SYNTHESIS OF AMINO- ACIDS 75 



A Neurospora mutant requiring threonine grew poorly 

 unless the medium was also supplemented with methionine, 

 cystathionine or homocysteine, indicating that all these com- 

 pounds were derived from a common precursor. The latter 

 is probably homoserine since this amino-acid can replace 

 both threonine and the sulphur-containing amino-acids 

 [46]. Moreover, if homoserine can couple with cysteine the 

 product would be cystathionine. As regards the incorpora- 

 tion of inorganic forms of sulphur into organic compounds, 

 it has been suggested that this proceeds via the synthesis of 

 cysteine by cysteine desulphurase, an enzyme known to 

 decompose cysteine into HgS, NH3 and pyruvic acid. 

 There is, however, no evidence that the enzyme can catalyse 

 the reverse reaction. In Asp, ntger, Asp. nidulans and P. 

 Tiotatum [27], thiosulphate, and not sulphide, may be an 

 important intermediate in the synthesis of cysteine. The 

 sulphur requirements of thiosulphate auxotrophs of Asp. 

 7iidulans were satisfied by cysteine- S-sulphonate (a thiosul- 

 phate derivative of serine) and growth was extremely luxuri- 

 ous in the presence of serine and thiosulphate [28]. Hocken- 

 huU therefore proposed that the route of sulphur utilization 

 involves reduction of sulphate to sulphite which is then 

 converted to thiosulphate, perhaps with the intermediate 

 formation of sulphoxylate, and that finally thiosulphate is 

 condensed with serine or some other Cg-compound. A 

 different sequence has been proposed for Ophiostoma multi- 

 annulatum [see 5] and N. crassa: 



Cs-compound (alanine?) plus S07~ — >- cysteic acid — > 



cysteine sulphinic acid — >- cysteine 



Lysine and threonine 



The route of lysine synthesis in Neurospora appears to be 

 different from that in Esch. coli. Lysine auxotrophs oi Neuro- 

 spora were able to utilize a-aminoadipic or e-hydroxy- 

 a-aminocaproic acid but not a-ketoadipic, a,a'-diamino- 

 adipic or a,£-diaminopimelic acid. The biosynthetic se- 

 quence in N. crassa is believed to be: 



a-aminoadipic acid — >- 



€-hydroxy-a-aminocaproic acid — >■ — > — > lysine [20] 



