Transduction is what?

были описаны в разное время. Transformation and transduction have been described at different times. The latter was opened in 1952. In this article we will consider what kinds of transduction exist, what features they possess and where this phenomenon is used.

First studies

In his student years N. Zinder, working in the laboratory of Lederberg, was studying the presence of conjugation in Salmonella typhimurium. He used 20 monaauxotrophic strains of it. When pairing them together, the scientist tried to detect prototrophic offspring. In 9 cases of 79 combinations, clones of such cells were detected. Due to the fact that none of the original strains formed repertants 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 Davis's experiment using a U-shaped tube separated by a glass filter that did not allow cells of microorganisms to pass through. The study used strains of Salmonella typhimurium 2Ahis- and 22Atrp-. A culture of strain 22A was introduced into one branch of the test tube, in the other - 2A. Both were in a concentration of 1 • 108 cells / ml. After a period of incubation, the part containing the 22A strain revealed prototrophic cells. They were formed at a frequency of 1 • 10-5. In another branch of the tube, prototrophic cells were absent.

The results of the experiment did not support the hypothesis of conjugation transfer of information in strains 22A and 2A.

Transduction: Experience

During subsequent testing, it was found that strain 22A was infected with phage P22. It is able to infect and lyse cells of Salmonella typhimurium 2A. Penetrating through the filter, he infected cells, reproduced and dissolved them. At the same time, a filtering agent was released (as Lederberg and Zinger called it). He, in turn, penetrated through the glass. Under the influence of the 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 secreted.

In particular, the ability to synthesize tryptophan was revealed. It was determined that the activity of the filtering agent is not lost during the treatment with its DNase. This eliminated the possibility of transformation. It was established that the properties of the filtering agent are identical to those of phage P22. From this it was concluded that the latter carries information from strain 22A to 2A, which acted as evidence of the hereditary role of nucleic acids.

Transduction as a concept was introduced to refer to this phenomenon of the transfer of genetic data.

Process Specificity

специфичное явление, при котором осуществляется перенос генетической информации от клетки-донора к клетке-реципиенту при помощи фага . Transduction is a specific phenomenon in which genetic information is transferred from the donor cell to the recipient cell by phage . It is based on the fact that during the multiplication of phages, their particles can form. Together with or instead of DNA, they include other fragments and have been called transducers. According to their adsorption properties and morphology, they are analogous to ordinary phage virions. However, if they infect new cells, the genetic determinants of the host's past are transferred. The mechanism of transduction, thus, is as follows. For the transfer of genetic determinants, it is necessary to multiply the phage on the cells of the donor-donor. After this, the resulting phagolysate is introduced into the recipient cells. The choice of transductants is carried out on selective media. In them, the original recipient cells can not grow.

Classification

When studying the phenomenon it was found that some phages have the ability to transmit different genes, while others - only specific. In accordance with this, two types of data transfer are distinguished:

1. Generalized transduction . явление предполагает передачу любого фрагмента хромосомы. This phenomenon involves the transmission of any fragment of the chromosome.
2. Specific transfer. In this case, only certain genes are transmitted.

Nonspecific (generalized) process

What features does transduction have in this case ? явление имеет место при наличии вируса, выступающего только в качестве переносчика материала. This phenomenon occurs in the presence of a virus that acts only as a carrier of the material. One of them is the aforementioned phage P22. Lederberg and Zinger worked with him. PBS1 B. Subtilis, Р1 E. Coli и проч. In addition, the phages with which generalized transduction is performed are PBS1 B. Subtilis, P1 E. Coli and so on. 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 in the course of their development and after the induction of the prophage. A certain amount of particles is packed with bacterial DNA. The size of its fragments is not more than the size of the head. In this case, various parts of the bacterial chromosome can be packed. Phage particles that carry DNA fragments are called defective.

Recombination

If the phagolysate, in which both normal and transducing fragments are present, treat 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. At the same time, there is no circularization. In other words, linear fragments from donor DNA recombine with DNA from the recipient. This process is controlled by the recA-gene. Thus, it is a general homologous recombination, which is accomplished through reciprocal (mutual) exchange of the corresponding homologous parts.

Specific process

Transduction of this type was discovered in 1956. Its peculiarity is that each phage transmits a very limited, specific region of the chromosome. If the phage is a "passive" transporter of genetic material, while recombination proceeds according to general patterns, then in this case it not only transmits information, but also ensures its entry into the chromosome. The most famous example is the process performed by phage λ. It is capable of infecting E. coli cells with further integration of DNA into the genome. This moderate phage in the lysogenesis of bacteria in site-specific recombination (which breaks and crosses the chains of the molecule) is built into the chromosome only at one site - between the bio- and gal-loci. Presumably, this is due to the "wrong" formation of the loop during the disintegration of the prophage. As a consequence, the region of the genome adjacent to it is cleaved from the structure of the chromosome and becomes a free phage. The embedded material is capable of replacing up to 1/3 of the genetic information. After packing the DNA of the phage, defective particles are formed.

Directions of use

The transduction of bacteria can be used:

1. When constructing strains of a specific genotype, isogenic, in particular. In this case, a small amount of transmitted particles provides the advantage of the transduction process before the conjugation process. The isogenic strains formed by generalizing transfer of material differ only in that part of the chromosome that is transferred by the defective phage.
2. To accurately map the genes of bacteria, establish the order of their placement in operons, the fine structure of certain determinants. This is done through a complementation test. It has been established 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 analogous mutants that are incapable of synthesizing the enzyme. It is not known whether they differ genetically. To identify the genotype, transduction is carried out, that is, the phage multiplication on cells in one population, followed by infection of the elements of the second. If large and small colonies of transducers are formed on the selective medium, it is concluded that the location of the mutations is in different genes.
3. When transducing plasmids and short fragments of donor chromosomes.

Additionally

The literature also often uses such a notion as "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.