OSWALD T. AVERY, COLIN M. MACLEOD, AND MACLYN MCCARTY 153 



The techniques used in the study of transformation appear to afford a sensitive 

 means of testing the validity of this hypothesis, and the results thus far ob- 

 tained add supporting evidence in favor of this point of view. 



If it is ultimately proved beyond reasonable doubt that the transforming 

 activity of the material described is actually an inherent property of the nucleic 

 acid, one must still account on a chemical basis for the biological specificity of 

 its action. At first glance, immunological methods would appear to offer the 

 ideal means of determining the differential specificity of this group of biologically 

 important substances. Although the constituent units and general pattern 

 of the nucleic acid molecule have been defined, there is as yet relatively little 

 known of the possible effect that subtle differences in molecular configuration 

 may exert on the biological specificity of these substances. However, since 

 nucleic acids free or combined with histories or protamines are not known to 

 function antigenically, one would not anticipate that such differences would be 

 revealed by immunological techniques. Consequently, it is perhaps not sur- 

 prising that highly purified and protein-free preparations of desoxyribonucleic 

 acid, although extremely active in inducing transformation, showed only faint 

 trace reactions in precipitin tests with potent Type III antipneumococcus 

 rabbit sera. 



From these limited observations it would be unwise to draw any conclusion 

 concerning the immunological significance of the nucleic acids until further 

 knowledge on this phase of the problem is available. Recent observations by 

 Lackman and his collaborators (25) have shown that nucleic acids of both the 

 yeast and thymus type derived from hemolytic streptococci and from animal 

 and plant sources precipitate with certain antipneumococcal sera. The reac- 

 tions varied with different lots of immune serum and occurred more frequently 

 in antipneumococcal horse serum than in corresponding sera of immune rab- 

 bits. The irregularity and broad cross reactions encountered led these in- 

 vestigators to express some doubt as to the immunological significance of the 

 results. Unless special immunochemical methods can be devised similar to 

 those so successfully used in demonstrating the serological specificity of simple 

 non-antigenic substances, it appears that the techniques employed in the study 

 of transformation are the only ones available at present for testing possible 

 differences in the biological behavior of nucleic acids. 



Admittedly there are many phases of the problem of transformation that 

 require further study and many questions that remain unanswered largely 

 because of technical difficulties. For example, it would be of interest to know 

 the relation between rate of reaction and concentration of the transforming 

 substance; the proportion of cells transformed to those that remain unaffected 

 in the reaction system. However, from a bacteriological point of view, nu- 

 merical estimations based on colony counts might prove more misleading than 

 enlightening because of the aggregation and sedimentation of the R cells ag- 



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