RECOMBINATION IN VIRUSES AND BACTERIA 



125 



not multiply and is diluted out by division. Second, the occurrence of 

 mutations from the defective state back to normal identifies defective 

 phages. These rare changes allow the release of some free normal phages 

 in a culture carrying a defective prophage, but that culture, although 

 lysed upon exposure to inducing agents, does not produce phages in the 

 process. Therefore we can see how mutation to the defective state 

 suppresses the viral aspect but not the other genetic functions of the 

 prophage. The sequence — virus to prophage to defective — is a change 

 from infectious heredity to stable heredity. The reverse mutation from 

 defective phage to prophage causes the special character of the infected 

 cell to become transmissible via the virus produced (Figure 5.6). 



TRANSDUCTION OF BACTERIAL GENES VIA BACTERIOPHAGE 



By phage infection, then, genes determining bacterial characters may 

 be transferred from one host to another. Lysogenic conversion is none- 

 theless quite different from transduction via temperate phages. In 

 transduction the phage is simply a vehicle for the transfer of a part of 

 the genotype of the donor bacterium to a recipient bacterium. When 

 streptomycin-sensitive bacteria, for example, are infected with phages 

 that have been grown on streptomycin-resistant hosts, a fraction (ca. 

 10~^), of the cells that survive infection become streptomycin-resistant 

 and can thenceforth act as donors of that character. During their multi- 

 plication some temperate phages incorporate within their coats not only 

 the DNA of their own genotypes but also some DNA from their bacterial 

 host. This host DNA is injected into the recipient and has a small 



a 



Antigen 15 



i 



2 X 10' 



1 C k - 



1 / Mutation 



Antigen 10 



< 10' 



Mutation 



Lysogenic cell 

 producing normal 

 phage 



Cell with 

 defective prophage 



^ 



Antigen 15 



Prophage 

 lost 



Prophage 

 regained 



FIGURE 5.6. Mutation to defectiveness and loss of propfiage in a Salmonella 

 lysogenic for phage f v/hich causes the formation of antigen 15 instead of antigen 10 

 produced in its absence (after Uetake, in: Luria, 1958, Ann. N. Y. Acad. Sci, 

 71:1085). 



