32. THE NUCLEIC ACIDS OF MICROORGANISMS 157 



purine, in the DNA of bacteria. 90 ' 91 At first, it was found in the DNA of 

 an E. coli strain which required thymine (15T~); with thymine being defi- 

 cient in the medium, it accumulated in rather considerable amounts in the 

 DNA of this bacterium. 90 Later investigations showed this base to be a 

 normal constituent of DNA in all bacteria studied, namely in E. coli (B/r, 

 K12, and 15T - ), A. aerogenes, Diplococcus pneumoniae, and M. tuberculosis 

 bovis. 91 ' 93 From the DNA of the above mentioned bacteria, not only the base 

 itself was isolated, but also the corresponding deoxyriboside and 5-deoxy- 

 ribonucleotide. 92 In the DNA of these bacteria, the molar content of 6- 

 methylaminopurine did not exceed 0.7% of the total content of the bases, 

 ranging from 0.4 % (in D. pneumoniae) to 2.7 % (in A. aerogenes) as mole % 

 of adenine. This base was not found in the DNA from the tissues of higher 

 plants and animals 91-93 ; it seems to be characteristic of microorganisms. 



The study of different strains of E. coli showed the normal content of 

 6-methylaminopurine in DNA of all three strains (B/r, K12, and 15T~) to 

 be the same (1.7-1.8% mole % of adenine). 92 In contrast, different species 

 differed widely in the content of this base. Thus, one may consider the quan- 

 titative distribution of 6-methylaminopurine in DNA as a feature specific 

 for the species. The authors think 92 ' 93 that under normal conditions this 

 base probably replaces a certain proportion of the adenine in the DNA 

 chain. 



The authors showed, 92 however, that a strain, requiring thymine (E. coli 

 15T~), when placed under conditions of either thymine deficiency or of the 

 inhibition of its formation by the increase of such thymine antagonists as 

 5-aminouracil, 2-thiothymine, or 5-chlorouracil, synthesizes DNA with an 

 increased content of 6-methylaminopurine (up to 15% of the adenine) and 

 a decreased content of thymine. Thus, under the conditions mentioned, 

 either 6-methylaminopurine substitutes for a portion of the thymine in the 

 DNA molecule or the DNA synthesized no longer fulfills Chargaff's regu- 

 larities. At any rate, the DNA formed under these conditions cannot be 

 brought into accord with the Watson-Crick structure, and is supposed 93 ' 94 

 to be abnormal. 



The data summarized here have a close connection with the problem of the incor- 

 poration of unnatural analogs of purine and pyrimidine bases into bacterial DNA. 

 This concerns, mainly, halogenated pyrimidines, such as 5-bromouracil, 5-chloroura- 



90 D. B. Dunn and J. D. Smith, Nature 175, 336 (1955). 



91 D. B. Dunn and J. D. Smith, Biochem. J. 60, Proc. xvii (1955). 



92 D. B. Dunn and J. 1). Smith, Biochem. J. 68, 627 (1958). 



93 D. B. Dunn and J. D. Smith, J^th Intern. Congr. Biochem., Vienna Symposium 7, 

 preprint No 10 (1958). 



94 F. H. C. Crick, in "The Chemical Basis of Heredity" (W. D. McElroy and B. 

 Glass, eds.). Johns Hopkins, Baltimore, 1957. 



