Generalized transduction and its process

July 11, 2018

Introduction

Bacteriophages not only infect and kill cells but also sometimes transfer DNA from one cell to another.
This process by which foreign DNA is introduced into a cell by a virus or viral vector is called transduction.
Phages capable of transduction are called transducing phages.
Donor strain is the original bacterial strain in which the transducing phage has multiplied.
The infected bacterial strain is called the recipient strain.
Cells that have received the DNA from another bacterium by transduction are called transductants.
Bacterial transduction can be defined as the transfer of DNA from one bacterium to another by a bacteriophage serving as a vector.
Most bacteriophages, the virulent phages, undergo a rapid growth cycle in their host cells.
However, in some bacterial viruses, the temperate phages behave as a kind of episomes in bacteria.
Such viral genomes integrated into the bacterial genome are known as pro-phages.
Bacteria carrying prophases are called lysogenic bacteria.
They can be induced with UV light or chemicals to go through a lytic cycle, resulting in cell lysis and the release of new phage particles.
Phage particles may become filled with chromosomal DNA, Phage DNA, or both.
Such aberrant phages can transfer bacterial DNA from one cell to another.
There are two types of transduction in bacteria. They are:

a) Generalized transduction.

b) Specialized transduction.

Generalized transduction

If all the fragments of bacterial DNA (i.e. from any region of the bacterial chromosome) have a chance to enter a transducing phage, the process is called generalized transduction.
In this transduction virtually any gene on the donor chromosome can be transferred to the recipient.
It was first discovered and extensively studied in the bacterium Salmonella enterica with phage P₂₂ and has also been studied with phage P₁ in E. coli.
During the lytic infection, viral enzymes hydrolyze the bacterial chromosomes into many small pieces of DNA.
The enzymes responsible for packaging viral host DNA into bacteriophages sometimes package host DNA accidentally.
The resulting virion is called a transducing particle.
On the lysis of the cell, these particles are released along with normal (i.e. potentially lytic) virions, and so the lysate contains a mixture of normal virions and transducing particles.
Because the transducing particles cannot lead to normal viral infection (they contain no viral DNA), they are said to be defective.
When this lysate is used to infect a population of recipient cells, most of the cells become infected with the normal virus.
However, a small proportion of the population receives transducing particles that inject the DNA they packaged from the previous host bacterium.
While this DNA cannot replicate, it undergoes genetic recombination with the DNA of a new host.
If the injected DNA is a plasmid, it may replicate after it enters the cell and thus be maintained.
If the incoming DNA contains a transposon, the transposon may hop, or insert itself into the host plasmid or chromosome.
Because only a small proportion of particles in the lysate are defective, and each of these contains only a small fragment of donor DNA, the probability of a given transducing particle containing a particular gene is quite low.
It might be typically only one cell in 10⁶to 10⁸ which is transduced for a given marker.
Transduction occurs very rarely for a number of reasons.
First, mistaken packaging of host DNA is itself rare, and the transduced DNA must survive in the recipient cell to form a stable transductant.
Since, each of these steps, has a limited probability of success, transductants can be detected only by powerful selection techniques.
Phages that form transducing particles can be either temperate or virulent.
It depends on the main requirement being that they have a DNA packaging mechanism that accepts host DNA and the DNA packaging occurs before the host DNA is completely degraded.
The detection of transduction is most certain when the multiplicity of phage to host is low that is a host cell is infected with only a single phage particle.

Transducing phages

Not all phages can transduce.
The phage must not degrade the host DNA completely after infection or no host DNA will be available to be packaged into phage heads.
The packaging sites or pack site, of the phage, must not be so specific that such sequences will not occur in host DNA.
Phage P₁ which infects Gram Negative bacteria is a good transducer because it has less pac site specificity than most phages and efficiently packages host DNA.
The Salmonella enterica serovar Typhimurium phage P₂₂ is also a good transducer and in fact, the first transducing phage to be discovered.
T₄ normally degrades the host DNA after infection but works extremely well as a transducing phage if its gene for the degradation of host DNA has been inactivated.
Because phage T₄ does not have pac site, it packages any DNA including host DNA with high efficiency.
In a large population of phages, there will be transducing phages carrying different fragments of the bacterial genome.
Coli phage P₁ can transduce a variety of genes in the bacterial chromosome.
Defective P₁ pages bearing coli DNA can be detected by the genetic markers present in the DNA.
For instance, if a thr cell is infected by a phage carrying a fragment of coli DNA with a thr⁺ gene, this thr^– gene may be integrated into the bacterial chromosome to result in a prototrophic recombinant detectable by growth in a medium devoid of threonine.