LYSOGENY

From Big Medical Encyclopedia

LYSOGENY (grech, lysis decomposition, easing + geneia an origin, creation) — the transferred property of a bacterial cell to form and allocate a bacteriophage against which this culture of bacteria is steady to the environment is hereditary. Existence of the bacteria which are spontaneously producing a phage is for the first time described in 1921 by Zh. Borde and Chuke (M. Ciuca). Deeper studying of L. it was begun in the 50th by A. Lvov.

The bacterial cultures having property to produce an infectious phage call lysogenic. It is established that the lysogenic bacterium repeatedly shares and grows, without allocating a bacteriophage on Wednesday. It appears in the environment of later lysis of a bacterial cell. The subsequent infection of bacterial cells with these phages can lead or to a lysis of a cell, or to establishment of a condition of lysogenicity. The Fag having such feature carry to group of so-called moderate phages. All attempts to find in a lysogenic bacterium inf. a particle of a bacteriophage or its antigen before formation of a full-fledged particle of a vegetative phage were unsuccessful. In this regard the conclusion is made that the bacteriophage in a lysogenic bacterium is in the special form different from a mature phage called by A. Lvov a prophase. Believe that the mechanism of formation of lysogenicity consists in implementation and fixing of DNA of a phage in a chromosome of a bacterial cell. Unlike it, pseudo-lysogenic cultures of bacteria can be easily exempted from the phages which are contained in them, napr, as a result of several resowings. The fact that moderated a phage can create virulent mutants (see. Bacteriophage ), the lost ability to lizogenizirovat bacterial cells, and nek-ry virulent phages (e.g., TZ) is created by the options with the weakened lytic activity though which do not have ability to lizogenizirovat points to a community of an origin of virulent and moderate phages.

The type of an infection, at Krom the phage infecting a cell turns into a prophase, on A. Lvov's terminology, transformation of a phage into a prophase — a reduction is called reductive, and. The ratio between quantity of the lysogenic cells formed at infection of bacterial culture with a moderate phage, and total number of the infected cells is called the frequency of lysogenization. Frequency of lysogenization depends on a stage of development of the bacterial owner and properties of a moderate phage. E.g., the frequency of lysogenization of bacterial cells a phage of PI changes from 0,2 to 0,8 during the keeping of the infected cells during 1 — 2 hour at t ° 20 — 25 ° respectively. The effect takes place if the infected cells are affected by low temperature not later than in 20 min. after infection. Apparently, in the cell infected with a moderate phage, formation of a condition of L. it is blocked at low temperatures at the first stages after infection when favorable conditions for lysogenization are created. The conditions influencing the frequency of lysogenization are not identical to various phages. The effect described above is not noted concerning a phage of P2. Christensen (J. R. Christensen, 1957) and Bertani (G. Bertani, 1951) found out that the frequency of lysogenization phages of P1, P2, P22 increases iod influence of a hlormitsetin, the antibiotic suppressing synthesis of protein.

The lysogenicity acquired by bacterial cells — very stable property remaining at cultures during the resowings for a number of years however it is possible to allow reversion of lysogenic cells in not lysogenic, to-rye, in turn, reinfections the phage which is in the environment can undergo and or to lyse, or again to become lysogenic. The possibility of release of a bacterium from a prophase and its transformation into not lysogenic is confirmed experimentally. Lederberg and Lederberg (J. Lederberg, E. M of Lederberg) showed that among the lysogenic bacteriums (a phage of X) which survived after radiation by UV rays the large number of not lysogenic bacteriums is found. They also established that emergence of streptomitsinorezistentny options is followed by disappearance of lysogenicity. Bertani showed that superinfection of lysogenic bacteriums a virulent mutant of the corresponding moderate phage causes emergence among the survived bacteria of not lysogenic individuals. Therefore, the number of lysogenic cells in culture will depend on two processes: speeds of reversion of cells in not lysogenic and speeds of their reinfection.

L. the eurysynusic phenomenon among different types of bacteria. On the biochemical, cultural, antigenic properties lysogenic bacteriums differ from stock bacterial culture a little. Superficial structures of lysogenic and not lysogenic bacteriums, apparently, are also identical. The lysogenic bacterium has ability to adsorb a bacteriophage on the surface, to-ry was used for lysogenization. However reductive infection at the same time is not observed and the cell does not lyse. Resistance of such bacterium to a homologous phage is called immunity of a lysogenic bacterium. The lysogenic bacterium is immune not only to an initial phage, but also to a nek-eye to its mutations. E.g., strain E. coli K12 (λ) immunen not only to a phage (λ), but also to mutants λ mi, λ with, etc.

