138 Arthur L. Koch 



attached to the 7-position it prevents bond formation at the 9-position. Con- 

 sequently, the methyl group must be removed if the molecule is to be incor- 

 porated into the nucleic acids. 



The isotopic data, as well as other information, are adequate to demonstrate 

 that there is not a single molecule of enzyme present in these bacteria capable 

 of removing this methyl group (3). Therefore it would appear that certain of the 

 mutagenic materials are not and cannot be converted into a form in which they 

 can be linked covalently to cell materials, not at least by the 9-A'^-glycosyl 

 bond which has been universally found in biological materials. 



III. PURINE METABOLISM IN ESCHERICHIA COLI 



The next possibility we investigated was that the mutagens act by inter- 

 fering with nucleic acid biosynthesis. First, however, it is necessary to discuss 

 the metabolism of the organism under study. Fig. 2 summarizes, from the 



Adenine 

 Hypoxanlhine 

 and derivatives 



Glycine 



CO, ''"'C '" ^^r 



Serine _- / \ 0" 



DR 



Glucose 

 Formate 

 Ammonia J ) _ "GR - '- G « ' 6D 



Guonine 

 Xanthine 

 and derivatives 



CELL WALL 



AD 



♦ONA 



Fig. 2. The purine metabolism of Escherichia coli 



available tracer data, the pathways of purine synthesis in growing cultures 

 of the test organism (4, 5, 6, 7, 8). C^*-labeled COg (4), glycine (8), and serine 

 or formate (unpublished) lead to the formation of RNA adenine, DNA adenine, 

 RNA guanine, and DNA guanine, all of equal specific activity. The activity 

 in the purines derived from CO2 and glycine is such as to indicate that the 

 well-accepted scheme for purine biosynthesis is the major pathway in tliis 

 organism (4). C^Mabeled adenine and hypoxanthine and their derivatives 

 yield adenine samples of equal, but lower, specific activity in both RNA and 

 DNA. From these facts it is inferred that there are three pools at which purine 

 metabolism branches, namely, a 'purine' pool which is common to all cellular 

 purines, and an 'adenine' and a 'guanine' pool which are precursors of the 

 corresponding purine in both types of nucleic acid. So far, attempts to find 

 a precursor which enters purine metabolism at some point beyond the 'adenine' 

 or 'guanine' pool have failed. Even when the intracellular adenine-C^"* ribo- 

 nucleotides were specifically labelled (5), the incorporation into the purines 

 of the ribose nucleic acid was equal to that in the deoxyribose nucleic acids. 

 It should be mentioned that in organisms under conditions of rapid growth, 

 the soluble intermediate pool concentrations relevant to this scheme are small 

 (5). It was impossible to demonstrate guanosine, adenine deoxyriboside, 

 guanine deoxyriboside or phosphorylated derivatives. 



