Linked Mutations Borne by Deoxyribonucleic Acid Controlling the Synthesis of Capsular Polysaccharide in Pneumococcusx.

S defined by PONTECORVO (1958) , recombination is ". . . any process which Agives origin to cells or individuals associating in new ways two or more hereditary determinants in which their ancestors differed . . .". According to this definition, genetic transformations of bacteria constitute a special mode of recombination. In transformation a new combination of hereditary determinants is produced following the exposure of a suitably marked recipient strain of bacteria to the deoxyribonucleic acid (DNA) extracted from a donor strain that differs from the recipient strain by one or more such determinants. Yet, in another sense, transformations are directed mutations, for they confer one or more genetic properties of the donor upon the recipient strain of bacteria. It seems more desirable, however, to treat transformation as a recombination resulting from the confrontation of a portion of the recipient genome with the homologous portion of the donor genome contained in the transforming DNA. The evidence for this point of view comes from a number of directions. In the first place, it has been demonstrated that, when the recipient cells are physiologically competent, they absorb transforming (donor) DNA from the medium. This has been shown (GOODGAL and HERRIOTT 1957; LERMAN and TOLMACH 1957; Fox 1957) in experiments using transforming DNA uniformly labelled with P3*. The radioactive phosphorus enters the competent recipient bacteria; the amount that is irreversibly bound is directly proportional to the frequency of transformed bacteria eventually produced; the radioactive label is recoverable specifically from the DNA fraction of the recipient cells. A second piece of evidence is the reversibility of transformation (TAYLOR 1949; HOTCHKISS and MARMUR 1954; RAVIN 1959). A bacterium with genetic property A can be transformed into one with the mutated property Q; using the appropriate DNA, the transformed bacterium can then be transformed back into a bacterium with the property A. Such reversibility shows that transformation cannot be regarded as simply the adding on of donor DNA to the DNA already present in the recipient genome, but rather as the replacement of a piece of recipient DNA by a corresponding (homologous) piece contained in the transforming DNA. A third line of evidence is discussed in this paper. Certain transformations are best explained by the assumptions that the recipient and donor bacteria are mutated for a