CONJUGATION AT BACTERIA

From Big Medical Encyclopedia

CONJUGATION AT BACTERIA (Latin conjugatio connection; bacteria) — a form of exchange of genetic material between bacteria at their cellular contact. To. at. it was found in 1946 by J. Lederberg and E. L. Tatum at a research of strains of double and triple auxotroph mutants E. coli K12 (see. Auksotrofnost , Auksotrofny microorganisms ). Mixing cultures of two strains growth of one of which required addition on Wednesday of methionine (Met-) and biotin (Bio-), and for growth of another — threonine (Thr-), a leucine (Leu-) and thiamin (Thi-), and sowing then examples of the mixed cultures on minimum, i.e. free of Wednesday of necessary connections, they found on it colonies of prototrophic bacteria (Met+, Bio+, Thr+, Leu+, Thi+). Frequency of manifestation of such bacteria was equal 1 on everyone 10 7 the sowed parent cells. Further studying showed that they are genetic recombinants, i.e. cells bearing a combination of genes of parent cells (see. the Recombination, at bacteria ). In the subsequent experiments, including electronic and microscopic observations, it was established that an indispensable condition for formation of such recombinants is the direct contact between two genetically differing bacteria (fig. 1).

Fig. 1 Diffraction pattern of the conjugating bacteria: 1 — the donor cell which is in a stage of division (the place of division is noted by shooters); 2 — a cell recipient; 3 — the pairing bridge via which genetic material of the donor is transferred to the recipient; 4 — bacterial flagellums.

During the studying of the bacteria (conjugants) participating in conjugation it is established that bacteria of one strains, so-called F + - bacteria (a bacterium of men's type), at conjugation work as donors of genetic material whereas other, so-called F - - bacteria (a bacterium of female type), are recipients (the letter «F» — the first letter of the word fertility — fertility). Crossings of strains of the donor and recipients (F + X F - ) are always fertile, i.e. lead to emergence of recombinants while crossings of two recipients (F - X F - ) or two donors (F + X F + ) are sterile. The research of process of formation of recombinants showed that at conjugation genetic material is transferred from the donor to the recipient. It allowed to draw a conclusion on existence of sexual differentiation among mutant strains E. coli K12. At the same time the role of a donor cell is limited to transfer to the recipient of genetic material (DNA) then its presence is optional.

The most important distinction between donor bacteria of F + and recipients of F - the fact that the first of them contain the additional genetic element called by a sexual factor, a F-factor, a F-episome (see is. Sexual factor of bacteria ), which is absent in retsipiyentny cells. While transfer of chromosomal genes in crossings like F + X F - happens to rather low frequency (about 10 - 4 — 10 - 5 on one parent cell), the sexual factor is easily transferred to recipients (with a frequency about 0,2 — 1,0) therefore they gain properties of donors. F + - cells spontaneously it is (very rare) or under the influence of some agents physical. or the chemical nature can lose a F-factor. On this basis the conclusion is drawn that a F-factor of F + - cells has extra chromosomal (cytoplasmatic) localization and it is capable to breed quickly relatively irrespective of replication of a bacterial chromosome. This conclusion was completely confirmed by means of physical. - chemical methods.

Significant progress in studying To. at. became possible after allocation from cultures E. coli K12 F + L. Cavalli and W. Hayes of clones of the cells capable to transfer F in crossings - - to cells a certain segment of a chromosome with a frequency of 1000 times higher, than initial bacteria. Appearance of the donors of this kind designated by a Hfr symbol (high frequency of recombination — the high frequency of a recombination, English), is considered as result of inclusion of a F-factor in a bacterial chromosome. In this regard the low frequency of transfer of a chromosome observed in crossing of F + X F - , connect with education in culture of F + - cells of a small number of cells of Hfr as a result of integration of a part of autonomous F-factors in this or that site of a bacterial chromosome. On the other hand, the integrated sexual factor of cells of Hfr is capable to be returned with a certain frequency to an autonomous state, sometimes including at the same time adjacent sites of a bacterial chromosome and giving rise to the third type of the donor cells called by intermediate. Process To. at., rather well studied during the use of donors E. coli K12 of the Hfr type, can be conditionally divided into several stages (stages): 1) accidental collisions of donor cells with recipients after mixing of their cultures and formation of initial (fragile) cellular contacts; 2) formation of effective cellular contacts; 3) mobilization of a chromosome (or a konjyugativny episome) on transfer, i.e. transition of the ring (closed) structure of a molecule of chromosomal (episomny) DNA to line (open) transferable structure; 4) genetic transfer (transfer of a chromosome or episome) and formation of a zygote; 5) formation of recombinants.

An important role in implementation of the initial stages To. at. play, obviously, specific threadlike educations on surfaces of donor cells, including «sexual fibers» (F-fibers, F-drank) which synthesis is controlled by genes of a F-factor.

Fig. 2. Diffraction pattern of a fragment of a bacterial donor cell: 1 — a part of a bacterial cell; 2 — «a sexual fiber» (F-fiber) — proteinaceous education which synthesis is controlled only by genes of a F-factor; the donorspetsifichesky bacteriophages specified by shooters are adsorbed on it; 3 — a bacterial flagellum, free from a donorspetsifichesky bacteriophage.

