48 



Nature of the Genetic Material 



mutation, an extreme rough agent has arisen, which can be ex- 

 tracted for prockiction of transformation. As a compHcation, some 

 strains of extreme rough have been found (Taylor) which cannot 

 be transformed into encapsulated (smooth), while others permit this 

 transformation. 



Other pneumococcus transformations have been reported by 

 Hotchkiss (1951). He found the same type of action as described for 

 the capsule in antigenic specificities and in resistance to streptomycin, 

 penicillin, and sulfa drugs. Again the specific DNA from donor cells 

 is required, and a number of experimental conditions. In addition, 

 only a small percentage of the treated cells transform. The resistant 

 donor cells are, in the case of streptomycin resistance (as well as in 

 the cases mentioned above), simple mutants. The penicillin-resistant 

 strain was, however, obtained by long selection and would therefore 

 be considered in Mendelian terms as polygenic. It is remarkable that 

 here the transformation does not succeed at once but occurs in three 

 distinct steps after three exposures, each producing only a certain 

 level of resistance. Table 2 summarizes these facts. 



TABLE 2 



Probable Distribution of Various Transforming Agents Among Different 



Kinds of Strains of Pneumococci Used in Transformation Studies 



(From H. Ephrussi-Taylor, 1951) 



Brackets = presence indirectly proved 

 P = penicillin, Str = streptomycin 



