GENERAL MICROBIOLOGY - T.P. Pyroh - 2004

19. BACTERIAL GENETICS: CONSTANCY, VARIATION, AND TRANSFER OF TRAITS

19.3. TRANSFER OF TRAITS AND GENETIC RECOMBINATION

19.3.5. Restriction and Modification

Bacteriophages exhibit host Specificity: they infect only a single bacterial strain or a limited range of related strains, species, or genera. This specificity is primarily determined by the receptor Properties of the bacterial Cell surface.

In addition, Bacteria possess other systems that govern the specificity of their interactions with phages. One such system is the restriction-modification system.

It has been established that bacteriophage lambda, capable of parasitizing Cells of Escherichia coli strain K and isolated following the lysis of these cells, can infect cells of another strain of this bacterium—strain V—with only very low efficiency. Moreover, only a few phage particles that survived in strain V readily parasitized cells of that same strain, yet lost The ability to infect strain K. These two strains represent very closely related variants of Escherichia coli. Seeking to uncover the reasons for this peculiar behavior of phage lambda, scientists discovered The phenomenon of restriction-modification in bacteria. The Essence of this phenomenon is as follows.

Cells of Escherichia coli continuously produce endonuclease Enzymes (Restriction Endonucleases, restriction enzymes) that recognize precise specific sequences within DNA molecules and cleave The Double Helix at these sites. These endonucleases degraded the phage DNA within the cells of strain V. However, this naturally raises several questions: does strain K lack restriction enzymes altogether? And why do the Restriction Enzymes in strain V not destroy their own DNA along with the phage DNA?

It turned out that strain K does possess restriction enzymes, but of a different type: they cleave DNA at different sites. Meanwhile, the cells of strain V contain a methylase enzyme that modifies (by methylation, attaching a methyl group) the base within the DNA region recognized by the restriction enzyme of that strain. The modified DNA region becomes inaccessible to the restriction enzyme; in other words, modification protects the DNA from its "own" restriction enzymes. All DNA synthesized within a bacterial cell is modified at the very moment of synthesis. Conversely, when foreign DNA that lacks methylation-based modification enters The Cell, it is degraded by restriction enzymes. Only isolated molecules of foreign DNA that manage to become modified can function within the cells. This is why only a small fraction of phage lambda DNA succeeds in surviving in strain V, protected as they are by the methylases of the host cell.

Today, several hundred different restriction enzymes synthesized by numerous bacteria are known. Table 19.2 characterizes some of the restriction enzymes used in genetic research. Restriction enzyme names are derived from the abbreviated Latin names of the microorganisms from which they are isolated. For example, EcoRI indicates that the restriction enzyme was isolated from Escherichia coli, strain R, and was the first to be discovered.

Class="center">Table 19.2.

Selected Restriction Endonucleases

Restriction enzymes are indispensable tools in Introduction/32.html">Genetic Engineering. They have made it possible to perform the recombination of DNA molecules in vitro (in a test tube).