GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

19. BACTERIAL GENETICS: CONSTANCY, VARIATION AND INHERITANCE

19.3. GENE TRANSFER AND GENETIC RECOMBINATION

19.3.4. Transformation

Genes can also be transferred from Cell to Cell without any intercellular contact or vectors. This mode of Gene transfer mediated by free soluble DNA extracted from donor Cells is called transformation. In Bacteria, this method of trait transmission was discovered before the others, playing a pivotal role in The history of science.

Discovery of the role of DNA as genetic material. In 1928, F. Griffith described The conversion of a non-encapsulated R-strain of Streptococcus pneumoniae into a capsule-forming strain (S-strain). Griffith simultaneously injected mice with a small amount of the avirulent R-strain and heat-killed S-strain cells. Virulent encapsulated bacteria of the S-type were subsequently isolated from the Blood of the dead mice. Thus, the killed S-cells transferred the hereditary capacity for capsule formation to the R-strain cells, which in turn passed it on to their progeny. The Nature of this "transforming principle" was successfully identified in 1944 by O. Avery, C.M. MacLeod, and M. McCarty, who proved it to be DNA. This discovery provided decisive evidence that Genetic information resides in DNA rather than Proteins.

Competence. Later, the possibility of genetic transfer via transformation was established for numerous bacterial species. Traits such as resistance to various toxins and prototrophy for specific Amino Acids can be transferred in this manner. However, transformation is restricted to bacteria whose cells are capable of taking up high-molecular-weight double-stranded intact DNA; such cells are termed competent. Therefore, competence refers to the physiological ability of cells to take up exogenous DNA. Competence depends on the physiological state of the cells, peaking in the middle of the exponential growth phase. Cell competence can be artificially enhanced by Treatment with calcium chloride or cyclic adenosine monophosphate (cAMP).

Although competent cells can take up foreign DNA of any origin, recombination occurs only if the DNA is derived from a closely related species, which enables homologous pairing and exchange between the host and donor DNA regions.