From Big Medical Encyclopedia

EPISOMES (Greek epi-on, atop - f-soma a body) — the extra chromosomal genetic elements of bacteria capable to breed (to be replicated) in a cell and to exist in two alternative states: integrated into a chromosome or extra chromosomal, cytoplasmatic.

The term «episomes» was entered in 1958 by F. Jacob and E. Wollman for designation of such genetic structures as temperate backteriophages (see the Bacteriophage) and a sexual factor of bacteria (see). These structures are capable to integrate with a certain frequency (to join) in a chromosome of a bacterial cell, becoming at the same time a part of chromosomal unit of replication (see), i.e. chromosomal replicon, and losing ability to autonomous reproduction. In the structural relation E. usually represent ring molecules DNA (see. Deoxyribonucleic acid). The integration (inclusion) of a bacteriophage into a bacterial chromosome leading to formation of lysogenic bacteriums (see the Lysogeny), can be followed by change of their toxigenicity and other properties important for development inf. process. At integration of a sexual factor (a F-episome, a F-plasmid) there is a formation to letok-donor like Hfr (English High frequency of recombination) capable to carry out transfer of chromosomal material (genes) at conjugation with retsipiyentny bacteria of the same view or with bacteria of other views and even childbirth (see Conjugation at bacteria). Further properties of episomes were found in some other the plasmids (see) which are of medical interest including in separate factors of medicinal stability of bacteria (see a R-factor), bakteriotsinogennost (see Bakteriotsinogenny factors), etc. Remains not clear whether ability to integration into a chromosome is property of limited number of plasmids or this their general property, a cut it can be under certain conditions revealed at all (or at the majority) plasmids. The community of episomny properties of temperate backteriophages and various plasmids forms the basis for the widespread assumption of the virus nature of an evolutionary origin of plasmids.

Mechanisms of integration various E. in a bacterial chromosome are connected with processes of a recombination of genetic material (DNA) of two independent replicons — Epi-somnogo and chromosomal (see the Recombination). On the example of the most studied E. (a coliphage a lambda, etc.) it is established that processes of integration E. are controlled by the genes which are as in most E., and in the respective chromosomal sites. An essential role in a recombination at bacteria is played, apparently, by the migrating (trans-located) fragments of genetic material — IS elements (see Transposons). However integrated E. are capable to be returned with a certain frequency to an autonomous (extra chromosomal) state as a result of the repeated recombinations leading to «cutting» their genetic material (DNA) from structure of chromosomal replicon. In this case they become independent replicons — ring molecules DNA, being copied in cytoplasm of a cell under control of own genetic systems. In the course of «cutting» E. inclusion in their structure of adjacent sites (genes) of a bacterial chromosome is possible that leads to formation of the so-called replaced episomes (the replaced plasmids, transdutsiruyushchy phages). Autonomous E. can be transferred from the bacteria supporting them — donors of genetic material to cells of re-tsipiyentny bacteria as a result of their infection with a phage, at conjugation or in the course of transformation (see Bacteria). Also transfer integrated E is possible. (sexual factor, prophase, etc.) as component of a bacterial chromosome of donor cells like Hfr or lysogenic bacteriums.

During the studying of processes of transfer of genetic material at bacteria with participation E. widely use methods of the genetic analysis of these bacteria. Structural-genetic organization E. investigate by means of various physical and chemical, radio biological, electron-but-microscopic and other methods. For differentiation of the autonomous or integrated state separate E. apply a number of chemical substances and influences (acridic dyes, nek-ry surfactants, pharmaceuticals, temperature influences, «starvation» of bacteria on a nek-eye to growth factors, etc.), to-rye in the doses which do not have significant effect on functioning of chromosomal replicon suppress replication of autonomous episomes. It leads to release of a considerable part of cells of bacterial population from autonomous E., but does not interfere with preservation of the integrated E.

Sposobnost E. to ensuring effective intraspecific and trans-species exchange of genetic material in processes of conjugation and transduction at various bacteria, including types, pathogenic for the person and page - x. animals, their role as important factor of evolution of these organisms defines. Episomny properties of a sexual factor of bacteria and other plasmids are widely used in genetics of bacteria for designing of strains cells-dono-ditch of Hfr. By means of the genetic analysis of such donors genetic maps of various bacteria, including the pathogenic sorts Salmonella, Shigella, Vibrio, Pseudomonas, etc. were made. Transfer separate E. in various bacterial cells and their subsequent integration with a chromosome can lead to change of nek-ry properties pathogenic and the opportunistic pathogenic bacteriums important for development inf. process or being of interest to establishment of a type of an infestant. So, e.g., in the course of the integration of a temperate backteriophage leading to lysogenization of bacteria change (lysogenic conversion) of antigenic properties at salmonellas, toxigenicity of causative agents of diphtheria, etc. is possible. By means of replaced E. the genes of bacteria of other types (childbirth) controlling separate metabolic and other properties, in particular resistance to antibiotics and other physiologically active agents can be transferred to cells of bacteria.

Bibliography: Ford of the Item. Plasmids, the lane with English, M., 1982, bibliogr.; And to about F. and Volmane. A floor and genetics of bacteria, the lane with English, page 406, etc., M., 1962; To at d l and y D. G. Vnekhromosom-nye factors of heredity of bacteria and their value in infectious pathology, M., 1977, bibliogr.; Achievements of modern genetics, under the editorship of N. P. Dubinin, century 7, page 3, M., 1978, bibliogr.; Heys U. Genetika of bacteria and bacteriophages, the lane with English, page 477, etc., M., 1965.

V. P. Shchipkov.