Chapter 25 

 TRANSDUCTION 



T! 



(he three preceding chapters 

 dealt with recombination of 

 genetic material in bacteria 

 by mechanisms usually involving relocation 

 of only a portion of a bacterial genome. 

 In transformation, the presence of a donor 

 organism is unnecessary for the entrance of 

 donor DNA; in conjugation, however, a seg- 

 ment of chromosomal DNA passes from 

 donor to recipient bacterium through a cy- 

 toplasmic bridge effected by the presence of 

 an F particle. The F particle itself under- 

 goes recombination not only when it is in- 

 fectious but when it enters and leaves the 

 bacterial chromosome. 



As already noted (p. 310), it is expected 

 that any homologous segment of DNA, no 

 matter how introduced into a bacterium, can 

 pair with and integrate into a bacterial chro- 

 mosome. A third possible mechanism ex- 

 ists for introducing homologous DNA into 

 bacteria. Bacteriophages or phages 1 arc 

 viruses that attack bacteria. After these vi- 

 ruses become attached to the bacterial sur- 

 face (see Figure 24-1), all or part of the 

 phage remaining external to a bacterium can 

 be shaken off by the shearing action of a 

 blendor. Such treatment does not alter the 

 course of the infection, however; that is, the 

 virus still produces its characteristic effect 

 on the bacterium. We can infer that the 

 part of the virus essential for this effect 

 actually enters the bacterial protoplasm, 



1 The Greek letter <$> (phi) is used to denote phage. 

 330 



whereas the part of the phage that remains 

 attached to the bacterial surface is unneces- 

 sary. These observations suggest a new 

 way by which homologous DNA may en- 

 ter a phage-infected cell. The virus might 

 carry a segment of DNA derived from a 

 previous bacterial host. This piece could 

 penetrate the new host at the same time as 

 part of the phage docs, the phage's entry 

 providing the opening for the bacterial 

 DNA. 



With this possibility in mind, consider a 

 series of experiments 2 involving the mouse 

 typhoid organism. Salmonella typhimurium. 

 This bacterium, like its close relative E. coli, 

 can also be cultured on a simple nutrient 

 medium. A large number of auxotrophic 

 strains of Salmonella are available, includ- 

 ing one that requires methionine (M~T+ ) 

 and another that requires threonine 

 (M + T~). When these two strains are 

 mixed and plated on a culture medium lack- 

 ing both methionine and threonine, proto- 

 trophic colonies appear in such large num- 

 bers that they cannot be explained entirely 

 as the result of mutation. Prototrophs are 

 also obtained when a liquid culture of the 

 M + T~ strain is centrifuged (to remove 

 most of the bacteria), the supernatant liquid 

 heated for 20 to 30 minutes (to kill any 

 remaining bacteria), and this liquid added 

 to the M~T+ strain. This procedure dem- 

 onstrates that living M + T donor cells are 

 not required to furnish the M+ factor 

 needed to establish prototrophy. So here 

 the production of prototrophs clearly docs 

 not result from conjugation. Moreover, the 

 filtrate retains its full M+ capacity after 

 treatment with DNase. Accordingly, this 

 is not a case of genetic recombination via 

 transformation. Since the M+ factor can 

 pass through filters that hold back bacteria 

 but not viruses, the factor is a "filterable 

 agent." The reverse experiment — using fil- 



'-' The following discussion is based upon the work, 

 of N. D. Zinder and J. Lederberg (1952). 



