During specialized transduction, certain specific regions of the bacterial chromosome are favored. This is due to integration of the bacteriophage into the host chromosome (see Ch. 17). If the virus enters a lytic cycle and manufactures virus particles, those bacterial genes nearest the integration site are most likely to be incorrectly packaged into the viral coats. As discussed in Ch. 17, when bacteriophage lambda (or l) infects E. coli, it sometimes inserts its DNA into the bacterial chromosome (Fig. 18.09). This occurs at a single specific location, known as the lambda attachment site (attl), which lies between the gal and bio genes. The integrated virus DNA is referred to as a prophage.
When lambda is induced, it excises its DNA from the chromosome and goes into lytic mode. The original donor cell is destroyed, and several hundred virus particles containing lambda DNA are produced. Just like generalized transducing phages, a small fraction of lambda virus particles contain bacterial DNA. There are, however, two major differences. First, only chromosomal genes next to the attachment site are transduced by lambda. Second, the specialized transducing particles contain a hybrid DNA molecule comprising both lambda and chromosomal DNA (Fig. 18.10). This hybrid molecule results from mistakes during excision of the lambda prophage from the chromosome. Chromosomal DNA to the right or to the left of the prophage, but not both, may be included in the transducing phage. In practice this means that either the gal or bio genes are picked up.
Mistakes in excision of lambda only occur at a rate of 1 in a million relative to correct excision. Furthermore, the defective excision must generate a segment of headful packaging Type of virus packaging mechanism that depends on the amount of DNA the head of the virus particle can hold (as opposed to using specific recognition sequences) lambda (or l) Specialized transducing phage of Escherichia coli that may insert its DNA into the bacterial chromosome lambda attachment site (attl) Site where lambda inserts its DNA into the bacterial chromosome P1 Generalized transducing phage of Escherichia coli P22 Generalized transducing phage of Salmonella prophage Virus DNA that is integrated into the host chromosome specialized transduction Type of transduction where certain regions of the bacterial DNA are carried preferentially
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