From Big Medical Encyclopedia

SEXUAL FACTOR OF BACTERIA (synonym: sex-factor, factor of fertility, F-factor) — the genetic structure of a bacterial donor cell responsible for its ability to conjugate with a bacterial cell recipient, free of a sexual factor, and to transfer to this cell the genetic material (DNA).

Item f. E. Lederberg and W. Hayes during the studying of conjugation was found in 1952 — 1953 by E. Lederberg, L. Ca valli, in Escherichia coli K12 (see. Conjugation at bacteria ). It was established later that P. f. exists in a bacterial cell in two alternative states in relation to a chromosome: either in autonomous, or in integrated. On this basis of P. f. it was referred to category of the genetic structures called episomes (see) or plasmids, also received the name «F-episome» or «F-plasmid».

By the chemical nature P. f. (F-episome) represents the supertwisted spiral of two-chained DNA, covalent bonds closed in a ring. The molecule of such DNA has length of 30,8 — 31,7 nanometers and a pier. the weight (weight) 62 «106 — — 64-106. In DNA P. f. distinguish several genetic funkts, areas. The tra-area (English transfer transfer) controlling ability of a donor bacterium to conjugation and providing transfer of genetic material is the most studied from them (P. f. or chromosomal genes) cell recipient. 21 structural genes entering this area are applied on the genetic map tra-about-lasti. These genes control synthesis of F-fibers (so-called sexual fibers) of a donor cell necessary for implementation of its conjugation with a cell recipient, and also synthesis of the enzymes participating in metabolism of DNA in the course of conjugation. These genes make one structurally functional unit — a tra-operon, possessing autonomous system of genetic regulation. Also the gene of tra S defining inability of the containing P. f is a part of this operon. donor cells to be recipients of genetic material at their crossing with the bacteria bearing similar P. f. However this restriction can be overcome during the receiving F — phenocopies of donor cells under the influence of nek-ry influences (e.g., cultivation of bacterial cells at an elevated temperature, in the conditions of starvation, etc.).

Other genetic area P. f. controls its ability to autonomous replication in cytoplasm of a bacterial cell. This process is also defined by the chromosomal systems of genetic regulation providing rather stable soderzhaniyep. t. in a bacterial cell (1 — 2 copy on one chromosome).

Autonomous replication of P. f. it can be broken at action on a cell of dyes of an acridic row, nek-ry surfactants and pharmaceuticals etc. that leads to elimination (loss) of P. f. at a part of cells of a donor strain.

Autonomous (extra chromosomal) P. f. it is capable (the truth, with a low frequency) to join in certain sites of a bacterial chromosome thanks to existence in its DNA of the nucleotide sequences, homologous to the nucleotide sequences of these sites of chromosomes (see. Recombination ). As a result of such inclusion of P. f. becomes a part of chromosomal replicon that leads to formation of donor cells like Hfr capable effectively to transfer to the recipient chromosomal genes. In cells of Hfr at the same time the recombination between homologous sites of the integrated P. f is possible. and bacterial chromosome. As a result of P. f. is returned to cytoplasm and passes into an autonomous state, having included in the structure more or less considerable adjacent site (or sites) a bacterial chromosome. Thus there are replaced P. f. (so-called F' - factors), the bearing sites of DNA of a chromosome, various in size and genetic structure. Transfer to a cell recipient included in P. f. chromosomal genes (F-duktion, sexduction) happens to rather high frequency considerably exceeding transmission frequency of other chromosomal genes.

At a research of the molekulyarnogenetichesky organization and funkts, P.'s features f. methods of the genetic analysis of the corresponding donor bacteria are widely used (see. Bacteria ), and also modern physical. - chemical, radio biological, electronic and microscopic and other methods of studying of DNA.

Ability of a F-factor of cells E. to provide to coli K12 effective transfer of genetic material to retsipiyentny cells use in genetics of bacteria by drawing up genetic maps of organisms of this look. As a result of crossings the F-episome managed to be transferred from cells E. coli K12 in a bacterium of different types and childbirth that allowed for nek-ry of them to design strains of Hfr-donor and to make genetic maps, including for pathogenic bacteriums of the sorts Salmonella, Shigella, etc. In addition to an avirulent strain E. coli K12 original P. f. with properties of episomes were found in separate strains opportunistic E. coli, and also at nek-ry pathogenic bacteriums (childbirth of Pseudomonas, Vibrio, etc.)? therefore there were new opportunities for the genetic analysis of determinants of pathogenic and antigenic properties of bacteria. As convenient model for such analysis also various replaced P. f can serve., the structure to-rykh contains separate fragments of DNA (group of genes) of a bacterial chromosome. There are experimental data about P.'s ability f. to provide intraspecific, interspecific and intergeneric pairing transfer of determinants of various properties of bacteria (including pathogenic properties and antigenic signs, medicinal stability, metabolic activity, etc.) which allow to assume that genetic exchange plays an essential role in evolution of natural populations pathogenic and opportunistic pathogenic bacteriums, in development of infectious and epidemic processes.

See also Plasmids , R-factor .

Bibliography: Jacob F. and Volman E. Paul and the geneticist of bacteria, the lane with English, M., 1962, bibliogr.; Kudlay D. G. Extra chromosomal factors of heredity of bacteria and their value in infectious pathology, M., 1977, bibliogr.; Meynell G. Bacterial plasmids, the lane with English, M., 1976; P e-Hove A. P. Genetika of bacteria, M., 1977.

V. P. Shchipkov.