transduction

n.
Transfer of genetic material or characteristics from one bacterial cell to another by the incorporation of bacterial DNA into a bacteriophage.

[From Latin transductus, past participle of trānsdūcere, to transfer. See transducer.]

transductional trans·duc'tion·al adj.

A mechanism for the transfer of genetic material between cells. The material is transferred by virus particles called bacteriophages (in the case of bacteria), or phages. The transfer method differentiates transduction from transformation. In transformation the genetic material (deoxyribonucleic acid) is extracted from the cell by chemical means or released by lysis. See also Bacterial genetics; Bacteriophage; Deoxyribonucleic acid (DNA); Transformation (bacteria).

The transduction mechanism has two features to distinguish it from the more usual mechanism of gene recombination, the sexual process. The most striking feature is the transfer of genetic material from cell to cell by viruses. The second feature is the fact that only a small part of the total genetic material of any one bacterial cell is carried by any particular transducing particle. However, in general transduction, all of the genetic material is distributed among different particles.

Transduction is not accomplished by all bacteriophages. It is done by some that are classified as “temperate.” When such temperate bacteriophages infect sensitive bacteria, some of the bacteria respond by producing more bacteriophage particles. These bacteria donate the transducing material. Other bacteria respond to the infection by becoming more or less permanent carriers of the bacteriophage, in a kind of symbiotic relationship; these are called lysogenic bacteria. Bacteria in this latter class survive the infection, and it is among these that transduced cells are found. The proportion of bacteria in any culture that responds to infection in either manner can be influenced by the particular environment at the time of infection. See also Lysogeny.

Certain phages carry out a more restricted kind of transduction. They carry only a specific section of bacterial genetic material; they transduce only a few genes. Retroviruses carry out specific or restricted transduction. It has long been known that these viruses can cause the formation of tumors (oncogenesis) in animals. It is now known that these viruses exchange a small portion of their genome for a mutant cellular gene that has a role in gene regulation or replication. These viruses carrying mutant genes infect cells, causing them to be transformed into tumor cells. See also Animal virus; Retrovirus.

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Transduction is one of three basic mechanisms for genetic exchange in bacteria. Like transformation and conjugation, transduction allows the movement of genetic information from a donor cell to a recipient. Unlike the other mechanisms, however, transduction requires the participation of a type of virus called a bacteriophage in order to accomplish this movement. While transduction has been studied in the laboratory since the 1950s, more recently scientists have shown that the process also occurs in nature and probably plays an important role in the evolution of bacteria.

While transduction is common to many bacteria (but not all), the processes can be divided into two basic mechanisms. Generalized transduction tends to transfer all bacterial genes with similar frequencies, or number of cells genetically altered as a function of the total number of potential recipient cells. Specialized transduction tends to transfer only specific genes. It is the life cycle of the particular virus involved in transduction that determines which mechanism will occur, because the transduction of bacterial chromosomal genes is, in fact, the result of an error in the mechanism for viral replication. Thus, to understand the processes of transduction, one must understand the basic mechanisms of viral replication.

Generalized Transduction

Generalized transduction is usually mediated by certain lytic viruses. A lytic virus is one that normally infects a host cell and redirects the resources of the host away from its own cellular replication toward viral replication and the eventual lysis, or breaking open, of the bacterial cell. Typical lytic viruses use the transcription and translation mechanisms of the host cell to express viral genes.

Viral genes are expressed in a specific order. First, the genes needed to hijack host cell metabolism are expressed. Next, the viral genetic information is copied many times. The third step involves transcription and translation of viral genes for protein components. The fourth step requires the insertion of viral DNA into the capsid, the protein shell of the virus. Finally, the virus expresses genes needed to break open the host cell, releasing hundreds of new viral particles that move on to infect new host cells.

