326 



A. N. BELOZERSKIÏ 



From the data in Table 3 it follows that DNA possesses a species specificit}', 

 i.e., each species is characterized by a particular DNA of specific nucleotide 

 composition. The nucleotide composition of the DNA of bacteria varies very 

 greatly, and among these organisms one may encounter representatives of a 

 pronounced 'AT' type (e.g., Clostridium perfringens) and representatives of a 

 distinct 'GC type (e.g., Actinomyces globisporus streptomycini), as well as all the 

 intermediate stages between these two types. 



If we take closely-related representatives of the intestinal group, such as 

 Escherichia coli, Salmonella typhosa, and Salmonella typhimurium, we find that 

 their nucleotide composition exhibits closeness. However, the small variations 

 observed in the values of the ratio (G + C)/(A + T) are quite real, since they 

 represent the mean value of a large number of determinations. This question 

 was carefully studied by us. The more remote species such as Alcaligenes faecalis, 

 Proteus vulgaris and Pseudomonas aeruginosa display increasing differences in 

 nucleotide composition both among themselves and in relation to the afore- 

 mentioned bacteria of the intestinal group. It appears highly probable that in 

 closely related species there are differences in nucleotide composition only among 

 some DNA molecules (assimiing the correctness of the assumption about the 

 heterogeneity of DNA in the same object) in view of which the total DNA 



Table 4 



RNA composition of different bacteria 



