Transduction - what is it?

Transformation and transduction was described at different times. The latter was opened in 1952 In the article, what are the types of transduction, what features they possess and where used this phenomenon.

First research

In his student years, N. Zinder, working in the laboratory Lederberga, studied the presence of conjugation from Salmonella typhimurium. He used 20 monoisotopic its strains. In pair their mixing, the scientist tried to find protothronos offspring. 9 times out of 79 combinations were identified clones of such cells. Due to the fact that none of the original strains is not formed of reportnow on minimal medium, Singer concluded that between them is conjugation, during which genetic information is transferred.

To confirm the hypothesis, together with the Lederberga he repeated the experiment of Davis using a U-shaped tube separated by a filter of glass, do not miss the cells of microorganisms. The study used strains of Salmonella typhimurium 2Ahis– and 22Atrp–. In one branch of the tube to maintain culture of strain 22A, in another – 2A. Both were at a concentration of 1 * 108 cells/ml. After incubation period in part of the contents of the 22A strain was identified prototroph cells. They were formed with a frequency of 1•10-5. In the other branch of the tube prototroph cells were absent.

The results of the experiment did not confirm the hypothesis conjugational transferring information from strains 22A and 2A.

Transduction: the experience

At the subsequent verification it was found that the 22A strain is infected with phage P22. It is able to infect and lyse cells Salmonella typhimurium 2A. Penetrated through the filter, the infected cells were reproduced and dissolved them. Along with this was the release of a filter medium (as it was called, Lederberg and Singer). He, in turn, penetrated through the glass. Under the influence of a filter medium some cells in the strain 22A has acquired a specific hereditary characteristics. They were similar to those present in strain 2A, which were allocated FA.

In particular, revealed the ability to synthesize tryptophan. It was determined that the activity of the filtering agent does not get lost in the processing of his Dnazol. It excluded the possibility of transformation. It was found that the properties of a filter medium are identical to the characteristics of phage P22. From this it was concluded that the latter transfers the information from 2A to 22A strain that acted as evidence of genetic role of nucleic acids.

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

Specific process

Transduction – - specific phenomenon, which is The transfer of genetic information from donor cells to the cell-recipient with phage. It is based on the fact that during multiplication of phages can be formed of their particles. Together with DNA or instead they include other fragments and was named transducers. According to their adsorption properties and morphology they are similar to normal takovym the virions. However, if they infect new cells, they transfer the genetic determinants of the last owner. The mechanism of transduction, therefore, is as follows. For the transmission of genetic determinants is necessary to conduct multiplication of phage on cells, stemma donor. This is followed by the resulting phage lysate to enter the cells of the recipient. The choice of transductant is carried out on selective media. In those cases, the original cells of the recipient can not grow.

Classification

In the study of the phenomenon it was found that some phages have the ability to transfer a variety of genes, and others – only specific. In line with this, there are two types of data transfer:

1. Generalized Transduction. This phenomenon involves the transfer of any portion of the chromosome.
2. Specific transfer. In this case, only certain genes are transferred.

Non-specific (generalized) the process

What features in this case has a Transduction? This phenomenon occurs in the presence of the virus, acting only as a carrier material. One of them mentioned phage P22. Worked with him, Lederberg and Singer. In addition, the phages, which is a generalized Transduction, PBS1 B. Subtilis, E. Coli R1, and so on. The process proceeds with the participation of their defective particles. The formation of these elements occursduring reproduction of the phages, accompanied by the disintegration of the bacterial chromosomal DNA. It should be noted that it may occur in lytic development after prophage induction. In a certain number of particles when it is packaged bacterial DNA. The size of the fragments is not larger than the size of the head. To Packed can different parts of the bacterial chromosome. Phage particles which carry the DNA fragments are referred to as defective.

Recombination

If the phage lysate, which contains both normal and transducers fragments, treating the cells of the recipient strain, the infection with normal phage usually leads to lysis (dissolution). But some cells infect defective components. In the structure receives the short section of double-stranded DNA from a donor. In this circularisation not happening. In other words, the linear fragments from the donor DNA recombine with DNA from the recipient. This process is controlled by the recA gene. Thus, it represents a common homologous recombination by reciprocal (mutual) exchange of appropriate homologous parts.

Specific process

Transduction of this type was discovered in 1956, Its feature is that each phage has a very limited, specific region of the chromosome. If nonspecific transduction, the phage-passive vector of genetic material and recombination proceeds by the General regularities, in this case, it not only transmits information, but also ensures its entry into the chromosome. The most famous example is the process undertaken by the phage λ. It is able to infect cells of E. Coli with the further integration of the DNA into the genome. This temperate phage with lysogenization bacteria at the site-specific recombination (in which the gap and cross connection chains of molecules) embedded in a chromosome exclusively in one area – between bio - and gal-loci. Presumably, this is due to the "wrong" formation of a loop during the disintegration of the prophage. As a consequence, the region of the genome, adjacent, Vasalemma from the structure of the chromosome and proceeds in the free phage. Embedded material is able to replace up to 1/3 of genetic information. After packaging the DNA of phage are formed defective particles.

Directions

Transduction of bacteria can be used:

1. When constructing strains of a specific genotype, isogenic, in particular. In this case, a small value of the transferred particles has the advantage that the transduction process before conjugation. Isogenic strains generated by using generalizations transfer material, differ only in that area of the chromosome, which is carried by the defective phage.
2. For precise gene mapping bacteria, determine the order of their location in the operon, fine structure of some determinants. This is done by complementation test. It was found that for the synthesis of certain products requires the work of several genes. For example, a process determined by the components of the structures a and b. For example, there are 2 phenotypically similar mutant, which is incapable of synthesis of the enzyme. It is unknown whether they differ genetically. To identify the genotype is transduction, i.e. the propagation of a phage on cells in one population followed by infection with the elements of the second. If during insertion into selective medium and formed large and small colonies translucent, it is concluded that the localization of mutations located in different genes.
3. When transducible plasmids and short fragments of chromosomes of the donor.

Details

In the literature is often used also such a thing as "signal transduction". It represents the signal transmission. The process takes place in a specific pattern. First, the external agent interacts with the cell receptor. After that activated effector molecule. It is located in the membrane responsible for the formation of secondary mediators. Their generation promotes activation of protein targets. They, in turn, trigger the following intermediaries.