76 THE BIOSYNTHESIS OF PROTEINS 



It must be emphasized that many cell constituents are still synthesized 

 normally, which indicates that basic metabolism and energy production are 

 not touched. The process of RNA synthesis also continues but confusion 

 of azaguanine for guanine leads to abnormal nucleic acids. Under these 

 conditions protein synthesis is obliterated, although the utilization of 

 amino acids by the bacteria is not prevented, neither their condensation 

 into peptides. Cell wall material for instance, including the peptides they 

 contain, continue to be made at a normal rate (Chantrenne and Devreux, 

 1958; Richmond, 1959; Roodyn and Mandel, 1960). 



In these cases, the analogue partly replaces the normal purine or 

 pyrimidine compound, and this results in the formation of modified RNA. 

 It is striking that in all the cases where such an incorporation of abnormal 

 bases into RNA occurs, protein synthesis is interfered with. Let us con- 

 sider these facts a little more closely. 



There are good reasons to believe that the inhibition of protein synthesis 

 is linked to the incorporation of azaguanine into nucleic acids. For instance 

 mutant strains of Streptococcus foecalis resistant to azaguanine have been 

 shown to differ from the wild strain in that the resistant mutant has lost 

 the ability to incorporate azaguanine (and guanine) into nucleotidic com- 

 pounds. The same difference was observed for sensitive and resistant lines 

 of a leukaemia (Brockman et al., 1957-58). In Tetrahymena geleii, inhibition 

 of growth by azaguanine is observed only in the presence of uracil which is 

 required for RNA synthesis in this organism (Kidder et ai, 1951 ; Heinrich 

 et al.y 1952). Recent research in the author's laboratory has shown that this 

 also applies to protein synthesis in the same organism. It is only when 

 azaguanine can be incorporated into RNA that protein synthesis is inhibited 

 by the analogue. On the other hand, kinetic studies on the incorporation of 

 azaguanine into RNA and of amino acids into protein indicate that in 

 Bacillus cereus the inhibition of protein synthesis is already fully expressed 

 at a time when the amount of RNA made in the presence of azaguanine 

 represents only about 10 per cent of the total RNA (Chantrenne, 1958). 

 Fractionation of bacterial content by centrifugation shows that at that time 

 the 'non-sedimentable' RNA is already heavily loaded with azaguanine, 

 whereas ribosome RNA contains relatively little of the analogue. This 

 observation, together with the facts reported above, suggests that the incor- 

 poration of azaguanine into a soluble RNA might be responsible for the 

 general inhibition of protein synthesis caused by the analogue. 



This is one more piece of evidence that the integrity of some soluble 

 RNA is a requirement for protein synthesis. 



The action of azaguanine on B. cereus can be completely prevented by 

 several purines or purine derivatives (Mandel, 1957). Thus in the presence 

 of an adequate concentration of guanosine, no azaguanine is incorporated 

 into RNA and no inhibition of protein synthesis is observed. Guanosine 



