276 



H. H. PLOUGH 



to block successive steps in the syntheses of particular amino acids or vita- 

 mins or more complex products, the one gene-one enzyme hypothesis offers 

 the most satisfactory preliminary explanation, even though the presence of 

 the particular enzyme as a gene product has not been demonstrated. Each 

 set of auxotrophic mutants offers data on the chain of synthetic processes to 

 some essential substance, and thus becomes a challenging biochemical prob- 

 lem. It is significant that many of those already studied in the fungi have 

 also been uncovered in E. coli, but every organism shows individual differ- 

 ences. So far in Salmonella we have investigated the biochemical steps in 

 only two such series of auxotrophs, but many others await study especially 

 as new mutants are added. 



TABLE 17.4 



UTILIZATION OF SULPHUR COMPOUNDS BY VARIOUS 

 AUXOTROPHS OF 5. TYPHIMURIUM 



The first of these sets of interacting synthetic steps which we have studied 

 is the cysteine-methionine auxotroph series. These mutants fall into many 

 of the same gradations described by Lampen, Roepke, and Jones (1947) for 

 E. coli, by Emerson (1950) for Neurospora, and by Teas (1950) for B. 

 subtilis. We have tested all of the apparent cysteine or methionine requirers 

 for their ability to reduce inorganic sulfur compounds as well as to utilize 

 organic precursors of methionine. The wild type strains can reduce sulphate, 

 sulphite, or sulfide, and can grow with no other source of S. It has been 

 shown, however, that none of the apparent cysteine requirers can reduce 

 sulphate, but some can reduce sulphite and some sulfide. Many, however, 

 must have cysteine or cystathionine (kindly supplied by Dr. Cowie) and 

 others require methionine as such. 



A summary of representative mutants isolated as cysteine or methionine 

 requirers and their abilities to grow on various compounds as the sole source 

 of S is given in Table 17.4. This can be visualized as in Figure 17.4 in terms 

 of a succession of steps, each catalyzed by an enzyme controlled by a gene 

 which is inactivated by the mutation numbered in parentheses. Such a 

 straight line series appears to run in the direction of the arrows from sulphate 

 to protein. When a mutation occurs, as at (5), it must be assumed that growth 

 requirements will be satisfied by any compound succeeding the break in the 



