Bacteria: Recombination (!) 



341 



donor and the one acting as recipient. More- 

 over, it was shown that transformation does 

 not involve the mediation of a virus. There- 

 fore, beyond any reasonable scientific doubt, 

 it must be DNA itself and alone which is the 

 transforming agent. 



Other work showed that transformation 

 can occur in either direction (that is A -> A' 

 and A' -> A) and that bacteria can become 

 transformed with respect to any chromosomal 

 gene they possess. A' cells can be trans- 

 formed to A" type which, in turn, provide 

 large quantities of A"-DNA capable of trans- 

 forming other A' cells to A". So, the DNA 

 of transformed bacteria can be extracted in 

 turn to provide greatly increased amounts of 

 the same transforming principle. One trans- 

 forming principle (A') can transform bacteria 

 having any one of several alternative pheno- 

 types (e.g., A or A")- If the A'-DNA, from 

 bacteria transformed from A to A', is used 

 to transform bacteria of a third genotype 

 (A"), the only transformations produced are 

 those involving the genes of the immediate 

 donor (i.e., only A' and no A transformants 

 are found). This demonstrates that trans- 

 formations are not transmissible changes in- 

 volving a simple addition of particular genetic 

 material to the genotype, but entail the loss 

 of host genetic material at the same time that 

 the new genetic material is acquired. Thus, 

 the genetic change in transformation is of a 

 replacement type. 



How does transforming DNA accomplish 

 this replacement? It is possible - to trace the 

 fate of transforming DNA by labeling its 

 phosphate groups with radioactive P^-. At 

 various times after exposure to this labeled 

 DNA, one portion of the treated bacteria is 

 killed and analyzed with respect to the pres- 

 ence of P^- in its DNA, while another portion 

 is tested to determine whether it has been 

 transformed. Only after bacteria have been 

 exposed to the DNA extract for a suitable 



2 Based upon work of L. S. Lerman and L. J. Tol- 

 mach (1957). 



period of time is the labeled DNA found in 

 the extract containing the host's chromo- 

 somal DNA. Moreover, the frequency with 

 which the host cell is transformed is directly 

 proportional to the amount of labeled DNA so 

 incorporated. 



The results mentioned in the last two para- 

 graphs indicate that the transformer DNA 

 actually enters the bacterium and replaces a 

 segment of the host's chromosomal DNA,^ 

 after which the newly introduced material 

 replicates as a normal part of the chromo- 

 some. Since it is DNA which alone carries 

 the genetic information for transformation, 

 we conclude that transformation provides 

 direct and conclusive evidence that DNA is 

 genetic material. Accordingly, chromosomal 

 DNA contains the chemical units of the genetic 

 material. All other transformation studies 

 support this conclusion. 



We should now reexamine the assumption, 

 made earlier in this Chapter, that transforma- 

 tion involves mutation. The first results on 

 transformation seemed to involve novel, rare 

 changes in the genetic material, and were 

 hence called mutations (see p. 137). We now 

 know that transformation involves replace- 

 ment of one segment of genetic material by 

 another. No new type of genetic material 

 appears; there is only a shuffling of already 

 existent genes. Thus, no novel genes are pro- 

 duced by transformation. Moreover, genetic 

 transformation has been found not only in 

 Pneumococcus, but in Hemophilis, Xanthom- 

 onas. Salmonella, Bacillus, Neisseria, and 

 Escherichia and other organisms as well. In 

 the case of Neisseria, DNA is regularly liber- 

 ated into the slime layer by autolyzing cells 

 which are found in aging cultures. Such 

 DNA is effective in transformation, as is the 

 DNA obtained from penicillin-sensitive pneu- 

 mococci lysed after treatment with penicillin. 

 Not only is transformation widespread, but 

 a given type may occur with frequencies as 



^ See H. Ephrussi-Taylor (1951) for specific evidence 

 for the latter observation. 



