- 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.
- 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 106 to 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.
- 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.
Generalized transduction and its process