284 H. CHANTRENNE 



and it was observed in our laboratory (Allinckx, unpublished) that azagua- 

 nine inhibits amino acid incorporation into the proteins of this protozoan 

 only when uracil is provided. This suggests that the harmful synthesis in 

 which azaguanine is involved depends on uracil. One of the lesions caused 

 by azaguanine thus probably concerns some RNA fraction undergoing a 

 relatively rapid metabolism. 



The possibility that 8-azaguanine might change the acceptor specificity 

 of soluble transfer RNA for certain amino acids was investigated recently 

 by Osawa (personal communication) : in soluble RNA as isolated by the 

 phenol method from azaguanine-inhibited B. ceretis, about 25*^0 of the 

 guanine molecules were replaced by the analogue. In spite of this, no 

 difference was observed between this abnormal soluble RNA and the 

 normal compound in their ability to bind P^C]-leucine or in the total 

 amount of ^^C bound from a mixture of radioactive amino acids in the 

 presence of a normal extract of bacteria. 



According to these data, azaguanine does not seem to interfere with 

 the activation of the amino acids, at least if the classical scheme of protein 

 synthesis involving Hoagland's activation enzymes is valid for B. cereus. 

 The possibility that the "incorporation enzymes" of Beljanski and Ochoa 

 [15] is involved should also be investigated, for these enzymes require the 

 participation of the four nucleoside triphosphates [16] and azaguanine 

 might possibly interfere at this point. 



If the activation steps are not disturbed by azaguanine, and if the 

 acceptor specificity of transfer RNA is not changed, then azaguanine 

 probably interferes with the passage of the amino acids from transfer RNA 

 to the ribosomes or with their condensation into polypeptides in the 

 genetically controlled structure. These are the most obscure steps of 

 protein synthesis, and azaguanine might be a useful tool for the study of 

 these steps. An important observation in this respect is that azaguanine, 

 which is incorporated into both soluble and ribosomal RNA, disturbs 

 the normal balance between these two groups of nucleic acids. The ratio of 

 sedimentable to non-sedimentable RNA decreases considerably as azagua- 

 nine is being incorporated (Otaka, in press). In our laboratory, current 

 research by J. Lahou indicates that azaguanine drastically reduces uracil 

 incorporation into ribosomal RNA, while non-sedimentable RNA accumu- 

 lates. The addition of guanosine, which restores protein synthesis, causes 

 an increase of sedimentable RNA, and part of this originates from non- 

 sedimentable RNA which was formed in the presence of azaguanine. 



There is certainly a block at this stage. Unfortunately, it has not been 

 possible to establish yet whether the inhibition of formation of ribosomal 

 RNA is the cause or the consequence of the inhibition of protein synthesis. 

 It is indeed conceivable that ribonucleic acids which would normally be 

 bound into ribosomes remain in the non-sedimentable fraction for lack 



