Transduction

Index Introduction Discovery Linkage data from transduction Specialized transduction Lytic and Lysogenic Cycle

Introduction Transduction is the process by which DNA is transferred from one bacterium to another by a virus. It also refers to the process whereby foreign DNA is introduced into another cell via a viral vector. Transduction does not require physical contact between the cell donating the DNA and the cell receiving the DNA (which occurs in conjugation), and it is DNAse resistant (transformation is susceptible to DNAse). Transduction is a common tool used by molecular biologists to stably introduce a foreign gene into a host cell's genome.

Discovery Transduction was discovered by Norton Zinder and Joshua Lederberg at the University of Wisconsin Madison in 1951. 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 phe− trp− tyr−, and the other was met− his−. 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.

Linkage data from transduction Generalized transduction allows us to derive linkage information about bacterial genes when markers are close enough that the phage can pick them up and transduce them in a single piece of DNA. For example, suppose that we wanted to find the linkage between met and arg in E. coli. We might set up a cross of a met+ arg+ strain with a met− arg− strain. We could grow phage P1 on the donor met+ arg+ strain, allow P1 to infect the met− arg− strain, and select for met+ colonies. Then, we could note the percentage of met+ colonies that became arg+. Strains transduced to both met+ and arg+ are called cotransductants. Linkage values are usually expressed as cotransduction frequencies (Figure). The greater the cotransduction frequency, the closer two genetic markers are.

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

Lytic and Lysogenic Cycle The lytic cycles one of the two cycles of viral reproduction, the other being the lysogenic cycle. The lytic cycle results in the destruction of the infected cell and its membrane. A key difference between the lytic and lysogenic phage cycles is that in the lytic phage, the viral DNA exists as a separate molecule within the bacterial cell, and replicates separately from the host bacterial DNA. The location of viral DNA in the lysogenic phage cycle is within the host DNA, therefore in both cases the virus/phage replicates using the host DNA machinery, but in the lytic phage cycle, the phage is a free floating separate molecule to the host DNA.