- Transduction refers to the transfer of a part of DNA between two bacteriums via a bacteriaophage.
- Transduction is mediated by viruses.
- During the replication of virus in a cell, a small piece of bacterial DNA gets incorporated into the bacteriophage. This DNA is then transferred to the recipient bacteria at the time of infection.
- Through a process known lysogenic conversion, the phage DNA from the recipient bacterial cell is integrated into the cell DNA.
- Lysogenic transformation confers a new property on the bacterial cell. For example, nonpathogenic bacteria may become pathogenic through lysogenic convert.
- Bacteriophages can encode diphtheria, botulinum, cholera, and erythrogenic toxins from one bacterium, via transduction.
- Bacteria infecting viruses are known by the shorthand bacteriophages (or phages).
- Virulent bacteriophages multiply within their bacterial host as soon as they enter. After the progeny-phage particles have reached a certain amount, they cause the host’s cells to lyse. This allows them to be released and infect other host cells. This process is called the “lytic cycle”.
- Temperate bacteriaophages, however, don’t kill their host right away. Instead, the host establishes a relationship called lysogeny with the phage. Lysogenized bacteria are also known as lysogens.
- Many temperate Phage species establish lysogeny via inserting their genomes into a bacterial chromosome. Prophages are the viral genome that is inserted into the host bacterium. This is not harmful to the host bacterium and the phage gene is copied along with the genome of the host cell.
- Temperate phages might remain inactive for several generations in their hosts. Under certain conditions, such as UV radiation, they can be made to switch to the lytic cycle. Once this happens, the prophage from the bacteria genome is removed and the lytic cycles proceeds.
Principle of Bacterial Transduction
- The mechanism of infection is the basis of transduction.
- The bacterial donor DNA can be incorporated into the bacteriophage by either the lytic or the Lysogenic cycles.
- New phages are created from the bacteria cell after the bacterial genome is integrated into the phage.
- These phages then infect host cells. Phages attach to a bacterial cell’s surface receptor and inject their donor DNA into the cytoplasm.
- The phage can integrate the DNA into the bacteria genome or create phage progeny by replicating in the cell cytoplasm.
Classification of Transduction/Types of Transduction
Two types of transduction exist:
- Generalized transduction.
- Specialized transduction.
A. Generalized Transduction
- Generalized transduction occurs most commonly during the lytic cycles of virulent and temperate phages, but can sometimes occur during the cycle of temperate bacteria.
- This happens when a small portion of the phage viral virions generated during the lytic cycles are aberrant and contain a random fragment from the bacterial genome rather than phage DNA.
- Each transducing Phage carries a distinct set of closely related genes. This represents a small part of the bacterial genomic DNA. The term generalized transduction is when transduction occurs through the interaction of phage populations.
- Each part of a bacterial genome is approximately equally likely to be transferred from donor to receiver bacteria.
- Generalized transduction refers to the random selection of donor DNA segments. This happens because cells are damaged by infection, and fragments of the same DNA as viruses are incorporated into the bacterial genome. This happens approximately 1 in 1000 times.
- Virus genomes are packaged during the assembly stage by the “headful method”. This means that only genomes larger than a specified size (i.e., the number of nucleotides in a given length) are packaged.
- Generalized transduction can cause a mistakenly packaged fragment of the host’s genome to be approximately the same size. Because it could encounter a vulnerable host cell, such a phage may be called a generalized transmitter particle. Once it has been released, it will eject any bacterial DNA it carries into the cell.
- However, it doesn’t initiate a lytic-cycle because it lacks virus genes.
- Transformation is where the DNA fragment must be released into the recipient’s cell. Once it has been incorporated into its chromosome, the transferred genes will remain intact.
- During transfer, DNA remains double-stranded. Both strands are integrated into a recipient’s genome. 70-90% of the DNA transferred is not combined, but can sometimes be retained and expressed temporarily.
- This bacteria is called an abortion transductant. They are partially diploids and contain nonintegrated, transduced genomic DNA.
Steps of Generalized Transduction
- The Phage infects the bacterial cell during the first stage of Generalized Transduction.
- Following this, the host DNA can be hydrolyzed into small pieces. Then, phage DNA or proteins can be made.
- The Phages are assembled and sometimes a phage has a piece from the host cell chromosome.
- Transducing phage injects its genetic material into a new recipient cells.
- Transduced DNA can be recombined with the chromosomes of the recipient cells.
Note: The recombinant bacterium has a genotype (his+ lys- ) that is dierent from recipient bacterial cell (his- lys-).
Types of Generalized transduction
Generalized transduction may be complete or abortive:
- Complete transduction is characterized by production of stable recombinants that inherit donor genes and retain the ability to express them.
- Abortive Transduction refers to transient expression of one or several donor genes without the formation of recombinant material.
- The donor DNA fragment is not replicable in abortive transcription, and only one bacterium has the donor DNA segment among the progeny.
- After each generation of bacterial proliferation, the donor genes products are gradually diluted in other progeny until the donor phenotype is no longer expressed.
- Selective medium allows abortive transductants to produce small colonies that can easily be distinguished from stable transductants.
- The frequency of abortive transmission is usually one to twofold higher than that of generalized transcription. This shows that cells infected by generalized phages are not capable of producing recombinant geny.
B. Specialized transduction
- Specialized transduction occurs when only a small portion of the bacterial DNA is carried by transducing particle.
- An error in the lysogenic process of temperate phages allows for specialized transduction. These phages insert their genomes in a specific spot on the host’s chromosome.
- Excision can sometimes be done incorrectly when a prophage leaves the host chromosome. The phage genome that results is a phage contains some portions of the prophage chromosome (about 5-10% bacterial DNA) near the integration site. It’s similar to F’ plasmids.
- The transducing particle, however, is not virus-friendly and cannot reproduce by itself.
- It will still inject the viral genome and any other bacterial genes into another bacterium. If the conditions are right, the bacterial gene may be stably incorporated.
Example of Specialized transduction
- The E. coli phage bacterium lambda is the most well-studied example specialized transduction.
- The lambda gene inserts into the host’s chromosome in specific locations called attachment or att spots.
- The bacterial and phage att site are very similar and can interact with each other.
- The att site of lambda lies between the bio and gal genes on the E.coli chromosome. This means that when lambda excises incorrectly, these bacterial genetics are most commonly present.
- The cell lysis product (lysate), which results from induction of a number of lysogenized E. coli bacteria cells, contains normal phage as well as a few defective transducing particle.
- These particles are called lambda gal if they possess the galactose usage genes, or lambda biif they have bio from the opposite side of the att.
The Mechanism of Transduction for Phage Lambda and E. coli.
Between the gal and bio genes is integrated lambda-phage. If it excites normally (top left), then the new phage contains no bacterial gene and is complete. Rarely, the excision is done asymmetrically (top center), meaning that either the gal gene or bio genes are taken up and some phage genetics are lost. The gal genes are the only exception to this rule. A defective lambdaphage is created that carries bacterial genetics and can transfer them onto a new recipient.
Steps of Specialized transduction
- After infection of the donor bacteria with the bacteriophage the phage genome is integrated into the bacteria’s chromosome in the lysogenic phase.
- Because of the inexact cutting of the DNA of phages, some of the bacterial genomes are also removed.
- After infecting a new host with a phage, the donor DNA is transferred to the recipient bacterium through the lysogenic process of replication.
- The recipient expresses the newly acquired trait.