Transformation (Genetics)

The transformation phenomenon was first observed in 1928 by Frederick Griffith , who worked with pneumococci ( Streptococcus pneumoniae ). He found that avirulent strains lacking a capsule can receive something even from dead virulent cells having a capsule, and as a result also become virulent. After 16 years, Avery, MacLeod and McCarthy showed that this same agent was DNA containing the genes necessary for capsule formation ^[2] . They isolated DNA from the virulent strain of S. pneumoniae and showed that the introduction of this DNA alone into the cells of the avirulent strain turns them into pathogens. The results of Avery and colleagues were initially met with skepticism, and finally they were recognized as reliable after describing the phenomenon of Joshua Lederberg's genetic transfer - conjugation (in 1947) and transduction (in 1953) ^[3] .

In 1970, it was experimentally shown that Escherichia coli Escherichia coli cells can capture the DNA of bacteriophage λ without after treatment with a solution of calcium chloride ^[4] . Two years later, it was shown that cells can capture plasmid DNA under similar conditions ^[5] . So the chemical transformation was invented. In the late 1980s, electroporation began to be used to transform bacterial cells, which in many cases turned out to be more effective than chemical transformation and was applicable for a larger number of strains ^[6] .

Many bacteria are capable of transformation, for example, Streptococcus , , Bacillus , actinomycetes , cyanobacteria and other bacteria. Thus, the antigenic variation observed in the causative agent of gonorrhea Neisseria gonorrhoeae is provided by transformation, in which the cells transfer to each other the genes of different variants of pili , due to which they are attached to the cells of the host organism ^[7] . In the normal state, the penetration of large DNA molecules into the bacterial cells is impeded by dense covers, therefore, in order to be able to transform, the cell must enter the so-called state of competence. Under natural conditions, competence acquires part of the in the logarithmic growth phase under the action of certain proteins (competency factors) acting through a two-component system . Chloramphenicol , which blocks protein synthesis , prevents competent cells from forming ^[8] . It is also possible that the density of the bacterial culture contributes to the development of competence, since this increases the concentration of competence factors. In Streptococcus mutans and in other species of the Streptococcus genus, transformation often occurs during the formation of biofilms ^[9] . In Bacillus subtilis, some genes involved in competency development are also involved in sporulation . The development of competence in the log phase is due to a lack of nutrients and the accumulation of a significant number of competency factors ^[7] . Transformation can be provoked by bacteriophages that cause DNA to escape from dying cells ^[10] , as well as damage to bacterial DNA ^[11] . The acquisition of competence is an extremely complex physiological process; in Bacillus subtilis, it requires the expression of about 40 genes ^[12] .

First, competent cells bind DNA to their surface using special receptors , and with linear fragments the cell transforms much more easily than ring ones. DNA is cleaved by nucleases to fragments weighing up to 4-5 million Yes , and only one of the two chains of fragments enters the cell. Some bacteria, such as pneumococci and Bacillus subtilis , can absorb DNA from a variety of sources, while others, such as Haemophilus , can only absorb DNA from cells of their species . Fragments having a mass of less than 500 kDa do not enter the cell ^{[8] [2]} .

After entering the cell, a single chain fragment is inserted into the genomic DNA of the recipient cell. Transformation lasts from 10 to 30 minutes and occurs in different bacteria with a frequency of about 1% ^[13] .

Under natural conditions, transformation allows bacteria to obtain genes from outside that can help adapt to these conditions. Thus, transformation is one of the mechanisms of horizontal gene transfer , along with conjugation (exchange of genetic material by cells during physical contact) and transduction, in which a DNA fragment is transferred by phage ^[14] . Since competence can be caused by DNA damage and often occurs under the influence of DNA damaging agents (for example, the antibiotic ciprofloxacin induces the formation of double-strand breaks in Helicobacter pylori ^[15] ), the transformation can serve as an adaptive mechanism that facilitates DNA repair . Obtaining a DNA fragment from outside (especially from a bacterium of the same type), the bacterium can use it as a matrix for repairing damage by homologous recombination ^[16] .

Transformation has become a routine method of molecular biology to generate a large amount of the required plasmid . To artificially introduce cells into a state of competence, there are two main approaches: electroporation , in which cells absorb DNA after a short-term voltage , and chemical transformation, in which various salts of divalent ions act on cells, for example, calcium chloride ^{[2] [17]} .

• Inge-Vechtomov S.G. Genetics with the basics of selection. - SPb. : Publishing house NL, 2010. - 718 p. - ISBN 978-5-94869-105-3 .
• Jeremy W. Dale, Simon F. Park. Molecular Genetics of Bacteria. - 4th Edition. - John Wiley & Sons, Ltd, 2004. - ISBN 0-470-85084-1 .