There are two distinct types of transduction. In generalized transduction fragments of bacterial DNA are packaged more or less at random in the phage particles. This is the case for bacteriophage P1 as described above (Fig. 18.08). Consequently all genes have generalized transduction Type of transduction where fragments of bacterial DNA are packaged at random and all genes have roughly the same chance of being transferred
When the bacteriophage lambda infects a host E. coli cell, it can integrate its phage DNA into the chromosome. The phage DNA will only integrate at a site called atfl, which is found between the bio gene and gal gene of the chromosome. Once integrated, the phage is referred to as a prophage.
Viruses that integrate into host DNA pick up fragments of host DNA that lie next to the integration site.
roughly the same chance of being transferred. In specialized transduction certain regions of the bacterial DNA are carried preferentially—see below.
For a bacterial virus to transduce, several conditions must be met. In particular, the phage must not degrade the bacterial DNA. For example, phage T4 normally destroys the DNA of E. coli after infection. However, mutants of T4 that have lost the ability to degrade host cell DNA work well as transducing phages. The packaging mechanism is also critical. Some phages, such as lambda (see below) use specific recognition sequences when packaging their DNA into the virus particle and so will not package random fragments of DNA. In other cases, packaging depends on the amount of DNA the head of the virus particle can hold. Such "headful packaging" is essential for generalized transduction.
Two examples of generalized transducing phages are P1, which works on Escherichia coli, and P22, which infects Salmonella. The ratio of transducing particles to live virus is about 1:100 in both cases, that is, for every 100 virus particles made, one will be packaged with bacterial host DNA. The likelihood of the transduced DNA recombining into the recipient chromosome is roughly 1 to 2 in 100. P1 can package approximately 2% of the E. coli chromosome (about 90 kb of DNA), whereas P22 is smaller and can carry only 1% of the Salmonella chromosome. Taken all together, about 1 in 500,000 P1 particles will successfully transduce any particular gene on the E. coli chromosome. This may seem a low probability but as both typical bacterial cultures and preparations of P1 contain about 109 cells per ml, transduction happens at useful frequencies in practice. P1 can also transduce DNA from E. coli into certain other gram-negative bacteria, such as Klebsiella.
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