TRANSFORMING PRINCIPLE 61 



polysaccliaride of different chemical structure from the poly- 

 saccharide of Type II Pneumococcus. Capsulated pneumo- 

 cocci will give rise under certain conditions to the growth of 

 non-capsulated or " Eough " strains. Avery and his colleagues 

 have shown that a rough non-capsulated Type II Pneumococcus 

 will grow as a capsulated Type III Pneumococcus [i.e. acquire 

 the enzyme necessary for the synthesis of the Type III 

 polysaccharide) if an extract of Type III organisms is added 

 to the medium. Careful investigation of the nature of the 

 " transforming principle " in the extract shows that it is a 

 desoxyribonucleic acid and it is active in a dilution of 1 part 

 in 6 X 10^ parts of medium. Further, once the Type II 

 organism has been transformed into a Type III organism, 

 it then continues to grow as a Type III organism, even when 

 grown in the absence of the desoxyribonucleic acid. In this 

 case the potential enzymic constitution of the organism has 

 been altered by the addition to the organism at a certain 

 stage of a minute amount of the nucleic acid, and it is tempting 

 to think that this is equivalent to adding a gene to the genetic 

 make-up of the organism. 



The studies, mentioned above, with Neurospora have shown 

 that alteration of a gene will result in the loss of an enzyme — 

 and, presumably, reconstitution of the gene will result in the 

 reappearance of the enzyme. The enzyme may thus be lost, 

 or gained, by spontaneous mutation or the process may be 

 artificially accelerated by irradiation or by treatment with 

 "mutagenic" substances such as mustard gas. This sort of 

 phenomena is well known in bacterial chemistry. Perhaps 

 the earliest case to be studied was that of Escherichia coli 

 mutabile: this is a variant of Esch. coli which will not ferment 

 lactose and when grown on lactose plates containing indicator, 

 produces white colonies, indicating no acid formation from 

 lactose. If, however, the incubation is continued, small red 

 papillae appear on the white colonies, indicating that new 

 cells are growing which have the ability to ferment the sugar. 

 If the non-fermenting culture is serially subcultivated several 

 times in lactose-containing medium, then the power to ferment 



