32. THE NUCLEIC ACIDS OF MICROORGANISMS 163 



yeast culture 128 has been questioned 127 and was not confirmed by later work 

 on microorganisms. Detailed investigations of the DNA composition at 

 different developmental stages of the bacteria showed its invariability with 

 a number of cultures, E. coli, 129 Salmonella enteritidis, 130 Azotobacler agile, m 

 Sarcina lutea, no> m Streptococcus pyogenes, 130 and B. cereus. 2 * Besides, as 

 was mentioned above (Section 11,1), no alteration in DNA composition 

 with the age of the actinomycetes was found in our laboratory. The same 

 was observed by Scherbaum for the cultures of Tetrahymena pyriformis 

 (Ciliata). 133 



When noting the invariability of DNA composition in the course of 

 growth, it should be borne in mind that the DNA of the bacterial cell may 

 be heterogeneous, i.e., it may consist of a number of molecules of different 

 composition and properties. 134136 Since the total DNA composition of a 

 culture does not show any change, this may favor the view, that despite 

 the age the same proportion of the DNA molecules of different composition 

 is kept invariable for a given species. 



(2) RNA. The study of RNA composition in the course of microbial 

 development was undertaken with the following organisms: E. coli, 129 A. 

 agile, 131 S. lutea 132 and B. cereus, 28 as well as with a representative of the 

 protozoa, namely Tetrahymena pyriformis. 133 As in the case of DNA, the 

 invariability of RNA composition was shown at different growth stages. It 

 is much more difficult to account for the invariability of the RNA composi- 

 tion in the course of growth than it is in the case of DNA, because in the 

 former, unlike DNA, considerable variations are found in the RNA content. 



This invariability can occur if the composition of all the RNA molecules 

 in the cell is roughly the same, i.e., if there is no considerable heterogeneity 

 of the RNA molecules within the cell with regard to their composition. This 

 is supported, also, by direct experimental evidence obtained by Wade and 

 Morgan, 66 who have shown the identity of composition of functionally 

 differing RNA fractions of E. coli, namely the "fluctuating RNA" which 

 is responsible for the basophilia of cells and is found in the light intracellular 

 particles, and the "constant RNA" which is connected with heavier nucleo- 

 protein particles. 



128 R. Abrams, in "Phosphorus Metabolism" (W. D. McElroy and B. Glass, eds.), 

 Vol. 2. p. 335. Johns Hopkins, Baltimore, 1952. 



129 A. S. Spirin, A. N. Belozersky, and A. Pretel Martines, Doklady Akad. Nauk 

 S.S.S.R. Ill, 1297 (1956). 



130 Ki Yong Lee, R. Wahl, and E. Barbu, Ann. inst. Pasteur 91, 212 (1956). 



131 G.N. Zaitseva and A. N. Belozersky, Mikrobiologia 26, 722 (1957). 



132 S. K. Dutta, A. S. Jones, and M. Stacey, J. Gen. Microbiol. 14, 160 (1956). 



133 O. Scherbaum, Exptl. Cell. Research 13, 24 (1957). 



134 G. L. Brown and M. Watson, Nature 172, 339 (1953). 



135 C. F. Crampton, R. Lipshitz, and E. Chargaff, J. Biol. Chem. 211, 125 (1954). 



136 L. S. Lerman, Biochim. el Biophys. Acta 18, 132 (1955). 



