What is bacteriophage transduction?

What is phage transduction?

Is a process where by bacteriophage(phage) is able to carry/transfer bacterial genes from one bacterium to another. Sometimes transduction led to transfer of Antibiotics Resistance Genes (ARGs) 
Alternative cell cycles of a temperate phage and its host. (After A. Lwoff, Bacteriological Reviews 17, 1953, 269.)


How was transduction discovered?

In 1951, Joshua Lederberg and Norton Zinder were testing for recombination in the bacterium Salmonella typhimurium by using the techniques that had been successful with E. coli. The researchers used two different strains: one was phetrptyr, and the other was methis. (We won’t worry about the nature of these markers except to note that the mutant alleles confer nutritional requirements.) When either strain was plated on a minimal medium, no wild-type cells were observed. However, after the two strains were mixed, wild-type cells appeared at a frequency of about 1 in 105. Thus far, the situation seems similar to that for recombination in E. coli.

However, in this case, the researchers also recovered recombinants from a U-tube experiment, in which cell contact (conjugation) was prevented by a filter separating the two arms. By varying the size of the pores in the filter, they found that the agent responsible for recombination was about the size of the virus P22, a known temperate phage of Salmonella. Further studies supported the suggestion that the vector of recombination is indeed P22. The filterable agent and P22 are identical in properties of size, sensitivity to antiserum, and immunity to hydrolytic enzymes. Thus, Lederberg and Zinder, instead of confirming conjugation in Salmonella, had discovered a new type of gene transfer mediated by a virus. They called this process transduction. In the lytic cycle, some virus particles somehow pick up bacterial genes that are then transferred to another host, where the virus inserts its contents. Transduction has subsequently been shown to be quite common among both temperate and virulent phages.

Types of phage transduction

There are two kinds of transduction: generalized and specialized.

Generalized transducing phages can carry any part of the chromosome, whereas specialized transducing phages carry only restricted parts of the bacterial chromosome.

How does Generalized transduction occur?

when a donor cell is lysed by bacteriophage virions, the bacterial chromosome is broken up into small pieces. Occasionally, the 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.

Because the phage coat proteins determine a phage’s ability to attack a cell, transducing phages can bind to a bacterial cell and inject their contents, which now happen to be donor bacterial genes. When a transducing phage injects its contents into a recipient cell, a merodiploid situation is created in which the transduced bacterial genes can be incorporated by recombination. This explanation was reported in 1965, by K. Ikeda and J. Tomizawa using P1 phage (E. coli phage)

How does specialized transduction occur?

The recombination between regions of phage and the bacterial chromosome is catalyzed by a specific enzyme system. This system normally ensures that the phage integrates at the same point in the chromosome and, when the lytic cycle is induced (for instance, by ultraviolet light), it ensures that the prophage excises at precisely the correct point to produce a normal circular phage chromosome. Very rarely, excision is abnormal and can result in phage particles that now carry a nearby gene and leave behind some phage genes.

Lambda is a good example of a specialized transducing phage. As a prophage, λ always inserts between the gal region and the bio region of the host chromosome . In transduction experiments, λ can transduce only the gal and bio genes. In λ, the nearby genes are gal on one side and bio on the other. The resulting particles are defective due to the genes left behind and are referred to as  (λ-defective gal), or λdbio. These defective particles carrying nearby genes can be packaged into phage heads and can infect other bacteria. In the presence of a second, normal phage particle in a double infection, the λdgal can integrate into the chromosome at the λ-attachment site. In this manner, the gal genes in this case are transduced into the second host. Because this transduction mechanism is limited to genes very near the original integrated prophage, it is called specialized transduction.
Specialized transduction mechanism in phage λ. (a) A lysogenic bacterial culture can produce normal λ or, rarely, an abnormal particle, λdgal, which is the transducing particle. (b) Transducing by the mixed lysate can produce gal+ transductants by the coincorporation of λdgal and a λ wild type.



Transduction occurs when newly forming phages acquire host genes and transfer them to other bacterial cells. Generalized transduction can transfer any host gene. It occurs when phage packaging accidentally incorporates bacterial DNA instead of phage DNA. Specialized transduction is due to faulty separation of the prophage from the bacterial chromosome, so the new phage includes both phage and bacterial genes. The transducing phage can transfer only specific host genes.

References
An Introduction to Genetic Analysis. 7th edition.Griffiths AJF, Miller JH, Suzuki DT, et al.New York: W. H. Freeman; 2000.

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