On the Species Specificity of the Nucleic Acids of Bacteria 'ill 



composition varies but insignificantly. On the other hand, we must not rule out 

 the possibility that the DNA molecules in closely related species with identical 

 nucleotide composition (e.g., S. typhosa and Shigella dysenteriae) differ in 

 nucleotide sequence to a certain extent. 



It must be mentioned that our findings and conclusion concerning species 

 specificity of DNA are in accord with the data in the recently published paper 

 of Ki Yong Lee, Wahl & Barbu [lo], which contains a large amount of experi- 

 mental research results concerning DNA specificity in bacteria. 



The data of the above-mentioned writers, together with our own data, lead 

 one to think that the nucleotide composition of DNA — on the one hand, owing 

 to its high specificity, and on the other, owing to a certain closeness of related 

 species — can probably be used as one of the systematic indications, at any rate 

 in bacteria. The following example could be mentioned: Bergey, in his hand- 

 book, refers Proteus morganii to the Proteus group, together with Proteus vulgaris. 

 In nucleotide composition, however, the DNA of Proteus morganii has nothing 

 in common with the DNA of Proteus vulgaris, and, on the contrary, is very 

 similar to the intestinal bacteria of the Salmonella-Shigella group. It may there- 

 fore be that the microbiologists who doubt the correctness of referring Proteus 

 morganii to the Proteus group are right. 



As for RNA, the data in Table 4 demonstrate that all the bacterial RNA's 

 belong to the same 'GC type. Moreover, all of them are extremely close in 

 nucleotide composition [4, 11]. When the nucleotide composition of the DNA 

 of bacteria varies very markedly, the nucleotide composition of their RNA, on 

 the contrary, is similar even in the remotest species and the variations observed 

 are insignificant. From this it follows that the nucleotide composition of RNA 

 displays no marked conformity with the nucleotide composition of DNA and 

 does not change considerably depending on the nature of an organism. 



Hence, whereas in relation to DNA we can speak of a rather marked specificity 

 even in studying its total composition, in the case of RNA it could be said only 

 of remote species, and even then to a lesser degree. Still, the identity of RNA 

 composition of the related species in no way excludes the possibility of its 

 specificity, since this specificity may be exhibited in nucleotide sequence in 

 different RNA while the total composition remains the same. Nevertheless, the 

 similarity of nucleotide composition in different RNA speaks in favour of a 

 relatively lower specificity of this nucleic acid as compared with DNA. 



However, the existence of a certain positive correlation between DNA and 

 RNA composition may be held thereby for highly probable [4]. This correlation 

 is expressed in a certain tendency toward the increase of the ratio (G + C)/ 

 (A + U) in RNA on passing from the species with the smaller value of the ratio 

 (G + C)/(A + T) in their DNA to those with the greater value of this ratio [4]. 

 It should be remarked that in our case positive correlation between RNA and 

 DNA composition is exemplified by twenty-one species of bacteria and that 

 further investigations are certainly necessary to prove that this correlation is 

 not fortuitous. Nevertheless, on the basis of the presented data we may suppose 

 that a certain portion of cellular RNA (and, apparently, only a small portion of 

 it, this fact being proved by the minute value of the regression (G + C)/(A + U) 



