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Transducing Normal phage phage

Recipient cell

Transductant

Donor bacterium

Transducing Normal phage phage

Recipient cell

Transductant

8.27 Genes can be transferred from one bacterium to another through generalized transduction.

Recombination

Recombination

Excisie Bacteriofaag

|.. .which are used to infect a recipient strain of bacteria that is a- b- c

Conclusion: Genes located close to each other are more likely to be cotransduced, so the rate of cotransduction is inversely proportional to the distances between genes.

|.. .which are used to infect a recipient strain of bacteria that is a- b- c

Conclusion: Genes located close to each other are more likely to be cotransduced, so the rate of cotransduction is inversely proportional to the distances between genes.

I 8.28 Generalized transduction can be used to map genes.

prophage may imperfectly excise from the bacterial chromosome, carrying with it a small part of the bacterial DNA adjacent to the site of prophage integration. A phage carrying this DNA will then inject it into another bacterial cell in the next round of infection. This process resembles the situation in F' cells, where the F plasmid carries genes from one bacterium into another (see Figure 8.16).

One of the best-studied examples of specialized trans-duction is in bacteriophage lambda (X), which integrates into the E. coli chromosome at the attachment (att) site. The phage DNA contains a site similar to the att site; a single crossover integrates the phage DNA into the bacterial chromosome ( FIGURE 8.29a). The X prophage is excised through a similar crossover that reverses the process (< Figure 8.29b and c).

An error in excision may cause genes on either side of the bacterial att site to be excised along with some of the phage DNA (IFIGURE8.29d and e). In E. coli, these genes are usually the gal (galactose fermentation) and bio (biotin biosynthesis) genes. When a transducing phage carrying the gal gene infects another bacterium, the gene may integrate into the bacterial chromosome along with the prophage ( FIGURE 8.29f), giving the bacterial chromosome two copies of the gal gene ( FIGURE 8.29g). These transductants are unstable, because the prophage DNA may excise from the chromosome, carrying the introduced gene with it. Stable transductants are produced when the gal gene in the phage is exchanged for the gal gene in the chromosome through a double crossover ( FIGURE 8.29h).

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