Generalized transduction

By what mechanisms can a phage carry out generalized transduction? In 1965, K. Ikeda and J. Tomizawa threw light on this question in some experiments on the E. coli phage P1. They found that when a donor cell is lysed by P1, the bacterial chromosome is broken up into small pieces. Occasionally, the newly forming phage particles mistakenly incorporate a piece of the bacterial DNA into a phage head in place of phage DNA. This event is the origin of the transducing phage.

A phage carrying bacterial DNA can infect another cell. That bacterial DNA can then be incorporated into the recipient cell's chromosome by recombination (Figure 5-27). Because genes on any of the cut-up parts of the host genome can be transduced, this type of trans-duction is by necessity of the generalized type.

Phages P1 and P22 both belong to a phage group that shows generalized transduction. P22 DNA inserts into the host chromosome, whereas P1 DNA remains free, like a large plasmid. However, both transduce by faulty head stuffing.

Generalized transduction can be used to obtain bacterial linkage information when genes are close enough that the phage can pick them up and transduce them in

5.5 Transduction

Recipient Bacterium

Transduced bacterium

Figure 5-27 The mechanism of generalized transduction. In reality, only a very smal minority of phage progeny (1 in 10,000) carry donor genes.

Recipient bacterium

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Transduced bacterium

Figure 5-27 The mechanism of generalized transduction. In reality, only a very smal minority of phage progeny (1 in 10,000) carry donor genes.

Recipient bacterium

a single piece of DNA. For example, suppose that we wanted to find the linkage between met and arg in E. coli. We could grow phage P1 on a donor met+ arg+ strain, and then allow P1 phages from lysis of this strain to infect a mer arg~ strain. First, one donor allele is selected, say, met+. Then, the percentage of met+ colonies that is also arg+ is measured. Strains transduced to both met+ and arg+ are called cotransductants. The greater the cotransduction frequency, the closer two genetic markers must be (the opposite of most mapping correlations). Linkage values are usually expressed as cotransduction frequencies (Figure 5-28).

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