Volume 3

Gene expression / Benjamin Lewin.

  • Lewin, Benjamin.
Date:
1974-1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

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    CHAPTER 1 Transformation Genetic Transfer by DNA Molecules Discovery of Transformation and the Transforming Principle Transformation was the first type of transfer of genetic information to be discovered in bacteria; and it was the subsequent identification of the transforming principle with deoxyribonucleic acid that led to the de velopment of the concept that genetic information is stored in the form of nucleotide sequences. Transformation was first observed in the study of pneumococcal infection of mice reported by Griffith in 1928. Pneumococ- cus causes pneumonia in man, but the bacterium is pathogenic for mice and so on infection may prove fatal. Virulence in pneumococci depends upon a component of the surface of the bacterium, the capsular polysac charide, which confers protection against phagocytosis in the animal and gives colonies of virulent bacteria a smooth (S) appearance. Several types of pneumococcus differ from each other in the constitution of their capsular polysaccharide and thus in the antigenic response which can be provoked against them. Mutants may be derived (from any type) which fail to synthesize the capsular polysaccharide; these R bacteria produce colonies with a rough surface and are avirulent when injected into mice. An appeal of this system for experimental analysis is that it is not difficult to distinguish R and S bacteria and to type the S strains according to their antigenic properties. As Griffith described the system, virulence and type characters are closely related in the pneumococcus. When pneumococci are grown in homologous immune serum, some descen- dents become attentuated in virulence and these can be recognized by their formation on solid media of a distinctive variety of colony known as the R form of pneumococcus. The virulent or S form of pneumococcus produces in fluid media, and still more abundantly in the peritoneal cavity of the mouse, a soluble substance which, although not itself antigenic, gives a copious precipitate with the appropriate antiserum. Each type of pneumococcus forms a special soluble substance which has no affinity for /