AVERY, MACLEOD, MCCARTY 



ing that highly purified and protein- 

 free preparations of desoxyribonucleic 

 acid, although extremely active in in- 

 ducing transformation, showed only 

 faint trace reactions in precipitin tests 

 with potent Type III antipneumococ- 

 cus rabbit sera. 



From these limited observations it 

 would be unwise to draw any conclu- 

 sion concerning the immunological 

 significance of the nucleic acids until 

 further knowledge on this phase of the 

 problem is available. Recent observa- 

 tions by Lackman and his collaborators 

 (25) have shown that nucleic acids of 

 both the yeast and thymus type de- 

 riv^ed from hemolytic streptococci and 

 from animal and plant sources precipi- 

 tate with certain antipneumococcal 

 sera. The reactions varied with diflFer- 

 ent lots of immune serum and occurred 

 more frequently in antipneumococcal 

 horse serum than in corresponding sera 

 of immune rabbits. The irregularity 

 and broad cross reactions encountered 

 led these investigators to express some 

 doubt as to the immunological signifi- 

 cance of the results. Unless special 

 immunochemical methods can be de- 

 vised similar to those so successfully 

 used in demonstrating the serological 

 specificity of simple non-antigenic 

 substances, it appears that the tech- 

 niques employed in the study of trans- 

 formation are the only ones available 

 at the present for testing possible dif- 

 ferences in the biological behavior of 

 nucleic acids. 



Admittedly there are many phases 

 of the problem of transformation that 

 require further study and many ques- 

 tions that remain unanswered largely 

 because of technical difficulties. For 

 example, it would be of interest to 

 know the relation between rate of re- 

 action and concentration of the trans- 

 forming substance; the proportion of 

 cells transformed to those that remain 

 unaffected in the reaction system. 



189 



However, from a bacteriological point 

 of view, numerical estimations based 

 on colony counts might prove more 

 misleading than enlightening because 

 of the aggregation and sedimentation 

 of the R cells agglutinated by the anti- 

 serum in the medium. Attempts to 

 induce transformation in suspensions 

 of resting cells held under conditions 

 inhibiting growth and multiplication 

 have thus far proved unsuccessful, and 

 it seems probable that transformation 

 occurs only during active reproduc- 

 tion of the cells. Important in this con- 

 nection is the fact that the R cells, as 

 well as those that have undergone 

 transformation, presumably also all 

 other variants and types of pneu- 

 mococci, contain an intracellular en- 

 zyme which is released during auto- 

 lysis and in the free state is capable of 

 rapidly and completely destroying the 

 activity of the transforming agent. It 

 would appear, therefore, that during 

 the logarithmic phase of growth when 

 cell division is most active and auto- 

 lysis least apparent, the cultural con- 

 ditions are optimal for the maintenance 

 of the balance between maximal re- 

 activity of the R cell and minimal 

 destruction of the transforming agent 

 through the release of autolytic fer- 

 ments. 



In the present state of knowledge 

 any interpretation of the mechanism 

 involved in transformation must of 

 necessity be purelv^ theoretical. The 

 biochemical events underlying the 

 phenomenon suggest that the trans- 

 forming principle interacts with the R 

 cell giving rise to a coordinated series 

 of enzymatic reactions that culminate 

 in the synthesis of the Type III capsu- 

 lar antigen. The experimental findings 

 have clearly demonstrated that the in- 

 duced alterations are not random 

 changes but are predictable, always 

 corresponding in type specificity to 

 that of the encapsulated cells from 



