chromosome, bacterial genes located up to several kilobases from the phage can get packaged into new virus particles that are transferred to new bacterial strains. If the transferred genetic material provides sufficient DNA for homologous recombination, the genetic material will be inserted into the recipient chromosome.
Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart WM (2000). Transduction. An Introduction to Genetic Analysis (7th ed.).