Transformation and transduction have been described at different times. The latter was discovered in 1952. In the article we consider what types of transduction exist, what features they possess and where this phenomenon is used.
First research
In his student years, N. Zinder, working in the Lederberg laboratory, was studying the presence of conjugation in Salmonella typhimurium. He used 20 monoauxotrophic strains of it. When mixing them in pairs, the scientist tried to detect prototrophic offspring. In 9 cases out of 79 combinations, clones of such cells were detected. Due to the fact that none of the initial strains formed repertories on a minimal medium, Zinger concluded that conjugation is carried out between them, during which hereditary information is transmitted.
To confirm the hypothesis, together with Lederberg, he repeated the Davis experiment using a U-shaped tube separated by a glass filter that did not allow the passage of microorganism cells. The study used strains of Salmonella typhimurium 2Ahis– and 22Atrp–. The culture of strain 22A was introduced into one branch of the tube, and 2A into the other. Both were at a concentration of 1 • 108 cells / ml. After an incubation period, prototrophic cells were detected in the portion containing strain 22A. They formed with a frequency of 1 • 10-5. Prototrophic cells were absent in the other branch of the tube.
The experimental results did not confirm the hypothesis of conjugative transfer of information in strains 22A and 2A.
Transduction: Experience
A subsequent check revealed that strain 22A was infected with phage P22. It is able to infect and lyse Salmonella typhimurium 2A cells. Penetrating through the filter, he infected the cells, reproduced and dissolved them. Along with this, the release of the filtering agent (as it was called Lederberg and Zinger) took place. He, in turn, penetrated through the glass. Under the influence of a filtering agent, some cells in strain 22A acquired specific hereditary traits. They were similar to those that were present in strain 2A from which FA was isolated.
In particular, the ability to tryptophan synthesis was revealed. It was determined that the activity of the filtering agent is not lost during the processing of its DNase. This ruled out the possibility of transformation. It was found that the properties of the filtering agent are identical to the characteristics of phage P22. From this it was concluded that the latter transfers information from strain 22A to 2A, which served as evidence of the hereditary role of nucleic acids.
Transduction as a concept was introduced to denote this phenomenon of genetic data transmission.
Process specifics
Transduction is a specific phenomenon in which genetic information is transferred from a donor cell to a recipient cell using phage . It is based on the fact that during the propagation of phages their particles can form. Together with or instead of DNA, they include other fragments and have been called transducing. In their adsorption properties and morphology, they are similar to ordinary phage virions. However, if they infect new cells, the genetic determinants of the past host are transmitted. The transduction mechanism, therefore, is as follows. To transmit genetic determinants, it is necessary to multiply phage on the cells of the donor strain. After this, the resulting phagolysate should be introduced into the cells of the recipient. The choice of transductants is carried out on selective media. In them, the original recipient cells cannot grow.
Classification
When studying the phenomenon, it was found that some phages possess the ability to transmit various genes, while others only specific ones. In accordance with this, two types of data transfer are distinguished:
- Generalized transduction . This phenomenon involves the transfer of any fragment of the chromosome.
- Specific transfer. In this case, only certain genes are transmitted.
Nonspecific (generalized) process
What features does transduction have in this case ? This phenomenon occurs in the presence of a virus that acts only as a carrier of material. One of them is the aforementioned phage P22. Lederberg and Zinger worked with him. In addition, the phages by which generalized transduction is carried out are PBS1 B. Subtilis, P1 E. Coli, etc. The process proceeds with the participation of defective particles. The formation of these elements occurs during the reproduction of phages, accompanied by the decay of bacterial chromosomal DNA. It should be noted that their formation can take place both during the lytic development and after the induction of the prophage. In this case, bacterial DNA is packed into a certain number of particles. The size of its fragments is not larger than the size of the head. In this case, various parts of the bacterial chromosome can be packaged. Phage particles that carry DNA fragments are called defective.
Recombination
If the phagolysate, in which both normal and transducing fragments are present, process the cells of the recipient strain, then infection with their normal phage usually leads to lysis (dissolution). But some cells infect defective components. Short parts of double-stranded DNA from the donor enter the structures. In this case, circularization does not occur. In other words, linear fragments from the donor DNA are recombined with the DNA from the recipient. This process is controlled by the recA gene. Thus, it represents a general homologous recombination, which is carried out through reciprocal (mutual) exchange of the corresponding homologous parts.
Specific process
Transduction of this type was discovered in 1956. Its feature is that each phage transmits a very limited, specific region of the chromosome. If in non-specific transduction the phage is a “passive” carrier of genetic material, and recombination proceeds according to general laws, then in this case it not only transfers information, but also ensures its entry into the chromosome. The most famous example is the process carried out by phage λ. It is able to infect E. Coli cells with further integration of DNA into the genome. This moderate phage during bacterial lysogenesis during site-specific recombination (in which the molecule chains are broken and cross-linked) is inserted into the chromosome only on one site - between the bio and gal loci. Presumably, this is due to "incorrect" loop formation during the disintegration of the prophage. As a result, the region of the genome adjacent to it is cleaved from the chromosome structure and becomes a free phage. Embedded material can replace up to 1/3 of genetic information. After packaging of phage DNA, defective particles form.
Directions of use
Bacterial transduction can be used:
- When constructing strains of a certain genotype, isogenic, in particular. In this case, the small size of the transferred particles provides the advantage of the transduction process over the conjugation process. Isogenic strains formed by generalization transfer of material differ only in that part of the chromosome that is transferred by a defective phage.
- For accurate mapping of bacterial genes, establishing the order of their placement in operons, the fine structure of some determinants. This is done through a complementation test. It was found that the synthesis of certain products requires the work of several genes. For example, the process is determined by the components of structures a and b. Suppose there are 2 phenotypically similar mutants that are unable to synthesize the enzyme. It is not known whether they differ genetically. To identify the genotype, transduction is performed, that is, the propagation of phage on cells in one population with subsequent infection of the elements of the second. If large and small colonies of transductants are formed upon removal to a selective medium, it is concluded that the localization of mutations is in different genes.
- When transducing plasmids and short fragments of the donor chromosomes.
Additionally
The literature also often uses the concept of "signal transduction." It is a signal transmission. The process follows a certain pattern. First, the external agent interacts with the cellular receptor. After that, the effector molecule is activated. It is located in the membrane and is responsible for the formation of secondary intermediaries. Their generation promotes the activation of target proteins. They, in turn, launch the following intermediaries.