ROLE OF NUCLEIC ACIDS 75 



^-galactosidase production is thus drastically reduced, the synthesis of a 

 protein closely related to the enzyme but catalytically inactive can be 

 demonstrated by serological reactions (Bussard et ah, 1960). 



The analogue inhibits growth of E. coli only when it can be incorporated 

 into nucleotidic compounds, since mutants lacking the enzymes for making 

 the nucleotide of fluorouracil are resistant to the analogue (Brockman et al., 

 1960). As the effects of the analogue are observed without delay, the damage 

 caused by fluorouracil incorporation must concern one of the RNA 

 fractions which are rapidly renewed (Naono and Gros, 1960). The damaged 

 RNA seems to be involved in the process of selection or of arrangement of 

 the amino acids, for the proline and tyrosine content of the total bacterial 

 protein material made in the presence of fluorouracil is lower than normal. 



The reason why certain enzymes are deeply affected by such errors in 

 amino acid selection, whereas other enzymes are rather insensitive, can be 

 visualized easily. It is clear that the active centre of a given enzyme, or the 

 folding which creates this centre, may eventually be affected by amino 

 acid replacements. If, however, the number of changes in the amino acid 

 sequence is not too large, it is conceivable that the enzyme retains a prac- 

 tically normal tertiary structure, together with its catalytic and serological 

 properties. Ribonuclease, for instance, can stand a considerable amount of 

 damage indeed, without losing its catalytic properties: certain peptide 

 bonds can be broken (Richards, 1958), a whole section of polypeptide can 

 even be removed (Anfinsen et al., 1955; Rogers and Kalnitzky, 1957) 

 without much change of activity. The resulting molecules are simply more 

 fragile than the intact enzyme. 



In contrast with thiouracil and fluorouracil, another analogue, 6-azau- 

 racil, which also inhibits bacterial growth, does not interfere with protein 

 synthesis more than with the formation of other cell constituents (Sells, 

 1959). It is important to realize that 6-azauracil is not incorporated into 

 RNA in any detectable amount (Handschumaker, 1957); it inhibits nucleic 

 acid synthesis by blocking the pathways of uridylic acid synthesis (Skoda 

 and Sorm, 1958, 1959). 



The rate of incorporation of 8-azaguanine into the nucleic acids varies 

 very much according to the organism (Matthews, 1957). Bacillus cereus has 

 received special attention because it takes up quite a large amount of the 

 analogue. Detailed studies by Smith and Matthews (1957), Mandel and 

 Markham (1958) showed that 8-azaguanine replaces up to 40 per cent of the 

 guanine in the RNA made in its presence, and that the composition of the 

 nucleic acid is not otherwise changed. Synthesis of RNA continues at a 

 somewhat increased rate in the presence of the analogue, DNA is made 

 at a closely normal pace, but protein synthesis is drastically inhibited 

 (Chantrenne, 1958, 1959; Chantrenne and Devreux, 1958, 1959, 1960; 

 Mandei and Altman, 1960). 



