56 DEOXYPENTOSE NUCLEOPROTEINS AND PROSTHETIC GROUPS 



TABLE 17 



5-METHYLCYTOSINE DISTRIBUTION IN FRACTIONS OF CALF THYMUS 

 SODIUM DEOXYRIBONUCLEATE* 



Fraction 



NaCl molarity in extraction of nucleohistone gel 0.40 0.45 1.7 2.6 

 Corrected mean proportions: 



Thymine 24.2 



Cytosine 23.9 



5-Methylcytosine 1.9 

 Molar ratio: 



Thymine to 5-methylcytosine 12.7 



Cytosine to 5-methylcytosine 12.6 



* The mean proportions of each constituent have been corrected on the 

 assumption that one-half of the nucleic acid P is contributed by the 

 pyrimidine nucleotides. In one hydrolysis experiment with fraction 4 a 

 minute amount of 5-methylcytosine (about 0.6 mole) was found. 



proportion of dinucleotides which have recently been separated 

 and analyzed^^. Unless the assumption is made that this enzyme 

 distinguishes specifically between cytosine and its 5-methyl 

 derivative, it is significant that the proportions of the various 

 dinucleotides are far from what randomness of incorporation 

 would predict. For instance, the frequency of methylcytosine 

 being Unked to guanine in a dinucleotide as compared with that 

 of cytosine being so attached is 20 times as high in calf thymus 

 nucleic acid, and 1 3 times as high in the wheat germ preparation, 

 than would have been expected for non-selective incorporation. 

 The unexpectedly high proportion of 5-methyldeoxycytidylic acid 

 that is linked to deoxyguanyhc acid^^-^^ makes one, in fact, 

 wonder whether it is not the adjoining nucleotide rather than the 

 one opposite that directs the incorporation. 



Two other findings could also be cited, though their detailed 

 discussion would take us too far, namely, (a) the incorporation, 

 from the medium, of 5-bromouracil or 5-iodouracil in the deoxy- 

 pentose nucleic acids of certain deficient bacteria or of bacterio- 

 phages in replacement of part of the thymine^^- ^^j and (b) the 



