TRANSPOSONS

From Big Medical Encyclopedia

TRANSPOSONS (a synonym Tp-elemen-ty) — the genetic elements capable to migration within replicon or from one unit of replication in another and to implementation to various sites of DNA. At prokariot the group of the migrating genetic elements includes so-called IS elements, actually T. and nek-ry bacteriophages (see the Bacteriophage).


Fig. The diagrammatic representation of transposon (TPZ), the central part to-rogo contains the gene of Ya controlling synthesis of enzyme (3 lactamelements, and the genes of tnpA and tnpR determining a transposition; the gene of tnpA controls synthesis of protein of a transposition, and a gene of tnpR — synthesis of protein - re-pressora, tnpA suppressing manifestation of genes and tnpR and which is carrying out the recombinational exchange between two GMZ happening in the website of a specific recombination (it is specified by an arrow).


In the mid-sixties 20 century during the studying of mutational damages (see Bacteria, the geneticist of bacteria) were found in bacteria earlier not the known types of such damages, to-rye were caused by implementation in regulatory elements of operons (see the Operon) the foreign nucleotide sequences of considerable length. These nucleotide chains had a sedimentation constant (see Sedimentation), characteristic IS, and IS elements received the name. The sizes of IS elements fluctuate from 750 to 1500 base pairs. Their trailer nucleotide sequences from several couples to several tens base pairs long represent identical segments of DNA with mirror relatively each other the sequence of nitrogen bases (so-called inverted repeats), to-rye are necessary for a transposition and a recombination of these elements. In structure of IS elements (IS-, IS4-, IS10-, IS50-, 18903 elements) found the genes coding protein synthesis, providing only process of a transposition.

Actually T. in experiments with the bacterial plasmids (see) containing genes of medicinal stability are found 20 century in the mid-seventies (see. Medicinal stability of microorganisms). They have more complex structure, than IS elements, and consist of three structural components: the central part and the inverted or direct repetitions limiting it, to-rye are often presented by IS-or IS-like elements. Actually T. designate the Tp symbol with number corresponding given T.: Tnl, Tp5, TpYu etc. The central part of Tp eoder-zhit the gene (or genes) determining any sign, napr, medicines resistance, ability to formation of toxin, utilization of a certain carbohydrate, etc. In the central part of Tp, in addition to the specified genes, there can be genes necessary for implementation of a transposition. So, as a part of the central part of TPZ three genes (fig.) are found. One of them (Ya) controls synthesis of enzyme r-lactamelements (KF 3.5.2.6 and 3.5.2.8), providing ampicillin resistance of bacteria, and two others (tnpA and tnpR) determine a transposition. A product of a gene of tnpA is the protein consisting of 1015 amino-acid remains necessary for implementation of the recombination stimulated by TPZ. A product of a gene of tnpR is protein, the peptide chain to-rogo is constructed of 185 amino-acid remains;

this protein works as a repressor, suppressing manifestation of genes of tnpA and tnpR (see Expressivity of a gene), besides, it carries out the recombinational exchange between two TPZ happening in strictly certain site (website) to-ry is between genes of tnpA and tnpR. T. of this kind is called TPZ-podob-nymi.

There are T., the organization to-rykh differs from structure of TPZ-like T. The repetitions limiting the central part of such T., have length reaching several hundred base pairs and often are independent IS elements. Such T. are called Tp9-like. Genes determining a transposition are at Tp9-like T. not in the central part, and as a part of the repetitions limiting it. For nek-ry T. (Tp5-, TnlO-like) proved functional non-equivalence of repetitions: the full genetic information necessary for a transposition, only one of them contains.

However for implementation of a transposition of both TPZ-like, and Tp9-like T. the intact trailer sequences of both repetitions since these sites of DNA are learned by the enzymes which are carrying out recombinational processes — a transposition, deletion (see), inversion (see), connected with T are necessary. Therefore trailer repetitions of all 1S and Tn-elements are necessary for a transposition as certain structures; besides, trailer repetitions of Tp9-like T. code functions, essential to a transposition.

Among bacteriophages, genomes (see) to-rykh can be implemented into various sites of DNA of a cell of the owner and have properties T., Mi's bacteriophage is most studied. The transposition of a genome of a phage of Mi is the integral component of process of its replication (see). As the transposition and the genomic reorganizations stimulated by Mi's phage, and replication of his genome demand participation of phage genes And yes V. Gen of Mi's phage, being in structure of konjyugativny plasmids, promotes recombinational interactions between these plasmids and a chromosome or other plasmids that leads to formation of recombinant structures (cointegrates) and mobilization of genetic material at conjugation at bacteria (see). Unlike IS elements and actually T. DNA of a phage of Mi does not contain in the structure of trailer repetitions. The right end of DNA of a phage of Mu (S-end) is presented long (apprx. 1500 base pairs) not coupled polynucleotide chains, to-rye are the fragments of DNA of the owner which joined a phage genome during a transposition in the course of replication. On the left end of DNA of a phage of Mi (S-end) there are nucleotide chains containing apprx. 100 base pairs of the same origin. At each new act of integration of a genome of a phage of Mi not coupled iolinukleotidny trailer sequences are lost.

