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Bacterial Transformation

	Bacterial Transformation During the forties, it was recognised that inheritance in bacteria was governed basically by the same mechanisms as those in higher eucaryotic organisms. It was also realized that bacteria represent a useful tool to understand the mechanism of heredity and genetic transfer and were therefore, increasingly used in genetic studies. The first observation that the bacterial properties can be changed by the use of heat inactivated cell material was however, made in .1928 by Griffith. Griffith had found that in Streptococcus pneumoniae (earlier called Pneumococcus), virulence to mice was related to the presence of a capsular material and the loss of the ability to produce the capsule made the bacteria avirulent.
Mutants lacking the capsular material were designated as rough (R) because colonies formed by these on solid media appeared rough as opposed to the colonies formed by the virulent, capsule forming strains which were smooth and shining (S). Griffith's experiments involved the infection of mice with heat killed and living preparations from two different strains of S. pneumoniae. When he injected the mice with either the dead "S" cells or a small number of living "R" cells no death occurred. However, when the mice were injected with a mixture of dead "S" cells and a small number of live "R" cells, the mice died.
From these experiments he concluded, that the dead "S" cells which contained the capsule contributed to the killing effect by the "R" cells, since neither of the preparations by themselves were effective. Although, these observations were not well understood at that time, the term "transformation" was used to describe this phenomenon whereby one type of cells were converted by contact with the dead cells of a second strain. The material responsible for causing transformation was thought to be the capsular polysaccharide.
However not until 1943 the material responsible for bringing about this change was identified. It was left to Avery, McLeod and McCarty in 1944 to identify the transforming principle in capsulated cells as the DNA. Their studies with purified DNA from the smooth cells of pneumoniae and its ability to transform rough cells in a test tube explained the observations made by Griffith in 1928. It was then possible to conclude that the heat killed encapsulated cells carried the information for the synthesis of the capsule which was transferred to the live noncapsulated cells. As a consequence cells that received the genetic material for capsule formation became encapsulated and virulent.
	This remarkable finding did not receive as much attention as it should have at that time, since most believed that proteins rather than nucleic acids are the genetic elements and that proteins in the DNA preparations were responsible for bringing about transformation. Since then however using highly purified DNA preparations and other genetic markers it has been shown beyond doubt that the transforming principle is DNA and not protein. The process of transformation has now also been demonstrated in several other bacteria such as Bacillus subtilis. Haemophilus influenzae, Rhizobium, E. coli, Streptococcus, Streptomyces etc.

The process of transformation in all these organisms has certain common features: (i) the purified donor DNA is first transported across the cell membrane into the recipient "competent cells" (cells, that can take up DNA), and (ii) the DNA then undergoes recombination with the recipient DNA and is then expressed.

The uptake process apparently is not very specific since it has been found that even calf thymus DNA can betaken in by bacterial cells but the subsequent process of integration is highly specific. Although the double stranded DNA is necessary for transformation, single stranded DNA can also penetrate the bacterial cells. Following uptake, by the recipient cells the transforming DNA undergoes modifications immediately and an "eclipse" period lasting for a few minutes is seen.

During this period the donor DNA cannot be recovered from the recipient cells. Using isotopically labelled transforming DNA it has been shown that during this Period the DNA exists in a single stranded form. This is followed by the integration of DNA into the recipient DNA in an area of homology. The process of integration apparently involves recombination and the loss of a region of recipient DNA. Although many details are known about this process, our understanding of the transformation process in bacteria, is yet difficult to generalize

The frequency of transformation for any single character is rather small since the amount of DNA that is taken up by the cells and integrated is small. Nevertheless, transformation using purified DNA preparations has been useful in locating genetic loci (in genetic mapping) as well as to understand the effect of a variety of physical and chemical treatments on the; biological functioning of DNA.

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