GENETIC IMPLICATIONS OF MUTATIONS IN S. TYPHIMURIUM 275 



strain of E. coli give rise to new strains having the auxotrophic mutants in 

 new combinations. These initial observations have been repeated in differ- 

 ent combinations and amply confirmed by the observations of many other 

 investigators. As Lederberg has suggested, these results are most reasonably 

 interpreted as due to bacterial union like a sexual fusion of gametes, fol- 

 lowed by an immediate reduction process involving segregation and genetic 

 recombination, suggesting linkage in a single chromosome system. More 

 recently Lederberg (1949) has found evidence of what appears to be a diploid 

 strain which gives highly aberrant segregation ratios. These require assump- 

 tions of such an extremely complex and involved type of chromosome 

 interchange that it becomes questionable whether some other explanation is 

 not after all more probable. 



In S. typ/iinmrium we now have more mutant strains carrying single 

 auxotrophic genes or multiple combinations of these than in any other 

 bacterial species except E. coli. This makes it especially important to test the 

 theory with our strains. Accordingly Miss Marie McCarthy has been mixing 

 these in varying combinations, and then plating out in heavy suspensions on 

 base medium supplemented so as to show up the transfer of one or more re- 

 quirements from one to the other original combination. 



Although more than a hundred such tests have been made and carefully 

 checked, the results have been unequivocally negative until very recently. 

 This work will be reported in detail in a later publication, but I will describe 

 it briefly here. Multiple strain #519-38-94-41 requiring tryptophane, me- 

 thionine, and histidine was mixed with #533-486-96-85 requiring leucine, 

 threonine, and arginine. On plating in appropriate media it was found that 

 in addition to the original parental combinations several colonies each gave 

 strains requiring two new sets of requirements. Recombination No. 1 re- 

 quired tryptophane, leucine, and threonine. Recombination No. 2 needed all 

 six amino acids: tryptophane, methionine, histidine, leucine, threonine, and 

 arginine. These new stocks have been retested, and there can be no question 

 of the fact that we have here two recombinations of the original stocks used. 

 Other recombinations have now appeared but reciprocal classes are never 

 found. Thus we have in Salmonella confirmation of the recombination results 

 found by Lederberg in the K12 strain of E. coli. In view of the irregularity of 

 such results both in E. coli and in Salmonella, it would seem wise to suggest 

 that some alternative explanation may yet prove to be more satisfactory than 

 recombination or chromosomal crossing-over. 



BIOCHEMICAL STUDIES OF AUXOTROPHIC MUTANTS 



The Neurospora studies of Eeadle and his associates as well as those of 

 Lindegren (1949) on yeast have made it evident that in studying the action 

 of auxotrophic mutants we are many steps closer to the initial determinative 

 activities of the genes themselves than is ordinarily true for characters in the 

 higher plants and animals. When a series of auxotrophic genes can be shown 



