SEGREGATIONS IN ESCHERICHIA COLI 521 



strains of E. coli (Boivin and Vendreley 1946) one might postulate that 

 genotypically distinct cells interact not through cell fusion, but through the 

 release of "transforming substances" diffusing through the medium. Such 

 transforming substances would have the property of inducing or directing mu- 

 tational changes in the cell receiving them so as to lead to what appear to be 

 recombination types. Our inability to separate such postulated transforming 

 substances from the cells themselves is not proof of their absence but could be 

 due to their lability in our hands. 



In previous publications, certain reasons were given for the rejection of the 

 transformation hypothesis in favor of a picture of cell fusion, and so forth. It 

 was not our intention thereby to state, with clairvoyant insight, that no in- 

 vestigator will be able to duplicate the results which we have reported, using 

 instead of living cells extracts specially prepared. It is, rather, our view that 

 since we have been able to demonstrate no appreciable point of difference be- 

 tween the features of gene exchange in this strain of E. coli and in the classical 

 materials of Mendelian experimentation, the most economical conclusion is 

 that the mechanisms involved are also similar. In the absence of more detailed 

 information on the behavior of transforming systems, a critic would be free to 

 impute to such systems all of the properties which have been found to charac- 

 terize the genetic system of E. coli, K-12. While this would be tailoring the 

 cloth to suit the customer, it cannot be disputed that the only conclusive 

 method by which it could be shown that cell fusion underlies gene recombina- 

 tion would be a direct cytological demonstration. The rarity with which the 

 presumed zygote occurs, however (as indicated by the low frequency of effec- 

 tive recombination types) is very discouraging to attempts to find and char- 

 acterize the "fusion-cell," at least in the present material. 



Certain genetic experiments were performed in an attempt to characterize 

 further the behavior of this system. On the transformation hypothesis, one 

 must attribute the rarity of the imputed transformations primarily to re- 

 stricted conditions for susceptibility to the transforming factors released into 

 the milieu. Otherwise, one would expect to find "transformations" for single 

 factors much more frequent than those involving more than two factors. A 

 glance at tables 4 and 6 illustrates that certain "multiple transformed" types 

 are much more frequent than singly transformed classes. Under these con- 

 ditions, one might also anticipate that genetic materials from two different 

 kinds of cells could mix in the medium and together transform a third. In a 

 mixture of three cell types then, one should find cases where genes from all 

 three have combined. Using Lac and V\ as markers, this type of experiment 

 was set up in several different ways, as summarized in table 7. Pairwise, proto- 

 trophs can be formed only from biochemically distinct and nonover lapping 

 parents. Combinations of B~M~ and of T-L~Br were arranged so that taken 

 two at a time they were heterozygous either for Lac or for V\ but not both. For 

 example, a mixture of B-M-Loc-Vf, T-L~BrLac-Vi a and T-LSrLac+Vf 

 was plated. Prototrophs could be formed by recombination between either of 

 the two latter and the former types. In one case, only V\ would be heterozy- 

 gous, and the expected types would be Lac~ Vi r and Lac~V\'. In the other, Lac 



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