^6 NITROGEN METABOLISM 



Unlike Neurospora, Esch. coli contains a,e-diaminopimelic 

 acid (DAP) and possesses a specific enzyme converting it 

 into lysine and CO 2 (p. 29). This decarboxylase is not 

 present in those lysineless mutants which accumulate large 

 amounts of DAP in their culture media. The lysine require- 

 ments of other mutants were satisfied by DAP but not by 

 a-aminoadipic or a-amino-£-hydroxycaproic acid, and the 

 conclusion was reached that DAP is the immediate pre- 

 cursor of lysine in Esch. coli. Moreover, DAP and threonine 

 may be derived from a common precursor, and a-amino- 

 butyric acid may be an intermediate in the synthesis of 

 threonine from homoserine (Fig. 5.3) [9]. 



Valine, isoleucine and threonine 



A mutant (161 17) of N. crassa would only grow when 

 provided with both L-valine and L-isoleucine, yet as far as 

 could be ascertained it diifered in only one gene from the 

 wild type parent. For optimal growth, the ratio of L-valine 

 to L-isoleucine was critical (7:3) and increasing the concen- 

 tration of either acid adversely affected growth. The a-keto 

 acids corresponding to valine and isoleucine ('ketovaline' 

 and 'ketoisoleucine') supported the growth of other mutants, 

 and 161 17 would grow in the presence of isoleucine and 

 'ketovaline' but not valine and 'ketoisoleucine'. Moreover, 

 'ketoisoleucine' inhibited the growth of another mutant 

 requiring only 'ketovaline'. Bonner has suggested that 

 16117 is unable to synthesize isoleucine and that the sub- 

 strate of the blocked reaction ('ketoisoleucine'?) accumulates 

 and competitively inhibits the synthesis of valine (from 

 'ketovaline'?), with the result that the mutation of one gene 

 appears to bring about the blocking of two reactions [3]. 

 After 161 17 has grown in the presence of isoleucine and 

 valine, the medium contains a,^-dihydroxy-^-ethylbutyric 

 acid, a substance replacing isoleucine for an auxotroph of 

 Esch. coli; no 'ketoisoleucine' could be detected [ib]. Iso- 

 leucine auxotrophs of N. crassa, B. suhtilis and Esch. coli 

 can be divided into three groups according to the com- 

 pounds which they can utilize: (i) only isoleucine, (2) 

 a,j5-dihydroxy-/5-ethylbutyric acid or isoleucine, (3) iso- 



