BIOCHEMICAL MUTANTS 97 



arginine for growth ; of the seven, one would grow on addition 

 of nothing but arginine, two would grow on citrulline or 

 arginine, and four would grow on ornithine, citrulline, or 

 arginine. This indicates that the biosynthesis of arginine from 

 some precursor N must take place according to the sequence : 



N -^ (four separate steps) -> Ornithine -> ? -> 



Citrulline -> Arginine. 



The same synthetic series was investigated by Bonner, using 

 mutants of Penicillium, and he was able to elucidate some of 

 the earlier stages of the sequence and to show that glutamic 

 acid is a precursor of arginine thus : 



O, ,NH^ HN NH, 



\/ ^ / 



C C 



I I 



COOH CH^NH^ CH^NH CH^NH 



CHz >► ? >- CHj ^ ? ^ CH2 ^ CHj 



CHNH2 Ih CHNH^ CHNHj CHNH; 



I J I ' I I 



COOH ^1 COOH COOH COOH 



HX — CHp ^ ^ A 



Glutamic | | Ornithine Citrulline Arginine 



acid H,C CH-COOH 



'\ / 



N 

 H 



Proline 



Thousands of such biochemical mutants of various micro- 

 organisms have now been isolated and the nutritional require- 

 ments of a small fraction of them discovered. Many of these 

 show a disability in the synthesis of an amino-acid and their 

 detailed investigation is yielding much information on the 

 biosynthetic precursors of substances such as valine, isoleucine, 

 methionine, tryptophan, lysine, etc. ^ It is not known yet 

 whether results obtained with one organism also apply to 

 another, although it is fairly certain that the biosynthesis of 

 tryptophan is very similar in Neurospora and Escherichia coli. 

 The synthesis of methionine from sulphate has been worked 