Thus, a phagoresistance of lysogenic bacteriums is caused not by the external exception of a phage connected with change of superficial structures of a bacterium and loss of fagoretseptor, and the internal mechanism excluding infection and a reproduction of a phage in a cell.

Transformation of a prophase into a mature bacteriophage in a lysogenic bacterium can spontaneously happen. The reasons causing this process are insufficiently studied so far. A certain frequency of transformation of a prophase into a phage is inherent in separate lysogenic strains. However at nek-ry lysogenic strains it is possible to activate this process. Such strains characterize as induktabelny, and the influence provoking this process as induction of lysogenic bacteriums. The inducing factors, except Uv-radiation and ionizing radiation, many substances having recovery properties are (thiolow-new and thioglycolic to - you, recovered glutathione and ascorbic to - that, hydroxyquinoline and nek-ry mutagen and carcinogenic substances, including nitrogenous iprita). Proceeding from a way of effect of oxin (8 oxyquinolines), A. Lvov considers that induction results from cation exchange, i.e. replacement of an ion of Co 2+ ion of Ca 2+ in a prophase or in the certain enzyme participating in conversion of a prophase in a phage.

As showed Bertani and Ionesco's works (N. of Ionesco), not all prophases have equally ability to induction, and even various prophases of the same strain of bacteria can have various sensitivity to the inducing agents. It is possible to consider that each system a prophase — the bacterium has own balance and that in one bacterium there can be two such systems independent from each other (polylysigenic systems). The fact that most of the inducing agents has cancerogenic properties is of special interest.

The first facts establishing connection of a chromosome of a bacterium with a prophase in lysogenic cells were received by Lederbergami, to-rye studied heritability of a lysogeny during the crossing of lysogenic donors with not lysogenic recipients E. coli (see. Conjugation at bacteria ). They showed that a sign of lysogenicity, i.e. the prophase, is transferred to the recipient, being linked to other hromosomalny markers. At the same time it was established that different prophases are linked to different chromosomal genes. So, a prophase of a phage λ and its options are transferred together with gal — an operon, a prophase of a phage of P2 — with genes of fermentation of xylose and methionine, and f 80 — with genes of synthesis of tryptophane.

At the heart of modern ideas of the mechanism of association of genetic material of a moderate phage, in particular a phage X with a chromosome bacteria, Campbell (1962) model, philosophy a cut lies consist in the following: 1) DNA of a phage of X after penetration into a cell turns into circular structure; 2) circular DNA of a phage of X possesses the site designated by PP1, homologous to the respective site on a chromosome of a bacterium, designated as BB1. Interaction of these sites with each other leads to a so-called reciprocal recombination (see. the Recombination, at bacteria ), as a result cover DNA of a lizogeniziruyushchy phage joins in DNA of a bacterium. Transformation of a lysogenic bacterium in not lysogenic, as well as prophage induction, comes down to reversible process, i.e. to an exception of a prophase of a chromosome. Both integration, and an exception of a prophase are fagospetsifichesky functions, to-rye are controlled by the virus genetic device.

It turned out that the phage of X and a phage of Wednesday 80 possess system of the integration necessary for effective inclusion of a prophase in a bacterial chromosome and switching off from it. The genes of a phage of X controlling this function were mapped in a chromosome of a phage on the right side from the site of PP. Also mutants of a phage λint were allocated and their inability to integrate into a bacterial chromosome, i.e. to pass into a phase of a prophase, as well as to be switched off from its structure at induction is shown (see a phage a lambda). Near a gene of int in a chromosome of a phage λ there is a gene of xis. The product of a gene of xis is necessary for process of an exit of a prophase of a phage λ from a bacterial chromosome. For implementation of lysogenization, except the products of a phage providing a saytspetsifichny recombination the productive infection, i.e. a lytic way of development of a moderate phage shall be blocked. It is carried out by means of the products of a phage coded by its regulatory genes. In 1957 it was revealed that the mutants of a phage forming transparent plaques cannot lizogenizirovat bacteria. These mutations arise in three genes of a phage λ: cI, SII and SIII. The products controlled by all three genes are necessary for establishment of a lysogenic state, but only the product determined by a gene of cI is necessary for maintenance of lysogenicity. Therefore, the mutation in a gene of cI blocks reaction of transition of the infecting phage genome to a condition of a prophase. The allele with + dominates over a mutant allele of cI from where follows that the gene of cI controls synthesis of the repressor suppressing a vegetative cycle of reproduction of a moderate phage and providing immunity of a cell. This repressor in the form of monomeric protein with a molecular weight (weighing) 27 000 for a phage λ was allocated and characterized. Carry a phage to the most studied moderate phages λ, PI, R2, R22 and nek-ry other.

