RECOMBINATION IN VIRUSES AND BACTERIA 



123 



fectious phages which they have formed (Figure 5.5). When temperate 

 phages adsorb on sensitive bacteria, one of two results may follow: either 

 the invaders may behave like virulent phages, multiply, and eventually 

 Ivse the cell; or instead, the invading DNA may be reduced to the 

 prophage condition, multiplying innocuously in tempo with the host. 

 This equilibrium is rarely broken spontaneously, but it can be disrupted 

 in most cells by the application of inductive stimuli such as radiation 

 or chemical mutagens and carcinogens. 



Although the adsorption of a temperate phage clearly does not kill the 

 host bacterium, the growth of a phage out of synchrony with a bac- 

 terium does. In this case the prophage is converted to a vegetative 

 condition in which it multiplies, eventually becoming enclosed in a pro- 

 tein coat equipped with a tail. These mature, infectious, temperate 

 phages are released by lysis of the host. 



During the vegetative growth of two related strains of temperate 

 phages, gene exchange may occur. This process has all the properties 

 described for virulent phages. Recombination may even take place be- 

 tween a vegetative phage and a prophage, or between two prophages in 

 the same lysogenic bacterium. Just how this recombination takes place 

 is yet to be discovered. Of even more interest is evidence which indi- 

 cates that some temperate or nearly temperate phages may recombine 

 with bacterial genes. Evidence for this comes from the fact that some 



f — Y — N Nonlysogenic r — v — \ 

 ^-^ — ' bacteria ^^->— >* 



oo 



Lysogenic 

 bacteria 



with 

 prophage 



GXD 



^C^^ 



FIGURE 5.5. The life cycle of a temperate phage (after Lwoflf, 1953, 6ac/. Rev., 

 17:269). 