Among the best studied of the generalized transducing viruses is P22, a virus of the bacterium Salmonella choleraesuis (subspp. typhimurium). In the course of its normal life cycle, the virus binds to a specific receptor on the surface of the bacterial cell, injects the viral DNA into the host cell, and begins to express viral genes. The ends of the viral DNA are redundant; that is, they have small sections in which the genetic information is duplicated. The DNA repair enzymes of the host cell joins these ends so that the viral DNA forms a circle of double-stranded DNA. A DNA nuclease enzyme then makes a nick in one of the strands of DNA. The unbroken strand then serves as a template, allowing the broken strand to extend itself. This forms a long stretch of DNA that repeats the genome of the virus many times.

The long repeating sequence of DNA is known as a concatamer. The genes for viral capsid proteins (the proteins that make up the coat of the virus) are then expressed, and empty viral capsids are produced. Next, a viral DNA-cutting enzyme cuts the concatamer of DNA at a specific sequence. Once the concatamer is cut, the end of the DNA is pushed into an empty viral capsid until the viral head is full. A new empty capsid then positions itself on the end of the concatamer and the process is repeated until all the viral DNA is packaged.

Generalized transduction occurs when the enzyme that normally cuts the concatamer cuts the host chromosome instead. The viral capsids cannot distinguish viral DNA from host DNA; thus, once the initial cut is made, the empty capsids are filled with host chromosomal DNA instead of viral DNA. Since each empty capsid picks up where the last ended, it is possible for all the genes on the host chromosome to be packaged at similar rates: thus, this process is generalized. When the host cell breaks open, the viral capsids containing host chromosomal DNA, now called transducing particles, are able to bind to a new host cell. The DNA they carry is injected into the new host, but since there are no viral genes included on the injected DNA, the newly infected host cell is not killed. If the original host cell (the donor) was genetically different from the new host cell (the recipient), the DNA recombination enzymes of the recipient cell will insert the new genes in place of the old, thus altering its own genetic makeup. This genetic change is integrated into the recipient genome and is passed on to future progeny cells.

Specialized Transduction

Specialized transduction results in the movement of only specific genes. The viruses that carry out specialized transduction are called lysogenic viruses. They have a mechanism for replication that is different from that of generalized transduction, for they integrate their DNA directly into the chromosome of its host's genome. Each time the host chromosome is duplicated, so is the integrated viral DNA. In many cases these viruses express genes that keep the viral DNA dormant; that is, the virus does not immediately replicate.

The best characterized model for a specialized transducing virus is phage Lambda of Escherichia coli. The Lambda virus begins its life cycle in much the same way as P22. A tail fiber on the end of the virus specifically binds to a receptor, called the maltose binding protein, on the surface of the E. coli cell. The viral DNA is then injected into the host cell. On the chromosome of the virus is a section of DNA that is almost identical to a DNA sequence found on the bacterial chromosome. The recombination enzymes of the E. coli host break and rejoin the viral DNA and host DNA together at this site, thus integrating the virus genome into the chromosome of the host bacterium.

A viral gene, the repressor gene, is then expressed and keeps the virus from activating its own replication. This integrated virus, called a prophage, can be maintained stably as a part of the host chromosome as long as the host cell remains healthy. If the host cell becomes damaged, enzymes are activated that destroy the Lambda repressor protein. Without this protein, the viral DNA will break out of the host chromosome and begin to replicate itself, much as a lytic virus does. Ultimately, the virus particles are packaged, released, and move on to infect a new host cell.

Specialized transduction occurs when the enzyme that cuts the viral DNA out of the host chromosome makes a mistake and cuts in the wrong place, removing some, but not all, of the viral genes. Since Lambda capsids fill by a "headful" mechanism, small bits of the host chromosome are packaged along with part of the viral genes. These viral particles are called defective particles because some of the viral genes are missing in the package and thus, when the virus infects a new host cell, not all the genes needed for viral replication are present. Lacking the ability to replicate, the virus cannot kill its new host. Because of this mechanism of viral packaging, specialized transducing viruses can pick up genes only on either side of the site where the virus integrates into the bacterial chromosome. Thus, while generalized transducing viruses can move any genes, specialized tranducing viruses move only specific genes.

