TRANSFORMATION:

Transformation involves the uptake of free or naked DNA released by donor by a recipient. It was the first example of genetic exchange in bacteria to have been discovered. This was first demonstrated in an experiment conducted by Griffith in 1928. The presence of a capsule around some strains of pneumococci gives the colonies a glistening, smooth (S) appearance while pneumococci lacking capsules have produce rough (R) colonies. Strains of pneumococci with a capsule (type I) are virulent and can kill a mouse whereas strains lacking it (type II) are harmless. Griffith found that mice died when they were injected with a mixture of live non capsulated (R, type II) strains and heat killed capsulated (S, type I) strains. Neither of these two when injected alone could kill the mice, only the mixture of two proved fatal. Live S strains with capsule were isolated from the blood of the animal suggesting that some factor from the dead S cells converted the R strains into S type. The factor that transformed the other strain was found to be DNA by Avery, McLeod and McCarty in 1944.

Fig. 49. Evidence of transformation

Transformation is gene transfer resulting from the uptake by a recipient cell of naked DNA from a donor cell. Certain bacteria (e.g. Bacillus, Haemophilus, Neisseria, Pneumococcus) can take up DNA from the environment and the DNA that is taken up can be incorporated into the recipient's chromosome.

By 1926 the quest to determine the mechanism for genetic inheritance had reached the molecular level. Previous discoveries by Gregor Mendel, Walter Sutton, Thomas Hunt Morgan, and numerous other scientists had narrowed the search to the chromosomes located in the nucleus of most cells. But the question of what molecule was actually the genetic material had not been answered.

In 1928 Frederick Griffith, in a series of experiments with   Diplococcus pneumonia   (bacterium responsible for pneumonia), witnessed a miraculous transformation. During the course of his experiment, a living organism (bacteria) had changed in physical form.

The pneumococcus bacterium occurs naturally in two forms with distinctively different characteristics. The virulent (S-strain) form has a smooth polysaccharide capsule that is essential for infection. The nonvirulent (R-strain) lacks the polysaccharide capsule, giving it a rough appearance. Mice injected with S-strain of the pneumococcus bacteria die from pneumonic infection within a few days, while mice injected with the R-strain bacteria continue to live. Injection with heat-killed S-strain bacteria also results in the mice surviving.

Griffith was surprised to find in his experiments that mice injected with a mixture of heat-killed S-strain and live but nonvirulent R-strain produced lethal results. In fact, Griffith discovered living forms of the S-strain bacteria in the infected mice. He hypothesize that the R-strain bacteria had somehow been transformed by the heat-killed S-strain bacteria. Some "transforming principle", transferred from the heat-killed S-strain, had enabled the R-strain to synthesize a smooth polysaccharide coat and become virulent.

Oswald Avery, Colin McCleod, and Maclyn McCarty (1934-1944) at the Rockefeller Institute, building on Griffith's work, showed that only DNA could cause the transformation. They isolated a cell-free extract from the S-strain bacteria and were able to transform living R-strain into a culture containing both S-strain and R-strain cells. The purified extract contained Griffith's "transforming principle". Through biochemical testing, they showed it to be deoxyribonucleic acid (DNA).

Fig. 50. Transformation (a) with DNA fragments and (b) with a plasmid

Fig. 51. Mechanism of Transformation

 

REFERENCES:

Text Books:

1. Jeffery C. Pommerville. Alcamo's Fundamentals of Microbiology (Tenth Edition). Jones and Bartlett Student edition.

2. Gerard J. Tortora, Berdell R. Funke, Christine L. Case. Pearson - Microbiology: An Introduction. Benjamin Cummings.

3. J. Krebs, E.S. Goldstein, Stephen T. Kilpatrick. Lewin's Genes X. Jones and Bartlett Publishers.

Reference Books:

1. Lansing M. Prescott, John P. Harley and Donald A. Klein. Microbiology. Mc Graw Hill companies.