From Big Medical Encyclopedia

PLASMIDS — the extra chromosomal genetic structures of bacteria representing the molecules DNA capable it is autonomous to be copied (to be replicated) and exist in cytoplasm of a cell. Nek-rye P. can also join (to integrate) into a chromosome with a certain frequency (see. Episomes ).

The term «plasmid» was introduced by J. Lederberg in 1952 for designation for the first time sexual factor of bacteria (see), Escherichia coli revealed in cells K 12. Further P. bearing the genes responsible for medicines resistance of cells were found in bacteria of different types (see. R-factor ), for synthesis of bacteriocines (see. Bakteriotsinogenny factors ), hemolysins (Hly-plasmid), enterotoxins (Eiit-plasmid), specific surface antigens, etc. Though the data on P. which are available now are received generally during the studying of bacteria, there are bases to believe that similar structures are eurysynusic in populations of the most various microorganisms. Extra chromosomal molecules DNA are found in the higher organisms as a part of mitochondrions and plastids (see. cytoplasmic inheritance ).

There are several approaches to classification of the Item. Depending on ability or inability to be transferred from one bacterial cell to another in the course of conjugation (see. Conjugation at bacteria ) distinguish P. konjyugativ-ny (transmissible, or transmissibelny) and nekonjyugativny (not transmissible). By transfer of some konjyugativny R-plasmids to cells of separate species of bacteria disintegration (dissociation) of these P. with formation of a «pure» transfer factor (a factor of RTF) and the nekonjyugativny plasmid bearing genes of medicinal stability is observed (see. Medicinal stability of microorganisms ). It allowed to assume that konjyuga-tivny P. arose in the course of evolution as result of association (cointegration) of transfer factors and nekonjyugativny P.

Nesposobnost of two related P. is stable to coexist in the same bacterial cell (the phenomenon of incompatibility of P.) forms a basis to classification to P. but groups of incompatibility. At the same time P. incompatible with each other combine in one group. Such classification probably reflects degree of phylogenetic relationship of separate P. what high degree of a homology of DNA of various P. of one group of incompatibility and low degree of a homology of DNA at compatible P. from various groups indicates.

By the chemical nature of P. represent the closed molecules DNA ring covalently about a pier. it is powerful (weighing) about 1-106 — 120-106. Nekonjyugativnye P., as a rule, have a pier. weight (weight) less than 10*10 6  ; a considerable part of such plasmid represents the genetic material providing its ability to be the independent replicated unit (replicon). Larger sizes have Konjyugativnye P. and along with the area controlling their replication contain so-called tra-area (English transfer transfer) which determines ability of bacterial cells to conjugate with each other. As a part of tra-area of a sexual factor of bacteria 21 various tra-genes are found and mapped, to-rye form uniform strukturnofunktsionalny unit (tra-one-ron) with independent system of genetic regulation. Nekonjyugativnye P. usually do not contain a tra-operon and therefore cannot independently be transferred from one cell to another at conjugation of bacteria. However such transfer is possible due to functioning of tra-genes of the konjyugativny P. which is together with nekonjyugativny P. in the same cell. Ability to replications (see), the konjyugativnost, incompatibility and other functions of bacterial P. to some extent are under control of chromosomal systems of genetic regulation. In addition to the specified genetic structures, as a part of P. there can be various genes giving to bacteria owners additional properties to-rye at certain living conditions provide to these bacteria temporary advantages in comparison with besplazmidny bacteria (cause, e.g., medicinal resistance, a kolitsinogennost, resistance to action of the ionizing and ultra-violet radiation, to effect of salts of heavy metals and so forth). Assume that such genes could get into composition of plasmid replicon as a result of genetic exchange between P. and a chromosome or between separate P. (see. Recombination ). Also essential role of the migrating (translocated) fragments of genetic material (transposons) in the course of evolution of the Item is established.

For studying structural funk-tsionalnykh separate P.' features methods of the genetic analysis of the bacteria containing these Items are used. At a research of the physicochemical and genetic organization of plasmid DNA widely apply also physical and chemical, radio biological, elektronnomikroskopichesky and other methods.

The possibility of bystry copying (replication) and effective intraspecific, trans-species and intergeneric pairing transfer of P. defines an important role of these genetic structures in the course of evolution of bacteria (see. Bacteria ). Items are convenient model for experiments on artificial reconstruction of genetic material and creation of new genomes of organisms. Due to the detection in populations of different types pathogenic and opportunistic pathogenic bacteriums of the plasmids controlling their virulent properties there is obvious P.'s role in inf. pathology and development of epidemic processes. Special value for development of problems of rational chemotherapy inf. diseases acquire data on features and dynamics of distribution of R-plasmids in modern populations of microorganisms. At diagnosis inf. diseases it is also necessary to consider a possibility of emergence of atypical forms of microorganisms as a result of trans-species (intergeneric) transfer of P. with determinants of separate metabolic functions (ability to ferment carbohydrates, to form hydrogen sulfide, etc.).

See also Gene , Genetic engineering .

Bibliography: Kudlay D. G. Vnekhromo-somnye factors of heredity of bacteria and their value in infectious pathology, M., 1977, bibliogr.; M e fi-not l of l of. Bacterial plasmids, the lane with English, M., 1976, bibliogr.; P e-x about in And. The item and Shch and p to about in V. P. Genetics of bacterial plasmids, in book: Usp. sovr, geneticists, under the editorship of N. P. Dubinin, century 7, page 3, M., 1978, bibliogr.

V. P. Shchipkov.