SEGREGATIONS IN ESCHERICHIA COLI 523 



platings in minimal agar medium, or after extraction of cells by Boivin's 

 method (Boivin et. al., 1946). No activity was found in the supernatant of a 

 suspension of Y-40 and Y-53 together or separately in the same minimal liquid 

 medium to which agar is added for plating experiments. The only manipulation 

 involved here consists of the removal of most of the bacteria by ordinary 

 centrifugation. It could thus be shown that the "activity" was associated with 

 the cells. Equally negative results characterized attempts to reveal transform- 

 ing activity on culture filtrates and cell autolysates prepared as crude fractions 

 according to Boivin's procedure. Finally, the addition of desoxyribonuclease 

 in a final concentration of .05 mg/ml to the mixing and plating medium had no 

 effect on the number of prototrophs which appeared in the cross of Y-40 

 and Y-53. Tests f° r tne destruction of enzymatic activity under these conditions 

 were, however, not done. 



The conclusions which we draw from these experiments are (a) that the 

 existence of transforming factors is exceedingly unlikely and (b) it would be 

 not worthwhile to go to extreme trouble to attempt to isolate such factors from 

 this system until the study of bona fide transforming systems has progressed 

 sufficiently that the genetical criteria already discussed might be applied. 



DISCUSSION 



Regardless of the stand that one takes on the issue of invisible zygotes versus 

 non-extractable transforming factors, it can be asserted that E. coli K-12 pro- 

 vides a useful tool for genetic analysis. The use of biochemical mutants as 

 parents allows crosses which are nearly as well controlled as in Neurospora. 

 The segregational behavior of mutant factors seems to be closely analogous to 

 that of higher forms, and seems to compel their admission into the same arena 

 as the genes of Drosophila. However, it would be premature to transfer these 

 conclusions to other genetic characters of other microorganisms, each of which 

 must be examined on its own merits. 



It may be wondered that the apparent recombination rate is so low. How- 

 ever, this is possibly not to be attributed to any sexual imperfections of E. coli, 

 but to the method of enumeration. It seems likely that an analogous com- 

 parison of the number of somatic and generative cells in an organism like the 

 oak-tree, or man (especially the female of the species) would give ratios similar 

 to those prevailing in E. coli. It is also possible that the optimal conditions for 

 zygote formation or germination have not yet been achieved and that by 

 special procedures the rate of zygote-formation may be accelerated to the level 

 where there might be some hope of finding it in the field of the microscope. 



Attempts to detect recombination in two other strains of E. coli, B 

 (Demerec and Fano, 1945) andL-15 (Roepke, Libby, and Small, 1944) by 

 analogous methods have been unsuccessful (Luria, 1947, Tatum and Leder- 

 berg, 1947). At least two strains then must be classified with the "Fungi Im- 

 perfecti." This dismal conclusion is, however, illuminated by the fact that 

 many heterothallic species have been eliminated from the Fungi Imperfecli 

 with the discovery of the appropriate opposite mating-types. At the present 

 time, one scarcely knows where to begin to look for the bacterial analogy. The 



161 



