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This lecture describes
- Bacterial transduction
- Generalized and specialized transduction
Transduction
Transduction
Transduction refers DNA transfer from one bacterial cell to another with the help of bacteriophages. Viruses can transfer host genes in two ways. In the first, called generalized transduction, DNA derived from virtually any portion of the host genome is packaged inside the mature virion in place of the virus genome. In the second, called specialized transduction, DNA from a specific region of the host chromosome is integrated directly into the virus genome—usually replacing some of the virus genes. This occurs only with certain temperate viruses. The transducing bacteriophages (otherwise referred as transducing particles) in both generalized and specialized transduction is usually non-infectious (did not kill the host) because bacterial genes have replaced all or some necessary viral genes.
Not all phages can transduce, and not all bacteria are Transducible.Desulfovibrio, Escherichia, Pseudomonas, Rhodococcus, Rhodobacter, Salmonella, Staphylococcus, andXanthobacter, as well asMethanothermobacterthermautotrophicus, a species ofArchaeaare some of the common prokaryotes having high frequency of transduction.
Generalized Transduction
Generalized Transduction
In generalized transduction, virtually any gene on the donor chromosome can be transferred to the recipient. Generalized transduction was first discovered and extensively studied in the bacterium Salmonella enterica with phage P22 and has also been studied with phage P1 in Escherichia coli. Zinder & Lederberg (1952) first identified generalized transduction in Salmonella typhimurium.
When a bacterial cell is infected with a phage, the lytic cycle may occur. However, during lytic infection, the enzymes responsible for packaging viral DNA into the bacteriophage sometimes package host DNA accidentally. The result is called a transducing particle. These cannot lead to a viral infection because they contain no viral DNA, and are said to be defective. On lysis of the cell, the transducing particles are released along with normal phages that contain the virus genome. Consequently, the lysate contains a mixture of normal virions and transducing particles. When this lysate is used to infect a population of recipient cells, most of the cells are infected with normal virus. However, a small proportion of the population receives transducing particles that inject the DNA they packaged from the previous host bacterium. Although this DNA cannot replicate, it can recombine with the DNA of the new host. Typically, only about 1 cell in 106 to 108 is transduced for a given marker.
Youtube video about generalized transduction
Specialized Transduction
Specialized Transduction
Generalized transduction allows the transfer of any gene from one donor to recipient, but at a low frequency. In contrast, specialized transduction allows extremely efficient transfer, but is selective and transfers only a small region of the bacterial chromosome. In the first case of specialized transduction (discoveredby E. Laderberg) was the galactose genes transduced by the temperate phage (lambda phage) in E. coli.
Steps in Specialized transduction
When lambda phage lysogenizes a host cell, the phage genome is integrated into the E. colichromosome (called prophage) at a specific site between gal and bio operons.
After insertion, viral DNA replication is under control of the bacterial host chromosome.
Upon induction, the viral DNA separates from the host DNA by a process that is the reverse of integration called excision.
Usually, the lambda DNA is excised precisely, but occasionally, the phage genome is excised incorrectly. Some of the adjacent bacterial genes to one side of the prophage (for example, the galactose operon) are excised along with phage DNA. At the same time, some phage genes are left behind in the bacterial chromosomal DNA.
This kind of altered lambda is referred as λdgal (referring defective galactose operon). λdgal is a defective phage because of the lost phage genes during mistaken excision.
Recall how the insertion and excision of prophage took in bacterial chromosome (left). Improper excision leads to specialized transduction.
However, a viable lambda virion known as a helper phage can provide those functions missing in the defective particle. When cells are coinfected with λdgaland the helper phage, the culture lysate contains a few λdgalparticles mixed in with a large number of normal lambda virions.
Upon infection, λdgal can introduce the gal genes to the recipients through homologous recombination.
Alteration of the phenotype of a host cell by lysogenization is called phage conversion. When a normal (that is, nondefective) temperate phage lysogenizes a cell and becomes a prophage, the cell becomes immune to further infection by the same type of phage. Such immunity may itself be regarded as a change in phenotype.
Video describing Specialized transduction (Youtube)
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