Generalized and specialized transduction

DNA transfer can occur among cells through the action of bacteriophages. Occasionally, the excision of a lysogenic phage’s genome from the host chromosome is imperfect. Instead of just phage genes the excision includes one or two host genes from locations adjacent to the site of phage recombination. When the altered phage infects another cell it is possible that the extra genes will complement mutated genes in the second host’s genome. This process is known as specialized transduction. In contrast generalized transduction can lead to the transfer of any gene in a host genome not just those adjacent to a site of lysogeny.

The example of a generalized transducing phage is the bacteriophage P1 of E. coli. Using this phage to transfer DNA among E. coli strains is a relatively quick and easy way of making chromosomal mutations and is sometimes abbreviated to P1 transduction.  The principle is outlined and for the most part follows the normal life cycle of a lysogenic bacteriophage. The difference occurs just  as the  pro- phage leaves  the  host  genome on  the  induction of the  lytic cycle.  The phage genome leaves completely and induces the degradation of the host chromosomes. The phage replicates inside the cell but during this process some random Bacterial DNA is packaged as if it were phage DNA.
When the donor cell is lysed, the P1 phage released can be used to infect a recipient. If the donor was a wild-type cell and the recipient was, for example, an auxotroph for histidine, some of the P1 phage may carry the DNA to correct the mutation. The small number of infected recipients will receive a fragment of wild-type donor DNA instead of phage P1 DNA and this can recombine with a recipient chromosome.

                                                   Figure:  Generalized transduction by phage P1.

While a useful technique in E. coli, where P1 phage has been engineered to increase the frequency of generalized transduction, few similar systems exist. The number of genes that can be transferred is limited to the maximum amount of DNA that can be packaged into a P1 phage head.