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Generalized and Specialized Transduction in Bacteria

In this generalized and specialized transduction in bacteria post we have briefly explained about generalized and specialized transduction in bacteria.

Bacterial Transduction is the transfer of bacterial genes from one bacterium to other by viruses. Example: Bacteriophage (Bacterial viruses). To understand the role of bacteriophage in gene transfer, the lifecycle of bacteriophage is described below briefly.

After infecting the host cell, a bacteriophage (phage for short) often takes control and forces the host to make many copies of the virus. Eventually the host bacterium bursts or lyses and releases new phages. This reproductive cycle is called a lytic cycle because it ends in lysis of the host.

Bacterial Transduction

The lytic cycle phases

Attachment: Attachment is achieved when specific proteins on the viral capsid or viral envelope bind to specific proteins called receptor proteins on the cell membrane of the target cell.

Penetration: Transduction occurs when a bacteriophage containing bacterial DNA infects a recipient bacterium and transfers this bacterial DNA to the recipient bacterial host cell.

Biosynthesis: After adsorption and penetration, the virus chromosome forces the bacterium to make viral components viral nucleic acids and proteins.

Assembly: Phages are assembled from the virus components. Phage nucleic acid is packed within the virus’s protein coat.

Release: Viruses can be released from the host cell by lysis, a process that kills the cell by bursting its membrane and cell wall if present.

Bacterial Transduction

Generalized and Specialized Transduction: The lytic cycle phases

Lytic cycle

Bacterial viruses that reproduce using a lytic cycle often are called virulent bacteriophages (e.g. T phages) because they destroy the host cell. The genome of many DNA phages such as the lambda phage, after adsorption and penetration do not take control of its host and does not destroy the host.

Instead the viral genome remains within the host cell and is reproduced along with the bacterial chromosome. The infected bacteria may multiply for long periods while appearing perfectly normal.

Each of these infected bacteria can produce phages and lyses under appropriate environmental conditions. This relationship between phage and its host is called lysogeny.

Bacterial Transduction

Generalized and Specialized Transduction: The lytic cycle

Bacteria that can produce phage particles under some conditions are said to be lysogens or lysogenic bacteria. Phages which are able to establish lysogeny are called temperate phages.

The latent form of virus genome that remains within the host without destroying the host is called the prophage. The prophage usually is integrated into the bacterial genome.

Sometimes phage reproduction is triggered in a lysogenized culture by exposure to UV radiation or other factors. The lysosomes are then destroyed and new phages released this phenomenon is called induction.

Sometimes, bacterial genes are incorporated into a phage capsid because of errors made during the virus life cycle. The virus containing these genes then infects them into another bacterium, resulting in the transfer of genes from one bacterium to the other. Transduction may be the most common mechanism for gene exchange and recombination in bacteria.

Generalized and Specialized Transduction: The lytic cycle

Generalized and Specialized Transduction

Based on how the DNA is packaged within the viral particle, there are two types of transduction:

Generalized Transduction

Generalized transduction occurs during the lytic cycle of virulent and temperate phages. During the assembly stage, when the viral chromosomes are packaged into protein capsids, random fragments of the partially degraded bacterial chromosome also may be packaged by mistake.

The resulting virus particles often inject the DNA into another bacterial cell but do not initiate a lytic cycle. Thus in generalized transduction any part of the bacterial chromosome can be transferred.

Once the DNA has been injected it may integrate into the recipient cell’s chromosome to preserve the transferred genes.

About 70 to 90% of the transferred DNA is not integrated but is often able to survive and express itself. However, if the transferred DNA is degraded gene transfer is unsuccessful.

Generalized and Specialized Transduction: Generalized Transduction

Steps

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.

Specialized Transduction

Specialized Transduction is also called restricted transduction in which only specific portions of the bacterial genome is carried by the phage. When a prophage is induced to leave the host chromosome, exicision is sometimes carried out improperly.

The resulting phage genome contains portions of the bacterial chromosome next to the integration site. When this phage infects another bacterium, it transfers the bacterial genes from the donor bacterium along with phage DNA.

Here only the bacterial genes that are close to the site of prophage are transferred. So, this transduction is called specialized.

Generalized and Specialized Transduction: Specialized Transduction

Steps

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.

Further Readings

Reference