14 SEX IN MICROORGANISMS 



of their mutation, the effect of physical and chemical agents on their 

 mutation frequency, and qualitatively in the types of biochemical 

 and enzymatic effects of their mutation (cf, Tatum and Perkins, 

 1950). 



After establishment of this functional analogy between genes 

 of bacteria and those of sexual forms, the next step logically was to 

 ask if the analogy could be carried further, and if a mode of inheri- 

 tance of bacterial characters similar to the Mendelian process in 

 higher types could be detected. Although a number of workers had 

 looked for character recombination in bacteria, and other microor- 

 ganisms, even as early as 1908 (Browning, 1908; for other important 

 references cf. Tatum and Lederberg, 1947), in most cases the biolog- 

 ical materials available were not adequate for a definitive experimen- 

 tal test, although some suggestive evidence of a circumstantial nature 

 has been obtained. The definitive nature of auxotrophic mutants and 

 the relative ease of their isolation and diagnosis provided ideal material 

 for testing the possibility of recombination of hereditary units in 

 bacteria. 



The independent occurrence and expression of auxotrophic 

 mutations in E. coli permitted building up multiple mutant stocks of 

 E. coli strain K-12 with several deficiencies by successive mutational 

 treatment (Tatum, 1945). In this way, for example, cultures Y-10, 

 requiring threonine, leucine, and thiamin, and 58-161, requiring biotin 

 and methionine, were obtained. For simplicity in considering its 

 capacity for synthesis of the factors concerned, strain 58-161 can 

 be represented as biotin— methionine— threonineH- leucineH- thia- 

 min+ (B— Af— T+ L+ Bi+), while strain Y-10 would similarly 

 be represented as B-f M-\- T— L— Bi— . In this representation, in 

 analogy with other organisms such as Nejirospora, the genes deter- 

 mining alternative characters (B+ and B— for example) are consid- 

 ered allelic. 



Accordingly, a sexual process in a mixed culture of these two 

 strains would involve reshuffling the indicated alleles at these five loci. 

 If this were at random, any recombination might be expected, and 

 might have been looked for. However, recombination to give a 

 nutritionally independent type (prototroph) would be most easily 

 detected, since it alone would grow on minimal medium, whereas any 

 dependent types, including both parental strains, would not. 



Experimental tests were carried out (Tatum and Lederberg, 