F-fibers have the proteinaceous nature, have specific antigenic properties and can be easily found at a submicroscopy thanks to ability to adsorb donorspetsifichesky a phage, to the Crimea cells recipients (fig. 2) are insensitive. It is supposed that F-fibers perform the recognitions function of a retsipiyentny bacterium and provide initial contact between a donor cell and a cell recipient, for the Crimea the rupture of the respective sites of their walls and formation of cytoplasmatic pairing «bridge» follows. The subsequent transfer of a chromosome from cells of Hfr in F - - bacteria are carried by strictly oriented and discontinuous (partial) character. Orientation of transfer of a chromosome of the donor depends on the place of integration of a sexual factor. In the place of integration of a F-factor the closed chromosome of the donor is broken off and turns into not closed (linear) structure, gaining ability to be transferred consistently to a retsipiyentny cell. As cells of each strain of Hfr take the strictly constant place of integration of a F-factor, the rupture of their closed chromosome always occurs in the same place, and its transfer begins with the same genetic locus — O-point. At the same time the sexual factor is transferred as the trailer (distal) site of a linkage group, and the recipients who received it can become donors like Hfr. The discontinuous nature of transfer of a chromosome of the donor is caused by accidental ruptures of the sites of a chromosome passing at this or that moment through the conjunction of the conjugating cells. Also artificial interruption of this process by means of various impacts on the conjugating cells is possible. As a result a considerable part of retsipiyentny cells receives not all chromosome of the donor, and only its some fragment that leads to formation of incomplete zygotes (merozygotes). The probability of transfer in a retsipiyentny cell of each specific gene will be that smaller, than further from O-point it is located on a chromosome, i.e. there is a gradient of transgenesis, located on length of a chromosome of the donor (in a linkage group). However the bacterial cell which received at conjugation genetic material from other bacterium (transconjugant) can give rise to a clone of recombinants only in case of the subsequent integration of this material with its chromosome. If the genetic material of the donor transferred to a cell behaves as a plasmid, i.e. remains in a cytoplasmatic state, then such cell is designated as a plasmid transconjugant. Along with a F-factor E. to provide to coli K12 process To. at. also other extra chromosomal elements which received the generalized name of konjyugativny plasmids of bacteria are capable. The R-plasmids bearing genes of medicinal stability of bacteria belong to their number (see. Medicinal stability of microorganisms , R-factor ), the col-plasmids bearing the genes controlling synthesis of colicines (see. Bakteriotsinogenny factors ).

In spite of the fact that To. at. it is studied only on the example of derivatives of an avirulent strain E in detail. coli K12, this phenomenon has probably extremely important value in evolution of different types of bacteria, providing them a wide range of combinative variability (see. Bacteria ). Genetic exchange in the form of conjugation is found in various representatives of the sorts Escherichia, Salmonella, Shigella, Vibrio, Pseudomonas, Pasteurella, etc. The available data on a possibility of crossings of donors E. coli K12 with strains of Shigella, Salmonella, Pasteurella, Proteus, etc. give the grounds to assume an essential role of intergeneric hybridization in emergence of atypical forms of pathogenic bacteriums. Also experimental data about transfer at intraspecific, interspecific and bigeneric crossing of various bacteria of the konjyugativny plasmids bearing determinants of medicinal stability, a bakteriotsinogennost, hemolitic activity, toxigenicity, antigenic properties, etc. are of great practical interest. Efficiency of experimental crossings of bacteria in an organism of various laboratory animals allows to consider that To. at. can occur not only in the environment, but also in a human body and page - x. animals.

The phenomenon of conjugation is widely used in genetics of bacteria for the purpose of mapping of genes, i.e. definition of their localization on a bacterial chromosome. On the basis of results of the interrupted crossings of a series of strains of Hfr which cells transferred to recipients various, but blocking each other chromosomal segments the conclusion was drawn on the closed (ring) structure of a chromosome E. coli K12 is also made its genetic map. The similar card is constructed for Hfr-strains of Salmonella typhimurium which were received during the crossings with E. coli K12 Hfr. In a similar way Hfr-strains of shigellas and opportunistic escherichias are received. The original konjyugativny plasmids capable to provide effective chromosomal transfer are found in strains of Vibrio cholerae, Pseudomonas aeruginosa and other pathogenic bacteriums. As a result of such researches premises for carrying out the genetic analysis of virulent, antigenic and other properties of bacteria are created.



Bibliography: Brown V. Genetics of bacteria, the lane with English, M., 1968, bibliogr.; About l d f and r D. M. Introduction to genetics of bacteria, M., 1966, bibliogr.; Meynell G. Bacterial plasmids, the lane with English, M., 1976, bibliogr.; Pekhov A. P. Genetics of bacteria, M., 1977, bibliogr.; X e y e U. Genetics of bacteria and bacteriophages, the lane with English, M., 1965, bibliogr.


V. P. Shchipkov.

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