Chapter *41 



BACTERIA: RECOMBINATION 

 (V. Transduction) 



Ti 



Ihe four Chapters preceding 

 this have dealt with the shuf- 

 fling of genetic material in bac- 

 teria. In the cases discussed, the new genetic 

 combinations typically involved the reloca- 

 tion of only a portion of a bacterial genome. 

 While, in the case of conjugation, a segment 

 of chromosomal DNA passes from donor to 

 recipient bacterium through a cytoplasmic 

 bridge formed by the activity of an F particle, 

 no organismal assistance is necessary for the 

 entrance of donor DNA in all of the cases of 

 transformation previously discussed. It has 

 been shown in the last two Chapters that the 

 F particle itself undergoes recombination, 

 not only because it is infectious, but because 

 it can enter and leave the bacterial chromo- 

 some, i.e., because it is an episome. More- 

 over, because of the episomal cycle, F may 

 acquire chromosomal memory and the chro- 

 mosome may acquire F memory. Such new 

 nucleotide combinations in turn foster further 

 genetic recombinations which result in a flow 

 of cistrons between F and chromosome. 



It has been reasoned earlier (p. 349) that 

 any homologous segment of DNA introduced 

 into a bacterium has the potentiality of pair- 

 ing with, and integrating into, a bacterial 

 chromosome. Have we exhausted the mecha- 

 nisms for introducing homologous DNA into 

 bacteria? Bacteriophages or phages ^ are 

 viruses that have the capacity to destroy 

 bacteria. After these fasten onto the bacteri- 

 um, all or part of the phage that is external 

 ^ The Greek letter 0, phi, is used to denote phage. 

 374 



to the bacterium can be shaken off" by the 

 shearing action of a Blendor. Nevertheless, 

 the course of the infection is unchanged by 

 such treatment, that is, the virus still produces 

 its characteristic eff"ects on the bacterium. 

 This can be taken to mean that the part of 

 the virus essential for these eff^ects actually 

 enters the bacterial cytoplasm, and that what 

 remains attached to the outside of the 

 bacterium is dispensable in this regard. In 

 view of this behavior by phage, two new ways 

 for the entry of homologous DNA into a 

 phage-infected cell can be envisaged. First, 

 the virus might carry externally, adsorbed to 

 its outer surface, a segment of DNA derived 

 from its previous bacterial host. This piece 

 may penetrate the new host at the same time 

 that the essential part of the phage enters, 

 the latter action providing a place of entry 

 for the bacterial DNA. 



The second mode of DNA entrance in- 

 volves the internal contents of the phage 

 which enter the bacterium. The part of the 

 virus which enters the bacterium may contain 

 DNA which possesses a nucleotide sequence 

 also found in the bacterial chromosome. 

 There are two possible origins for such homol- 

 ogous DNA. It might be (1) a segment of 

 DNA, not normally a part of the virus, which 

 originated in the previous host chromosome 

 (in which case the phage may or may not be 

 defective in its own DNA or RNA content), 

 or (2) a segment of DNA that is normally 

 found as a (continuous) part of the viral 

 DNA. 



With this introduction, let us examine the 

 results of a series of experiments - employing 

 the mouse typhoid organism, Sahnonella 

 typhimiirium. This bacterium is a close 

 relative of E. coli, and also can be cultured on 

 a simple medium. A number of auxotrophic 

 strains of Sahnonella have been obtained, in- 

 cluding one that requires methionine {M'T+) 

 and another that requires threonine (M+T~). 



^ The following discussion is based upon the work of 

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



