Transduction is the transfer of bacterial DNA from a donor to a recipient bacterium via a virus particle. The virus particle that infects bacteria is called a bacteriophage or phage, and the phages used for the transfer of DNA are called transfusing phages.
Not all phages are transducing phages. The process of transduction can transfer DNA regions of tens to hundreds of kilobases. Due to the high specificity of phages for cell surface receptors, transduction has the narrowest host range of DNA transfer among the methods of bacterial genetic exchange.
Transduction involves the carrying over of DNA (or gene transfer) from one organism to another by an intermediate agent, which is usually a bacteriophage. Transduction has an advantage over conjugation in that transduction doesn’t require physical contact between the cell donating and the DNA and the cell receiving the DNA.
Similarly, the process of transduction is resistant to the DNase enzyme while the transformation process is susceptible to DNase. Transduction is a standard process employed by many molecular biologists to introduce a foreign gene into a host cell’s genome.
Bacterial Transduction Principle
The principle of transduction is based on the mechanism of infection of the bacteriophage. In transduction, the bacterial donor DNA is incorporated into the bacteriophage either through the lytic or lysogenic cycle.
After the bacterial DNA is incorporated into the phage, new phages are released from the bacterial cell. These phages then infect the host bacterial cell. Phages attach to a specific bacterial cell surface receptor and inject their DNA containing the donor DNA into the cytoplasm of the host bacterial cell.
Depending on the phage, the DNA integrates into the bacterial genome, replicates in the cytoplasm as a plasmid, or replicates immediately producing phage progeny.
Types of Transduction
Based on how the DNA is packaged within the viral particle, there are two types of transduction:
In generalized transduction, phage mistakenly packages bacterial DNA instead of their own phage DNA during phage assembly. This results in an infectious virus particle containing bacterial DNA, but one that can no longer replicate in the bacterium due to the loss of all of the phage DNA.
The phage particle then attaches to a bacterial cell surface receptor and injects the packaged DNA into the cytoplasm of the bacterium.
If the bacterial DNA in the phage is from the bacterial chromosome, the DNA recombines with the homologous DNA of the bacterial recipient to generate stable transductants. This process requires a host recombinase, recA.
However, studies have indicated that the majority of transduced DNA is not stably integrated into the bacterial genome but rather remains extrachromosomal. Generalized transduction is used for mapping genes, mutagenesis, transferring plasmids and transposons, and determining whether different genera of bacteria have homologous genes.
In specialized transduction, the phage undergoes lysogeny usually at specific locations in the bacterial genome called attachment sites. During this process, the phage genome usually integrates into the bacterial chromosome as virus replication is repressed during lysogeny.
The phage genome then excises from the bacterial genome and, due to imprecise excision and recombination, adjacent bacterial genes are also excised. Unlike a generalized transducing phage, a specialized transducing phage contains both phage and bacterial genes.
During the subsequent infection, the newly acquired gene is inserted into the bacterial genome along with phage DNA to form a new round of lysogeny. Specialized transduction is independent of host homologous recombination and recA but requires phage integrase.
Specialized transduction is instrumental in the isolation of the genes in molecular biology, and in the discovery of insertion elements, which often serve as attachment sites for phage DNA integration.
Steps of Generalized Transduction
The phage host cell (donor cell) is first infected with the phage, during which, the phage DNA enters the cytoplasm of the bacteria. During the lytic cycle of the viral replication, the phage DNA, along with the bacterial chromosome is broken down into smaller pieces.
Some part of the bacterial chromosome is then packaged into one of the viral capsids when that is released by lysis of the bacterium. The transducing phase with the bacterial chromosome now infects a second bacterium (recipient bacterium), and the donor DNA enters the cytoplasm of the second bacterium.
In the presence of a host recombinase recA, the donor DNA recombines with the homologous DNA of the bacterial recipient to generate stable transductants.
Steps of Specialized Transduction
After the infection of the donor bacterium with the bacteriophage, the phage DNA is integrated into the bacterial chromosome during the lysogenic cycle. Due to the imprecise cutting of the phage DNA, some part of the bacterial chromosome is also excised.
The phage containing some part of the bacterial chromosome then infects a new host, and the donor DNA is incorporated into the recipient bacterium during the lysogenic cycle of the replication. The recipient then expresses the newly acquired genetic trait.
Examples of Bacterial Transduction
A good example of a generalized transducing phage is P1, which can transduce E. coli DNA to numerous Gram-negative bacteria.
E. coli phage lambda is a classic example of a specialized transducing phage that integrates its DNA precisely between operons encoding enzymes responsible for galactose (gal) and biotin (bio) utilization in the E. coli chromosome.