Generalized Transduction

Generalized transduction is the process by which any bacterial gene may be transferred to another bacterium via a bacteriophage, and typically carries only bacterial DNA and no viral DNA. In essence, this is the packaging of bacterial DNA into a viral envelope. This may occur in two main ways, recombination and headful packaging.

If   bacteriophages   undertake the lytic cycle of infection upon entering a bacterium, the virus will take control of the cell's machinery for use in replicating its own viral DNA. If by chance bacterial chromosomal DNA is inserted into the viral capsid which is usually used to encapsulate the viral DNA, the mistake will lead to   generalized transduction.

If the virus replicates using 'headful packaging', it attempts to fill the nucleocapsid with genetic material. If the viral genome results in spare capacity, viral packaging mechanisms may incorporate bacterial genetic material into the new virion. The new virus capsule now loaded with part bacterial DNA continues to infect another bacterial cell. This bacterial material may become recombined into another bacterium upon infection.

When the new DNA is inserted into this recipient cell it can fall to one of three fates

1. The DNA will be absorbed by the cell and be recycled for spare parts.
2. If the DNA was originally a plasmid, it will re-circularize inside the new cell and become a plasmid again.
3. If the new DNA matches with a homologous region of the recipient cell's chromosome, it will exchange DNA material similar to the actions in   conjugation.

This type of   recombination   is random and the amount recombined depends on the size of the virus being used.

Fig. 47. Generalized Transduction

Specialized Transduction

Specialized transduction is the process by which genes that are near the bacteriophage genome may be transferred to another bacterium via a bacteriophage. The genes that get transferred (donor genes) always depend on where the phage genome is located on the chromosome. This second type of recombination event which is the result of mistakes in the transition from a virus' lysogenic to lytic cycle is called   specialized transduction , and non-viral DNA is carried as an insertion/substitution. If a virus incorrectly removes itself from the bacterial chromosome, bacterial DNA from either end of the phage DNA may be packaged into the viral capsid. Specialized transduction leads to three possible outcomes:

1. DNA can be absorbed and recycled for spare parts.
2. The bacterial DNA can match up with a homologous DNA in the recipient cell and exchange it. The recipient cell now has DNA from both itself and the other bacterial cell.
3. DNA can insert itself into the genome of the recipient cell as if still acting like a virus resulting in a double copy of the bacterial genes.

When the partially encapsulated phage material infects another cell and becomes a "prophage" (is covalently bonded into the infected cell's chromosome), the partially coded prophage DNA is called a "heterogenote".

Esther Lederberg,   Larry Morse,   Herman Kalckar,   Michael Yarmolinsky, and   Yukinori Hirota   went on to do detailed studies of   Galactosemia.   Specialized transduction was used in these studies for gene mapping. At about this time, Esther Lederberg,   Julius Adler, and   Enrico Calef   were also engaged in similar research involving   Maltophilia.

Example of specialized transduction is   λ phages   in   Escherichia coli   discovered by   Esther Lederberg  as well as   Fertility Factor F, also discovered by Esther Lederberg.

 

Fig. 48. Specialized transduction