Bacteriophage Transduction
Some, but not all, bacteriophages are able to transduce bacterial DNA. The mechanism by which bacterial DNA hitch-hikes in the capsids of transducing bacteriophage depends on the life-cycle of the particular bacteriophage. We can define two broad categories of transduction:
GENERALIZED TRANSDUCTION
In this process, any part of the bacterial chromosome may be transduced. Examples of generalized transducing phage are bacteriophage P22 in Salmonella and bacteriophage P1 in E. coli.
SPECIALIZED TRANSDUCTION
In this process, only certain special pieces of the bacterial chromosome can be transduced. Bacteriophage l is the classic example of a specialized transducing phage.
All three of the bacteriophage named as examples above are TEMPERATE phage - i.e., they have 2 distinct life-cycles (lytic and lysogenic). There is no relationship between whether a phage is temperate or not and whether or not it transduces DNA.
Generalized transduction
We will use bacteriophage P1 as a model system for generalized transduction.
When this phage infects E. coli, it can grow either as a lysogen -- followed by subsequent lytic growth -- or it can enter a lytic growth cycle right away. Either way, when the phage is in the lytic growth cycle, it replicates its own DNA chromosome as long linear concatemers (i.e., one genome length after another on a single molecule of DNA).
Note that the phage P1 prophage state (in the lysogen) is independent of the host chromosome -- P1 does not integrate into the host chromosome when it forms a lysogen.
This mode of replication presents a problem: these concatemers must be processed into single genome length pieces. This processing requires a specific endonuclease. The resulting genome length pieces are then enclosed into a new bacteriophage capsid.
When the bacteriophage first enters a host cell and adopts a lytic mode of growth, it also synthesizes an endonuclease which cleaves the bacterial DNA. This makes a certain amount of sense for the phage since it wants the host cell to express phage genes and not host genes.
However, these fragments of bacterial chromosome are still present in the cell when the bacteriophage packaging machinery-- including the specific endonuclease -- starts to package the genome length pieces into new bacteriophage capsids. However, the packaging machinery cannot distinguish between a molecule that is bacterial DNA in origin and one that is bacteriophage DNA. All the packaging machinery cares about is that the molecules are the correct length.
Thus, bacteriophage capsids will contain both bacteriophage DNA (most of the time) and bacterial DNA (about 1 or 2 percent of the time).
If a preparation of bacteriophage is then used to infect fresh cells, capsids containing bacterial DNA will adsorb to the new host cell surface and will inject their capsid DNA into the cell. If the DNA is bacterial in origin, no infection will result. However, if the bacterial DNA contains a genetic marker that is different from one in the new host genome, then this can be detected if the phenotype of the new host cells changes as a result of recombination.
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Specialized transduction
Specialized transduction depends on the induction of an INTEGRATED prophage. Bacteriophage lambda, the classic example of a specialized transducing phage, integrates into the E. colichromsome at one very specific site -- called attB -- which is located between the bio and gal operons.
Just as F factors can integrate and excise, so can bacteriophage lambda. Bacteriophage lambda integrates when it is undergoing a lysogenic growth cycle and it excises when the prophage is induced and switches to a lytic growth cycle.
Just as F factors can excise incorrectly to form F' (F prime) factors, bacteriophage lambda can also excise incorrectly. When it does, bacterial DNA adjacent to the site of integration can be incorporated into the phage genome. Thus bacteriophage lambda can transduce the bio or gal operons. When this happens the phage will lose some of their own genetic information. As a result, they are defective phage.
