THE PROTEINS OF MICRO-ORGANISMS 



343 



of great value in the study of the synthetic mechanisms 

 of cells involves the use of mutants of the moulds Neuro- 

 spora crassa or N . sitophila. When asexual spores of the 

 moulds are treated with X-rays or ultra-violet light, 

 germinated and crossed with the heterothallic strain of 

 opposite sex, mutants arise which lack the ability to 

 bring about certain syntheses which the normal strains 

 can perform. Besides the strains which can no longer 

 synthesise some of the growi^h factors, others have been 

 produced which cannot make argim"ne, lysine, leucine, 

 valine, methionine, tryptophane, proline or threonine. 

 By using mutants in which particular stages in a synthesis 

 are blocked it becomes possible to trace the course of 

 synthetic processes. For instance, the cycle of formation 

 of arginine from ornithine via citrulliue : — 



NH2 



I 



(CH2)3 



CH.NH, 



COOH 



CO.NH, 



I 



NH 



+CO2 + NH3 



Gene 2 

 Gene 3 



(CH2)3 



I 

 CH.NH2 



COOH 



XH2 



C=NH 

 +NH3 I 

 > NH 



Gene 1 



Prote 



(CH,)3 

 CH.NHo 



COOH 

 (Ornithine) (Citrullinc) (Arginine) 



has been worked out using mutants lacking genes 1, 2 

 or 3 and thus incapable of carrying out the corresponding 

 stage in the synthesis. Similarly tryptophane has been 

 shown to arise by the following route : — 



^ / ^— COOH ^ 



Gene 1 | | ^^ Gene 2 



(Anthranilie acid) 



I I -1- CH2OH.CH.NH2.COOH — 



N 

 (Indole) 



(Serine) 

 CH2CH.NH2.COOH 



N 

 (Tryptojjhane) 



23 



