Transduction

Transduction:

Most bacteriophage the virulent phages, undergo a rapid lytic growth cycle in their host cells. They inject their nucleic acid, usually DNA into the bacterium,where it replicates rapidly and also directs the synthesis of new phage proteins. Within 10-20 min depending on the phage, the new DNA combines with the new proteins to make whole phage particles, which are released by the destruction of the cell wall and lysis of the cell. However, some bacterial viruses, the temperate phages, which ordinarily do not lyse the cell, carry DNA that can behave as a kind of episome in bacteria; like other episomes, such as the F factor, these viral genomes can become integrated into the bacterial genome; they are then known as prophages. We can define bacterial transduction as the transfer by a bacteriophage, serving as a vector of a portion of DNA from one bacterium to another.

This phenomenon was discovered by Zinder Lederberg in 1952 when they searched for sexual, conjugation among salmonella species. They mixed autotrophic mutants together and isolated prototrophic recombinant colonies from selective nutritional media. When U-tube experiment was carried out with a parental autotrophic strain in each arm and separated by a microporous fritted glass filter, prototrophs appears in one arm of the tube.Since the filter prevents cell to cell contact but allowed free passage of fluid between the cultures, it could be concluded that some phenomena other than conjugation are involved. The bacteriophage was released from a lysogeneic culture. The phage passed through the filter and infected the other strain, lysings it. During replication in the some of it. It then passed through the filter again, carrying part of donor's genetic information and imparting it to the recipient strain. Transduction ate two types which are given as below:

1. Generalized transduction
2. Specialized transduction (Restricted transduction)

Generalized transduction:

If all fragments of bacterial DNA from the donor have a chance to enter a transducing phage, the process is called generalized transduction. In generalized transduction most bacteria Phages eg. Coliphage P_i undergo a rapid lytic growth cycle in their host cells. During lytic growth cycle head, tail, DNA and all the components of bacteriophages are synthesized inside the host cell. The chromosome of the host cell degraded into several fragments. During assembly, DNA of the virus is packaged inside the head (capsid) to form complete virus particle. In cases, DNA of virus packaged into the head in the head and in this cases normal virus particles are produced. In some cases, fragments of donor DNA are packaged into head instead of viral DNA due to an error in the assembly process. In this case transducing bacteriophages is produced which contains bacterial DNA instead of its own DNA. When such transducing bacteriophage enters into surrounding cell to carry out next replication cycle, it transfers donor DNA into the cells and recombination occurs. In this transduction, any pieces of donor chromosomes can enter into virus during assembly because donor chromosome in fragmented into many pieces. Generalized transduction, like bacterial conjugation and transformation, also provides a means for mapping bacterial genes, since the fragments transferred by a bacteriophage are often large enough to contain hundreds of genes. The mapping technique involves providing to the phage-infected a bacteria growth medium that selects for those recombinants that have inherited a given genetic maker from bacterialDNA carrier by transducting phage.

Specialized transduction:

It is the transduction in which only certain donor genes can be transferred to the recipient. In specialized transduction, bacteriophages carry out lysogenic cycle in donor cells. In lysogenic cycle bacteriophage, DNA of the host and remains there for a generation. When such cells are exposed to high dose of UV light or any strong chemicals, bacteriophages DNA becomes free and then the lytic cycle begins. Usually upon the induction of the cell viral DNA comes out, replicated and makes normal phases. However, occasionally the viral DNA is excised improperly and carries adjacent pieces of the bacterial chromosome leaving portions of itself behind and in this case transducing phages produce after the lytic cycle. When such phage infects the cell it carries with it the bacterial genes that have become part of it. Such gene can then recombinant with the homologous DNA of an infected cell (recipient cell).

The best studied specialized transduction is in the phase of áµÅ¾ of e. Coli. The location of áµÅ¾ prophage in the bacterial chromosome is almost always next to the bacterial “gal genes”.

Significance of transduction:

1. It is used in gene mapping.
2. Specialized transduction is the good tool to study diploidy in bacteria.
3. Phage conversion: when a normal template phage lysogenize a bacterial cell and its DNA is converted to prophage, the lysogenic bacterial cell is immunized to further infection by the same type of phage. This process is called phage conversion.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.