LEDERBERG AND TATUM 



193 



Others. Sex plays its greatest role in heredity in that it provides for 

 the recombination of genes into new., ^^experimentar series, which 

 jnake up the raw 'material itpoji which natural selection acts. Leder- 

 berg and Tatinn reverse the origi?ial procedure in that they utilize 

 the phenonie7wn of recombiTiation to demo?istrate the existe?ice of 

 sex i?i bacteria. Since bacteria reproduce rapidly and abu?ida?nly , the 

 discovery of sexual processes in their reproduction made them as 

 important and valuable as a research tool as Drosophila and Neu- 

 rospora. A co??siderable series of papers followed this original a?j- 

 nouncement, and the interested reader can find many of them in 

 the book cited above. 



Analysis of mixed cultures of 

 nutritional mutants has revealed the 

 presence of new types which strongly 

 suggest the occurrence of a sexual 

 process in the bacterium, Escherichia 

 coli. 



The mutants consist of strains which 

 differ from their parent wild type, 

 strain K-12, in lacking the ability to 

 synthesize growth-factors. As a result 

 of these deficiencies they will only 

 grow in media supplemented with 

 their specific nutritional requirements. 

 In these mutants single nutritional re- 

 quirements are established as single 

 mutational steps under the influence of 

 X-ray or ultra-violet.^ -^ By successive 

 treatments, strains with several re- 

 quirements have been obtained. 



In the recombination studies here 

 reported, two triple mutants have been 

 used: F-10, requiring threonine, leu- 

 cine and thiamin, and F-24, requiring 

 biotin, phenylalanine and cystine. 

 These strains were grown in mixed 

 culture in 'Bacto' yeast-beef broth. 

 When fully grown, the cells were 

 washed with sterile water and in- 

 oculated heavily into synthetic agar 

 medium, to which various supplements 

 had been added to allow the growth 



1 Tatum, E. L., Froc. Nat. Acad. Sci., 31: 

 215, 1945. 



2 Tatum, E. L., Cold Spring Harbor Syjn- 

 posia Quant. Biol., vol. 11, (in the press, 

 1946). 



of colonies of various nutritional types. 

 This procedure readily allows the de- 

 tection of very small numbers of cell 

 types different from the parental 

 forms. 



The only new types found in 'pure' 

 cultures of the individual mutants were 

 occasional forms which had reverted 

 for a single factor, giving strains which 

 required only two of the original three 

 substances. In mixed cultures, how- 

 ever, a variety of types has been found. 

 These include wild-type strains with 

 no growth-factor deficiencies and 

 single mutant types requiring only 

 thiamin or phenylalanine. In addition, 

 double requirement types have been 

 obtained, including strains deficient in 

 the syntheses of biotin and leucine, 

 biotin and threonine, and biotin and 

 thiamin respectively. The wild-type 

 strains have been studied most inten- 

 sively, and several independent lines of 

 evidence have indicated their stability 

 and homogeneity. 



In other experiments, using the 

 triple mutants mentioned, except that 

 one was resistant to the coli phage Tl 

 (obtained by the procedure of Luria 

 and Delbriick ^), nutritionally wild- 

 type strains were found both in sensi- 

 tive and in resistant categories. Sim- 

 ilarly, recombinations between bio- 



3 Luria, S. E. and Delbriick, M., Genetics, 

 28:491, 1943. 



