32. THE NUCLEIC ACIDS OF MICROORGANISMS 185 



definite conclusion on the connection between this or other types of vari- 

 ability and changes in the nucleic acid composition. 



4. Conclusion 



In the light of the data od the specificity of DXA composition, we can- 

 not but link up this specificity with species specificity in general. Moreover, 

 the comparison of the data on DXA with those on RXA leaves the im- 

 pression that DXA is much more closely involved in the specific species 

 characters than RXA, whereas RXA seems to be concerned with more 

 general and less specific functions of the cell. We do not rule out, however, 

 the possibility that this connection of DXA with the specific features of the 

 cell is mediated through some small part of the cellular RXA, as may be 

 gathered from certain compositional correlations discussed before. 



It may be appropriate to ask a question: can we adhere to the view of a 

 direct genetic connection between DXA and RXA in the cell without any 

 reservation? It is certainly difficult to assume, in view of the substantive 

 differences in DXA composition, that they will determine the synthesis of 

 so identically composed RXA. Xevertheless, it is not excluded that some 

 small portion of RXA, perhaps the functional analog of nuclear RXA, de- 

 pends on DXA, being determined by its structure; or it may be directly 

 transformed into DXA and vice versa. But still, with respect to the main 

 bulk of cellular RXA, we regard any possibility of its direct transformation 

 into DXA and vice versa as hardly admissible. Even if the nucleic acids 

 are interdependent in the cell, this must be, apparently, by means of a more 

 indirect mechanism. 



It should be stressed, in conclusion, that the discussion concerning the 

 specificity of nucleic acids rested entirely on the composition of either the 

 total DXA or the total RNA. Even in the case of a very similar or identical 

 composition of the nucleic acids of two organisms, there certainly remain 

 unlimited possibilities for a very close specificity, in regard perhaps not 

 only to species but even to strain; a specificity that may express itself 

 structurally through the nucleotide sequence. Therefore, in order to eluci- 

 date the chemical mechanisms of heredity and of specific protein synthesis, 

 one must not confine oneself to the study of only the nucleic acid composi- 

 tion but proceed to the solution of the problem of the nucleotide sequence. 

 Unfortunately, up to the present, only preliminary attempts have been 

 made in this respect, which are based on more or less indirect approaches. 174 " 

 176 With respect to bacteria in particular, such attempts were made in two 

 different laboratories by different methods, namely, on the DXA from 

 Mycobacterium phlei 176 and from M. tuberculosis, bovine type. 82 



174 H. S. Shapiro and E. Chargaff, Biochim. et Biophys. Acta 23, 451 (1957). 



175 H. S. Shapiro and E. Chargaff, Biochim. et Biophys. Acta 26, 608 (1957). 



176 A. S. Jones, M. Stacey, and B. E Watson, J. Chem. Soc. p. 2454 (1957). 



