THE SYNTHESIS OF NEW SUBSTANCES 



379 



the normal strain would lead to the synthesis of the substance concerned, 

 depends on a gene which has mutated and is no longer able to carry out 

 its proper function. 



By collecting and testing strains with different nutritional requirements, 

 much may be learnt about the chains of reactions. An example of this is 

 illustrated in Fig. 17.4. The chain of reactions by which arginine is 

 synthesised may be broken by mutations i to 7. Mutants i to 4 will 

 grow if supphed with any of the substances which occur later (ornithine, 



ornithine cltrulline 



NHp CONHp 



I I 



Sugar 1,2,3,4 fcH ) _All_^ NH "^ 



&NH3 — ^^V^z/s > ^» 



^2/3 

 I 



CHNH- 

 I 

 COOH 



CO2 ^ urease 

 2NH3 



, proteins 



Figure 17.4 



Metabolic pathways leading to the synthesis of arginine, with indications of 

 the points where different genes in Neiirospora (i, 2, 3, 4, 5, 6, 7) interfere. 



citrulline or arginine) ; 5 and 6 can grow if provided with citrulline or 

 arginine, but ornithine does not make good their deficiency, while 7 

 requires arginine and cannot make do with either of the other two 

 substances. Very many similar situations have been discovered. 



Since many of the steps in these sequences are knov^m or believed to be 

 carried out by means of enzymes, it is natural to assume that the funda- 

 mental activity of the genes involved is the production of these enzymes, 

 and that when the gene mutates the enzyme is either not produced at all 

 or appears in an inactive form. In most cases this suggestion remains a 

 hypothesis; it is rarely that the enzymes have actually been mvestigated, 

 and when this has been done it has sometimes been found that the relevant 

 one is not absent from a strain in which some reaction-sequence is inter- 

 rupted, but is present though not effective (Wagner 1949). Nevertheless 

 the postulated gene-enzyme relationship has been generalised and made 



