236 ROBERT L. SINSHEIMER 



This accessory element, to which has been given the name "prophage," 

 once incorporated into the bacterial genome is then reproduced each time 

 the bacterial genome is reproduced, and in this way is passed on to all 

 the descendants of the modified cell. Such a bacterial culture, carrying 

 a prophage, is said to be "lysogenic." 7 ' 185 



Analysis of bacterial matings involving cells carrying prophage indicates 

 that these distinctive accessories are located at a specific site on the bac- 

 terial chromosome. Prophages from different species of temperate viruses 

 are in general found at different sites on the chromosome. Some species 

 of temperate phage (P 2 ) have apparently more than one possible site of 

 incorporation. One cell may carry two or more prophages. If two are pres- 

 ent they may be at the same site or at different sites. 



The presence of the prophage invariably confers new properties upon 

 the lysogenic cells. The most striking of these is the potential of the pro- 

 phage to break free of metabolic control, to assume the status of vege- 

 tative phage, and to initiate an unrestricted virus synthesis culminating 

 in lysis and the release of many mature phage particles This process — the 

 expression of the inherent phage potential — occurs spontaneously in lyso- 

 genic cell cultures at a low rate of the order of 10~ 4 per cell generation. 

 Hence, such cultures always contain a significant amount of free phage 

 particles. 



Some temperate phages when carried in the lysogenic condition are "in- 

 ducible"; the application of appropriate agents to a lysogenic culture of 

 such phage will cause a transformation from prophage to vegetative phage 

 in a large proportion (up to 90%) of the cells, resulting in a subsequent 

 mass lysis. 



It is of interest that the most effective agents for induction are also 

 well known mutagens, including ultraviolet irradiation, X- or 7-irradia- 

 tion, nitrogen mustards, and hydrogen peroxide. 185 Many species of tem- 

 perate phage are, however, as prophage, resistant to all known modes 

 of induction and can only be obtained as a consequence of spontaneous 

 liberation. 



The presence of prophage confers other properties upon the lysogenic 

 cell. Such cells are almost invariably immune to attack by other phages 

 of the same species. Immunity may also be produced to some unrelated 

 phages. This immunity appears to be a physiological condition which pre- 

 vents either the prophage or superinfecting phage from multiplying vege- 

 tatively. 



In some instances the presence of prophage confers upon the cell an 

 ability to make toxin, 186 or a change of colony form from smooth to rough, 187 

 or a change of somatic antigen. 188 



186 A. Lwoff, Bacteriol. Revs. 17, 269 (1953). 

 186 N. B. Groman, /. Bacteriol. 69, 9 (1955). 



