Genetic transformation of streptomycin resistance in Xanthomonas phaseoli.

Transformation of resistance to antibiotics was observed in Diplococcus pneumoniae by Hotchkiss (1951, 1952a, 1952b) and in Haemophilus influenzae by Alexander and Leidy (1953) and Alexander et al. (1954). In all these species only certain strains were capable of being transformed. The ability to be transformed has been termed competence; refractory strains are not competent. Austrian (1952) has suggested that the structure of the cell surface determines the competence of pneumococci. This could be considered as strain competence. In Haemophilus influenzae, only a portion of a given population of R cells proved to be competent for transformation. Alexander and Leidy (1951) suggested that such cellular competence may arise by mutations which introduce a specific somatic antigen that functions as a receptor. Alexander et al. (1954) found that the proportion of competent cells varied with the phase of the growth cycle of the recipient population. The fraction of competent cells was at a maximum during the early stationary phase. The same proportion of cells proved to be competent for both transformation of colony type and streptomycin resistance, and this proportion was characteristic of the strain. Different deoxyribonucleic acid (DNA) preparations competed for the same competent cells inasmuch as the fraction of cells transformed for each character was proportional to the ratio of concentration of the two DNA preparations. However, the total number of transformed cells was independent of this ratio. In contrast, Hotchkiss (1954) has concluded that there is no specific mutation to competence. By comparing the yields after short and long exposures to DNA, Hotchkiss demonstrated that cells become transformable, remain so for about 15 min, and then again lose this ability although the total fraction of transformable cells remains