In genomes of plasmids, bacteriophages and in bacterial chromosomes there are sites of preferable implementation of T. certain types. Various T., in turn, differ on degree of specificity of integration into a DNA target. The exact molecular mechanism of interaction of DNA of transposons with a DNA target is not known. It is established that implementation of T. occurs so that the segment of a DNA target consisting of several nucleotide couples in a point of integration of T. doubles and after completion of integration of the copy of this segment limit from each party implemented by T. Depending on a type of T. from 5 to 11 — 12 base pairs of a DNA target are exposed to doubling. At a transposition of a row T. observe association of a genome, from to-rogo there is a transposition (a donor genome), to a genome, in to-ry the T is implemented.

(retsipiyentny genome or target). This association is called cointegration, it can be a buffer stage of a transposition or its final stage. Value of formation of kopntegrat consists that during process of cointegration unrelated genomes can combine, and, therefore, the microorganism can gain new properties in the most various combinations.

Implementation of T. in genes causes disturbance of their structural continuity and leads to emergence of mutations. These mutations are characterized by stability and full defunctionalization of the corresponding gene (see). Implementation of T is frequent. causes also blocking of function of the genes located distalny from promoter (see the Operon), than T. This phenomenon, so-called polar effect, is connected with presence at T. the websites (sites) of termination of a transcription (see). Nek-rye T. show polar effect irrespective of orientation of implementation whereas implementations of other T. polyarna only in one of two possible orientations. In certain cases implementation of T. leads to gene activation, located on the course of a transcription from the place of implementation. This effect also depends on orientation of implementation and is connected or with existence of promoter in structure of T., or with creation during the act of implementation of the new nucleotide sequence having properties of promoter. The promoters which are available as a part of T., can be in the trailer sequences forming an inverted repeat or to belong to the structural genes localized in central speaks rapidly. Presence of T. in a certain segment of DNA gives it new properties, of to-rykh genetic instability and the increased ability of this segment to recombine with other segment of DNA containing homologous T are main. The genetic instability connected with presence of T., it is expressed in the increased frequency of deletions and inversions of the genetic material which is directly adjoining T. Deletions and inversions of this sort do not depend on existence of a homology between the recombining sites and on work of systems of the general recombination. Trailer points of deletions and inversions are located so that one of them always strictly matches a trailer nucleotide of T., and another is out of T. or in it. The provision of the second trailer point is defined by probability of a recombination between the end of T. and this or that website of DNA. This probability is not identical, i.e. is available expressed the website specificity of the recombinational processes stimulated by T. The deletions and inversions arising in DNA of the bacteria, their plasmids and bacteriophages containing T., play an important role in formation of new genetic structures, they are essential to evolutionary process and can be widely used for genetic experiments.

The increased recombinational ability of sites of DNA containing T., it is caused also by the fact that homologous T., DNA which are in various (nonhomologous) sites, provide a possibility of the general recombination between these sites with use of DNA T. as areas of a partial homology. This type of a recombination is as a matter of fact homologous, but is the cornerstone of structural interaction of unrelated segments of DNA and the whole genomes. Such interaction plays a key role in evolution of microorganisms since at the expense of it essentially new combinations of genes as a part of a genome are created.

Medical - biol. value T. is defined first of all by the fact that various determinants of medicinal stability, to-rye owing to ability of T often are their part. to movement between unrelated genomes get into structure of plasmids and are inherited by various activators inf. diseases. Species of microorganisms, traditionally sensitive to certain pharmaceuticals, gain resistance to them over time (see. Medicinal stability of microorganisms). The ampicillin and tetracycline resistance which extended at Haemophilus influenzae determined according to Tp1 and TpYu is an example. Even more often find causative agents of gonorrhea (see), steady against penicillin and its derivatives — to the antibiotics which are traditionally applied to treatment of this disease. It is also connected with distribution of the plasmids supporting Tnl. Emergence of strains of activators inf. the diseases steady against traditional pharmaceuticals, seriously complicates treatment. If earlier medicinal stability often arose at causative agents of such infections as dysentery (see) and prick - an infection (see), at infectious complications after operative measures and burns, now emergence of T. and the plasmids carrying them at such activators as N. influenzae and H. gonorrhoeae, conducts to a possibility of distribution of T. among a wide range of the people who are out of clinic that is qualitatively new stage of representations

about a wedge, the importance of T. and plasmids.

Medical value T. it is not exhausted by distribution of medicinal stability of microorganisms. In structure of the central part T. the genes determining synthesis of enterotoxins (Tp681), utilization of carbohydrates (Tn-raf, Tp951) or ability to pairing transfer of genetic material (Tp916) can enter. Inheritance by cells of a contagium of the listed signs as a part of one or several plasmids containing also T., defining medicinal stability, will lead to emergence of activators with properties, significantly other than the properties known earlier. Existence of T. in a cell of this activator complicates mikrobiol. diagnosis of an infection (emergence of earlier absent ability to utilize certain carbohydrates), more severe disease (ability to synthesis of toxin) causes and considerably complicates medicinal therapy (antibacterial drugs resistance).

All this defines need of careful studying of T., control of their distribution, a right choice of ways of therapy of infectious diseases (see) and developments of rational measures of fight against intrahospital infections (see). Bibliogrilyina T. S. Mechanism of a genetic transposition, Geneticist, t. 17, No. 1, page 7, 1981, bibliogr.; Smirnov G. of B, both Ilyina T. S. IS-ales-cops and their role in a genetic recombination, in the same place, t. 13, No. 4, page 696, 1977; Kleckner N. Transposable elements in prokaryotes, Ann. Rev. Genet., v. 15, p. 341, 1981, bibliogr. G. B. Smirnov.

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