Bacteria: Recombination {V) 



211 



FIGURE 41-1. Large and minute colonies of 

 Salmonella, representing complete and abortive 

 transductions, respectively. {Courtesy of P. E. 

 Hart man.) 



about ten times as many minute colonies (see 

 Figure 41-1). These minute colonies did not 

 appear in platings of auxotrophic mutants on 

 deficient medium. They were also not 

 attributable to any interaction between auxo- 

 trophs and colonies of normal or transduced 

 prototrophs located elsewhere on the plate. 

 It was possible to show, in a variety of cases 

 and by various methods,*' that each minute 

 colony contained but a single genetically 

 prototrophic cell. Minute colony formation 

 was demonstrated to have the following 

 origin. The initial cell of the minute colony 

 received through phage infection the segment 

 of DNA containing the gene for prototrophy 

 under test. However, this gene (1) failed to 

 be integrated, (2) failed to replicate, (3) but 

 retained its functional capacity to produce a 

 phenotypic effect. As a consequence, a 

 partial hybrid or heterogenote was produced 

 in which the injected cistron for prototrophy 

 was functional. Because prototrophic cis- 



"• By B. A. D. Stocker, J. Lederberg, N. D. Zinder, 

 and by H. Ozeki (1956). 



tronic product was made, the cell was able to 

 grow and then divide. However, only one of 

 the first two daughter cells received the extra 

 chromosomal segment, or exogenote. The 

 daughter cell without the exogenote was able 

 to grow and divide only until the prototrophic 

 cistronic product became too scarce; on the 

 other hand, the heterogenotic daughter cell 

 could continue to grow and divide, in turn 

 producing only one heterogenotic daughter 

 cell. In this way, a minute colony is produced 

 which contains a single genetically proto- 

 trophic cell. This consequence, of the failure 

 of complete transduction, is called abortive 

 transduction. 



Hypothetically, there are two possible fates 

 for the exogenote in an abortive transduction. 

 Eventually, the exogenote might either be 

 lost (by mutation) or it might undergo inte- 

 gration to result in a complete transduction. 

 The latter alternative has, so far, not been 

 found to occur. Regardless of its ultimate 

 fate, we can agree that the exogenote is 

 cistronic in nature. Note that the exogenote 

 is still considered to be a segment of genetic 

 material, even though it does not self-rep- 

 licate. But, remember, self-replication was 

 an assumed characteristic of the total genetic 

 material, and that no such capacity was re- 

 quired of a cistron when this was defined. 



In most transduction studies, an excess of 

 phage is used. In such experiments trans- 

 duced cells are always found to have simul- 

 taneously become lysogenic. This means 

 that the cell being transduced has received not 

 only the exogenote segment but an apparently 

 complete genome of a phage as well, the 

 former resulting in genetic recombination, 

 the latter in immunity and eventual lysis of 

 some progeny (lysogeny). Is the phage 

 particle whose contents enter the host cell and 

 make it lysogenic, the same particle which 

 introduces the exogenote? This need not be 

 so, since the use of high concentrations or 

 multiplicities of infecting phage means that 

 the host could have been penetrated by the 



