74 



NITROGEN METABOLISM 



a complete source of this element. Mutants exacting towards 

 various sulphur compounds are the easiest to produce and 

 isolate. Of four methionineless mutants of A^. crassa, only- 

 one specifically required methionine, homocysteine was just 

 as effective for two of the mutants, whilst the other grew on 

 cysteine, homocysteine or methionine. The culture filtrate 



l: 



NH2 CH2 



COOH CH.NHj 

 COOH 

 cystathionine 



CHjSH 



— yCHj _ 



CH.NHj 



COOH 



hornocyitaine 



CHjSCHj 



(fHjSH 



syi 



CHjOH 



CH.NH, 

 I ^ 



COOH 



COOH 

 methionine 



r3 



CHOH 

 I 

 CH.NHj 



COOH 



(jiHjS.SOaH 



CH.NH2 



COOH 



c ysteine-s-iul phonate 



homoierine «/- ominobutyric ocid threon in e 



. ' ll( 



CHOH ». y 



CHg^HjCHa 

 ^ C.OH 

 CHOH 



COOH 

 o(-keto .i3 h ydroxybutyric acid 



CHjOH HjSjOg^— H^SOjV-HzSOav- H2Sq4 



CH.NHj 



COOH *— 



^>A=1 



FIG. 5.3. 



HCOOH 



CH.NHj 

 COOH 

 glycine 



-Pathways for the synthesis of cysteine 

 and isoleucine 



COOH 

 di hydroxy-^ - ethyl 

 butyric ac'i?" 



i 

 CHg^CHjCHa 

 CH 



CH,NH2 

 COOH 



isoleucine 



, methionine 



of one homocysteine auxotroph was found to contain a sub- 

 stance capable of supporting the growth of the other homo- 

 cysteine autotroph and also of the mutant which would 

 grow on cysteine. This substance was isolated and identified as 

 cystathionine [29], and the suggested biosynthetic sequence 

 is shown in Fig. 5.3. Methionine is probably synthesized by 

 methylation of the homocysteine produced by the cleavage 

 of cystathionine (see p. 149). 



