Fate of Transferred Donor DNA upon Infection of a Recipient Host

According to the definition of transduction, transfer of donor DNA must be followed by the expression of donor genes in the recipient bacteria: expression of the acquired gene functions is the basis of the monitored phenotype of the transductants. To produce this phenotype, the donor DNA must be maintained and ideally also replicated in the trans-ductant clone.

If the donor DNA finds some homology in the recipient chromosome, the homologous segments can undergo general recombination. This type of genetic conversion is often involved in generalized transduction. It can cause stable integration also of DNA segments carried in specialized transducing phages, e.g. gal genes in A gal.

Another way of propagating acquired donor DNA is autonomous replication. This requires appropriate replicon functions. These can be provided by the vector phage. An example is specialized transducing PI carrying a transposon. Upon lysogenization of the recipient, this phage establishes its prophage as a plasmid.

Phage PI is also able to give generalized transduction for entire plasmids unrelated to the PI genome. Such plasmids can become established as independent replicons after their transfer into recipient bacteria.

Rather than relying on their own replication functions, genomes of specialized transducing phages can also integrate into the recipient chromosome and then become further propagated together with the latter. This is the case when Agal lysogenizes its host. Thereby the intact Agal hybrid genome integrates into the bacterial chromosome by site-specific recombination. Alternatively, integration of hybrid transducing genomes can also be obtained by other types of recombinational events, e.g. if the transducing phage carries an IS element which finds its homologous counterpart in the recipient chromosome, so that general recombination can provide a means for the production of cointegrates. Alternatively a transposon carried in a transducing phage has a low probability of transposing, after its transfer, into the recipient chromosome and this can ensure its further inheritance.

As discussed above, transferred donor DNA is not always propagated in the progeny of the infected recipient cell. It can indeed also be maintained intact without undergoing replication, possibly being protected against nucleolytic degradation by specific proteins. This then gives rise to abortive transduction, in which the acquired DNA is unilinearly inherited and the donor gene functions become expressed and subsequently diluted in the cytoplasm of the cells of the growing clone of the transductant.

Obviously, the survival of the infected recipient cells is a prerequisite for the detection of transductants. This condition is particularly relevant for virulent phage strains such as T-even and T1 phages. By applying particular infection conditions, it could be shown that lysates of these phages also contain phage particles packaged with host DNA and therefore can give transduction.

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