SUMMARIZING REMARKS 153 



The inspection of Table 34 will show that in the nucleotide 

 sequences under comparison the relative contribution of 5- 

 bromouracil to the sum of 6-ketopyrimidines equalled the relative 

 abundance of the analogue in the total nucleic acid. For instance, 

 in Prep. 2 the halopyrimidine accounted for 56% of the sum of 

 the 6-ketopyrimidines of the nucleic acid (Table 32) and the 

 same figure was found for its relative frequency in the solitary 

 and dinucleotide units (Table 34). Similar results were obtained 

 with the other mutant strain, not listed here in detail; and in this 

 case the agreement extended even to the trinucleotides^^. In ad- 

 dition, the frequencies of thymine in the various pyrimidine 

 nucleotide clusters released from the nucleic acid specimens 

 synthesized in the presence of thymine alone were quite similar 

 to the frequencies of the sum of thymine and 5-bromouracil in 

 the coiTesponding fractions from preparations produced in the 

 presence of the analogue. 



/. The concepts of pleromerism and homotopy and the neighbor 



problem 



The definitions of the suggested terms "pleromer" and *'homo- 

 tope" have been given above. The compilation in Table 35, by 

 no means complete, gives a few examples of such pleromeric 

 relationships in deoxyribonucleic acids. There can be little doubt 

 that in all these instances the established pairing relationships 

 would have been destroyed, had the minor pleromer been omitted 

 from the computations. Only one satellite normally occurring in 

 nature, viz., 5-methylcytosine, could be included in the table. 

 Another minor component — if it can be classified as a normal 

 constituent — namely, N-methyladenine, plays a baffling role. It 

 has been reported to occur, among other instances, in the deoxy- 

 ribonucleic acid of a thymine auxotroph of E. coli in not in- 

 considerable quantity ^^. Under special conditions of growth, its 

 concentration increases, somewhat in measure with a decrease in 

 the amount of thymine. It is, however, at the moment difficult to 

 see through what mechanism a 6-ketopyrimidine and a 6-methyl- 

 aminopurine could function as pleromer s. 



References p. 159 



