SYNTHESIS OF AMINO- ACIDS 69 



the growth of a mutant incapable of reaction a will be sup- 

 ported by B, Cor D\ if incapable of b, by C or D\ and if in- 

 capable of c, the mutant will only grow in the presence of D. 



For convenience, mutants are described either by adding 

 the suffix '-less' to the substance required for growth, e.g. 

 arginineless denotes exacting towards arginine, or by using 

 the term auxotroph, e.g. an arginine auxotroph. 



Growth in the presence of suboptimal amounts of the 

 product of a blocked reaction may result in the excretion of 

 the precursor of this reaction into the medium. Consequently 

 when two related mutants are streaked near one another on 

 a solid medium containing suboptimal amounts of required 



HOOC(cH2)2CH(nH2)COOH NH2CONhCcH2)3Ch(nH2)cOOH 



Qlutomic acid \ / citrulline 



NH2(cH2)3Ch(nH2)cOOH 

 ^ ornithine 



proline 



CH,— CH, / rjjHj NH2 



CO ^CNH(cH2)3Ch(nH2)COOH 



CHj^CHCOOH NM2 UU arginine 



NH urgQ 



FIG. 5.1. — The arginine cycle and the route of arginine synthesis 

 in Neurospora crassa and Penicillium notatum 



nutrients, one may secrete a substance which is used directly 

 by the other or changed by it into a form which can be 

 utilized by the former mutant. The enhanced growth which 

 results is readily visible and this phenomenon of 'syn- 

 trophism' has been widely exploited by Davis [7] (Plate I). 



Arginine synthesis 



Seven genetically distinct arginineless mutants of A^. crassa 

 were isolated and of these, four grew on ornithine, citruUine 

 or arginine; two on citrulline or arginine and one on arginine 

 only. The organism also possessed arginase and urease and 

 it was concluded that there is an 'arginine cycle' in Neuro- 

 spora (Fig. 5.1) comparable to that described by Krebs in 

 mammalian liver. It can be deduced frorn the genetic data 

 that there are at least four steps in the synthesis of ornithine 



