LYSOGENY 369 



3. Properties of Temperate Phages 



Temperate phages may be studied by the usual techniques 

 in which sensitive bacteria are infected with phage. Although 

 a certain fraction of the infected bacteria becomes lysogenic, 

 conditions can be obtained in which this fraction is sufficiently 

 low not to impair quantitative analysis. In this way one can 

 determine the characteristics of multiplication, such as adsorp- 

 tion, latent period, burst size, etc. 



Temperate phages, like other phages, are composed of DNA 

 and proteins. However, in opposition to what has been ob- 

 served with the virulent phages T2 and T5, temperate phages 

 previously inactivated by ultraviolet light do not kill sensitive 

 bacteria. Their protein coat appears therefore to be devoid 

 of any lethal action on bacteria. 



Many mutations have been described which affect various 

 properties of temperate phages, such as host range, plaque type, 

 plaque size, etc. Among these mutations, are some of a particu- 

 lar interest because they affect the ability to lysogenize. Plaques 

 formed by temperate phages are turbid because their center 

 is occupied by the growth of lysogenic cells. On the contrary, 

 plaques formed by certain mutants are clear because such phages 

 are unable to lysogenize (Dooren de Jong, 1931; Burnet and 

 Lush, 1936; Boyd, 1952a). They have lost the temperate 

 character. 



Genetic recombination has been analyzed in several tem- 

 perate phages (Murphy, 1953; Jacob and Wollman, 1954; 

 Kaiser, 1955; Levine, 1957). The main features are essen- 

 tially the same as those observed with T2 and T4 (Chapter 

 XVIII) and the Visconti-Delbriick (1953) theory can be ap- 

 plied to them all. In every case studied so far, all the known 

 mutations can be mapped on a single linkage group and the 

 average number of rounds of mating is rather low, 0.5 or less. 

 The frequency of recombination can be increased by exposing 

 the phage particles to ultraviolet light before crossing (Jacob 

 and Wollman, 1955). 