The phage allocated to Lederbergami (1953) from a strain E. coli K12, is widely used during the studying of genetic recombinations of regulation of expression of genes and in genetic engineering. Morphologically it is close to a phage of T5. On an antigenic structure the phage has no similarity with one of phages of group T. Stage of latency is equal in optimal conditions to 45 min., an average crop — from 80 to 130 particles. In DNA of a phage usual nitrogen bases are found. Various options of a phage which are characterized by a variety of a form of negative colonies are known (mi option — small colonies with an aura, option s — small colonies without aura, m5 and mb options — colonies of the average size, etc.).

The phage of PI is allocated to Bertani from naturally lysogenic strain E. Lisbon's coli. It is morphologically similar to a phage of T5. On an antigenic structure the phage of PI has no similarity with one of phages of group T. Stage of latency is equal in optimal conditions to 47 min., and an average crop — 150 — 300 particles. The virulent option of phage PI (PI — vir) which is forming transparent negative colonies, but lost ability to lysogenization is known.

The phage of P2 is also allocated from Lisbon's strain. On an antigenic structure differs from phages of a series T, but it is similar to a phage of PI, differing from it on a number of the major biol, signs (particle size, type of negative colonies, growth at low temperatures etc.). These two phages mutually exclude each other in experiences of multi-infection that allows to carry them to various types. It is morphologically similar to a phage of T1. The options of a phage P2 which are characterized by various morphology of negative colonies are known. From a phage of P2 also virulent options are received.

The phage of P22 is allocated by N. D. Zinder and Lederberg from a lysogenic strain of Salmonella typhimurium. In a form and the size of a head it is similar to a phage of T1, but has shorter tail. Stage of latency is equal in optimal conditions to 30 — 32 min., productivity — more than 1000 particles. DNA of a phage P22 contains usual nitrogen bases. Virulent options of a phage P22, and also the options different from an initial phage and from each other in a form of negative colonies are described.

During the studying of L. the possibility of existence at bacteria of the inherited masked infection was for the first time shown, mechanisms of activation of this infection are opened, one of mechanisms of a phagoresistance of bacteria is revealed and new mechanisms of exchange of genetic material at bacteria are opened.

In nek-ry cases lysogenization of bacteria is followed by sharp hereditary changes of their metabolism. Such type of variability of bacteria which is observed only at lysogenization unlike transductions (see), call lysogenic conversion. In 1951 Mr. V. Freeman showed that not lysogenic avirulent strains of a diphtheritic stick as a result of lysogenization nek-ry moderate phages turn into toxicogenic. It is proved that such transformation takes place only at lysogenization certain moderate phages. From two studied phages the phage a beta had ability to change toxigenicity, and the phage scale did not influence this property of a diphtheritic stick. In experiences with multi-infection of a diphtheritic bacterium both phages it was shown that the converging activity is the ancestral feature inherent in separate strains of a phage, and optional it is found in different strains of the same look. Along with it the facts indicating a role of bacteria in implementation of the converging activity of a phage are received. It means that lysogenization the converging phage in a nek-swarm of a part of bacteria is not followed by emergence of toxigenicity. Therefore, emergence of toxicogenic strains of a diphtheritic stick at their lysogenization depends as well on properties of the bacterial owner. Along with transformation of a diphtheritic stick from toxicogenic in not toxicogenic other cases of lysogenic conversion, in particular changes of cultural signs at you are described. megatherium and antigenic properties at salmonellas. The mechanism of lysogenic conversion is studied insufficiently. Nevertheless it demonstrates that the prophase fixed in the hereditary device of bacteria one way or another changes biosynthetic processes of a bacterial cell. Knowledge of essence of this phenomenon is important for interpretation of the mechanism of relationship between a virus and the owner since conversion is an example of a perversion of metabolism of a cell under the influence of the latent phage localized in it, and conversion of not toxicogenic bacteria in toxicogenic is important in epidemiology and a pathogeny of diphtheria. Since as a result lysogenic conversion changes virulent, serological or cultural properties of bacteria, it emphasizes importance of studying of this phenomenon for the decision nek-ry epidemiol, and diagnostic problems of practical microbiology.

L. is also convenient model for studying of many questions of variability and heredity of bacteria.

See also Bacteria, genetics ; Bacteriophage ; Virogeniya .



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D. M. Goldfarb.

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