GENETIC IMPLICATIONS OF MUTATIONS IN S. TYPHIMURIUM 277 



synthetic chain, unless a second mutation has occurred. This does not hold 

 for methionine which cannot be utilized in mutants #2 and 4 (Table 17.4). 

 Such a result suggests that cysteine is enzyme controlled through a gene 

 which is inactivated by the mutation numbered in parentheses. Cysteine is 

 ordinarily made from methionine (as has been shown for the mammal) and so 

 the reverse dotted arrow marked (1) is shown in the figure. It is hardly likely 

 that a second mutation is indicated for the mutants cited as showing two 

 blocks, but rather that certain mutations cause inhibition of more than one 

 enzyme system. A more comprehensive scheme for the cysteine-methionine 

 synthesis based on the Neurospora work has been given by Emerson (1950). 

 It is certain that more is involved in the series of reactions shown in Figure 

 17.4 than the furnishing of essential sulphur for cysteine and methionine. 



^ Protein 



(7) (6) (5) 

 SO4 »> SO3 *► S Cysteine 



HOMOSERINE 

 (1)/ \ (4) 



y Cystathionine 



? ^ d) \ 



Protein -< Methionine < Homocysteine 



Serine 



Fig. 17.4 — Possible chain of reactions involving sulphur-containing compounds. (Mutant 

 blocks indicated by numbers in parenthesis.) 



Sulphate, sulphite, and suliide, as well as cysteine itself, may act as H 

 acceptors, cooperating with dehydrogenases involved in the respiratory or 

 energy producing activities of the organism. That the organism reduces more 

 sulphate than is necessary for the S required in the amino acids is indicated 

 by the fact that Salmonella forms a readily testable excess of H2S. We are 

 attempting to trace the course of the sulphur by the use of the radioactive 

 isotope S^''. Last summer Dr. T. P. Ting and the writer were able to show that 

 (NH4)2S*04 is taken up by the wild type 533 organisms and not at all by a 

 cysteine requiring mutant, thus confirming our growth tests (see also Cowie, 

 Bolton, and Sands, 1950). 



We hope to continue this work using labeled sulphur in sodium sulfide or 

 barium sulfide, which should be utilized by wild and mutants number (7), 

 (6), (5) (Fig. 17.4). Finally it should be possible to determine by quanti- 

 tative tests how much S^* is combined into bacterial protein and how much 

 passed out in HoS. Comparisons between different strains in oxygen utiliza- 

 tion are being made with the W'arburg respirometer. As already shown in 



