32. THE NUCLEIC ACIDS OF MICROORGANISMS 181 



belongs, as a rule, to a comparatively slight GC-type. Moreover, when 

 these data are compared with the RNA composition of higher organisms, 

 animals 111 ' 155 ' 157 and plants, 89 ' 158 - 160 one is struck by the close similarity 

 of the RNA composition in the entire organic kingdom. 



As an exception, there may be quoted the data on the RNA composition 

 in a representative of protozoa, namely, T. pyriformis, the RNA of which, 

 according to Scherbaum's data, 133 belongs to the high AU-type. It may be 

 that in this group of organisms greater variations in the RNA composition 

 will be found. 



c. Correlation between the Composition of DNA and RNA 



Comparison of the analytical data on DNA and RNA composition in microorgan- 

 isms will show that the RNA composition does not change as considerably as that of 

 the DNA in relation to the systematic position of the species. From these data, it 

 follows that the RNA composition displays no marked conformity with that of the 

 DNA. 



However, when studying in parallel the DNA and RNA composition in 19 species 

 of bacteria, we succeeded in finding a certain positive correlation. 146 ' 161 This sta 

 tistically significant correlation was expressed with a tendency toward the increase 

 of the ratio (G -f- C)/(A + U) in RNA on passing from the species with the smaller 

 value of the ratio (G + C)/(A -f- T) in their DNA to those with greater values for 

 this ratio. 



It should be said that in our case positive correlation was shown with a compara- 

 tively small number of species of bacteria, and that further investigations are cer- 

 tainly necessary to prove that this correlation is not fortuitous. Nevertheless, on 

 the basis of the data presented, it was suggested 146 ' 161 that a certain portion of the 

 cellular RNA, and, apparently, only a small portion of it (this fact being proved by 

 the minute value of the regression of the RNA composition as compared with that 

 of DNA) may fully correlate in its composition with that of DNA. On the other hand, 

 the main bulk of cellular RNA, conceivably, does not exhibit any correlation with 

 DNA, its composition being similar in very different species. It could be thought that 

 the part of RNA correlating with DNA is the connecting link in the transmission of 

 the hereditary information from DNA to other cellular substrates, proteins in par- 

 ticular. The specific synthesis of the larger part of the RNA, on the contrary, may 

 not be under the immediate control of DNA; and may be carried out indirectly, 

 through some other links and substrates of the metabolism. 



3. Nucleic Acid Composition and Variability 



With respect to bacteria, there is vast microbiological literature available 

 on hereditary resistant forms which are obtained experimentally by dif- 



157 D. Elson, L. W. Trent, and E. Chargaff, Biochim. et Biophys. Acta 17, 362 (1955). 

 168 R. Lipshitz and E. Chargaff, Biochim et Biophys. Acta 19, 256 (1956). 



159 K. K. Reddi, Biochim. et Biophys. Acta 23, 208 (1957). 



160 G. A. Medvedev and N. N. Zabolotsky, Izvest. Timirjazev. SeVskokhoz. Akad. No. 

 3, 207 (1958). 



161 A. N. Belozersky and A. S. Spirin, Nature 182, 111 (1958). 



