124 CELL HEREDITY 



semi-temperate phages, when plated on a certain bacterial host, yield 

 mutant types which, more frequently than chance will allow, occur to- 

 gether in a single cell as though they had both arisen in a single event. 

 As the progeny phages are repeatedly plated on the same bacterial strain, 

 they become stable. But immediately upon plating on another strain 

 mutants appear once again. It is as though the phages, by recombina- 

 tion with the bacterial host, picked up new genes from it. After re- 

 peated "inbreeding, the phages became isogenic with that part of the 

 host with which they shared genetic homology. But immediately upon 

 "outcrossing," new genes could be assimilated once again. Genetic 

 homology with the host may be the reason for the strong resistance of 

 temperate phages to irreversible damage by ultraviolet light. Whether 

 there is really an exchange with the host chromosome or with some 

 cryptic prophages on that chromosome is a problem for the future. But 

 there may be less difference between phage and bacterial genes than was 

 previously thought. 



For example, some genes located in the prophage DNA influence 

 traits of the host bacterium, including colony morphology and the pres- 

 ence of particular antigens or enzymes. This phenomenon is called 

 lysogenic conversion. For example, all toxigenic strains of the diphtheria 

 bacillus carry a specific prophage; when it is lost by cultivation in anti- 

 phage serum, a strain is selected which is not toxigenic. When this 

 strain is lysogenized again, it once more becomes toxigenic. Lysogeniza- 

 tion with a different strain of phage is without effect. However, the 

 genes of the two phages may be exchanged in a cross during vegetative 

 reproduction, and the second phage may receive the gene conferring the 

 capacity for toxin production on the host bacterium. These and other 

 temperate phage genes may be mapped and are found to exist in a linear 

 order. One group of these genes is concerned with the ability of the 

 phage DNA to reduce to prophage. Phages bearing one of these genes in 

 mutant form behave as virulent and invariably lyse the host unless pres- 

 ent with another phage also mutant, but at another locus. Such "hetero- 

 zygous" pairs complement one another and allow reduction of the viru- 

 lent phage to the prophage condition. 



Mutations also occur which make the prophage defective, that is, able 

 to infect the host but not to multiply or to give rise to vegetative phages 

 which lyse the host cell. The cryptic existence of defective phage is 

 demonstrated first by the retention of the immunity against related 

 phages that characterizes the presence of prophages. Immunity is the 

 character a prophage confers upon its host whereby, even though a re- 

 lated phage may be adsorbed and inject its DNA, that DNA cannot 

 develop into vegetative phage and rarely into prophage; it simply does 