Uses in Research

Generalized transducing viruses are the most useful in mapping bacterial chromosomal genes. Since the amount of DNA that is packaged by the virus is determined by the size of the head of the virus, each viral particle holds the same amount of DNA. The initial cutting of the host chromosome is a random event, giving all genes approximately the same probability of being packaged and transferred. Each piece of DNA that is packaged will be the same length, meaning that the closer together two genes are, the higher the probability that the two genes will be present on the same fragment of packaged DNA. In other words, the closer together the genetic markers are, the higher the frequency of cotransduction. Therefore the distance between closely linked chromosomal genes can be calculated by measuring the frequency that two genes or genetic markers are cotransduced.

When the distance between two genes is greater than the size of the viral genome, it is physically impossible for the two genes to be packaged in the same viral capsid. Thus, these genes are said to be "unlinked" with regard to viral mapping. Since most transducing viral capsids can hold only from about fifteen to fifty genes, transductional mapping of bacterial chromosomal genes is most effective for genes that are relatively close to one another.

Historically, viruses, including transducing viruses, have played an important role in defining the basic principles of molecular biology. Perhaps the most important contribution to the study of transduction was that made by Alfred Hershey and Martha Chase. During the 1940s and 1950s there was still a great deal of controversy over whether DNA or protein was the genetically inheritable material. Hershey and Chase recognized that the simplicity of the virus, consisting of DNA wrapped in a protein coat, was the ideal model to directly address the question of the basis for inheritance.

They began their experiments by growing viruses on host bacteria in media containing radioactive forms of sulfur and phosphorus. The radioactive sulfur labeled the protein components of the virus, while the radioactive phosphorus labeled the DNA portions. This allowed them to independently track the protein and DNA. After separating the radioactively labeled viruses from their host cells, they used the viruses to infect host bacteria that were not radioactively labeled. After infection, they separated the bacterial cells from the growth media. Radioactive phosphorus (viral DNA) was found inside the host cells, while the radioactive sulfur (viral proteins) was found outside the cell. This indicated that only the DNA of the virus enters the host cell, while the protein was left on the outside.

Further details of their experiment make the case for DNA even more strongly. When viruses are grown on bacteria in a thin layer on the surface of agar plates, each virus will create a clear spot called a plaque, indicating infection. Hershey and Chase had two different mutants of their virus that resulted in plaques that looked different from the normal viral infection.

When the infected bacteria were replated, normal plaques were seen, indicating that the two different new mutants had both infected the same host cell and that recombination between the virus DNA occurred within, making a virus that had repaired both mutations. Consequently, since only DNA had entered the host cells and genetic change had occurred in the viruses, DNA had to be the inheritable material. Proteins could not be the source of inheritance because the viral proteins never entered the host cells.

Bibliography

Curtis, Helen, and N. Susan Barnes. Invitation to Biology, 5th ed. New York: WorthPublishers, 1994.

Ingraham, John, and Catherine Ingraham. Introduction to Microbiology, 2nd ed. PacificGrove, CA: Brooks/Cole Publishing, 1999.

Madigan, Michael T., John Martinko, and Jack Parker. Brock Biology of Microorganisms, 10th ed. Upper Saddle River, NJ: Prentice Hall, 2000.

Streips, Uldis N., and Ronald E. Yasbin. Modern Microbial Genetics, 2nd ed. Hoboken, NJ: John Wiley & Sons, 2002.

—Gregory Stewart

Transduction is the process by which a vector (usually a bacteriophage) carries DNA from one bacterium to another bacterium. It can be used experimentally to map bacterial genes.

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The transfer of a genetic fragment from one bacterium to another by bacteriophage.

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