MUTATION AND PHENOTYPIC VARIATION IN PHAGES 307 



tions are of special interest because the action of proflavine has 

 been traced to the maturation step in phage growth. 



Sertic (1929a) described a phage whose plaques were sur- 

 rounded by a spreading halo caused by a rapidly diffusible iysin. 

 Sertic and Gough (1930) reported hereditary variation with 

 respect to this character. Another example is described by 

 Adams and Park (1956), who cite additional pertinent literature. 

 In their case the Iysin was identified as an enzyme hydrolyzing 

 the capsular material of Friedlander's bacillus. The enzyme 

 was found both free in phage lysates and attached to phage 

 particles. It was not found in uninfected bacteria. Phage 

 variants were obtained diff"ering in the amount of enzyme pro- 

 duced, with corresponding differences in plaque-type. The 

 possibility of an enzyme under genetic control by the phage is 

 strongly indicated. The role of the enzyme in adsorption of the 

 phage is not yet clear. 



Several results of Streisinger's intercrosses between T2 and T4 

 have been mentioned, but the most remarkable remains to be 

 described. These two phages diflfer in three characters pertinent 

 in the present context (Streisinger and Weigle, 1956). The 

 DNA of T2 contains 0.77 mole of glucose per mole of hydroxy- 

 methylcytosine, whereas that of T4 contains 1.0 mole (Sin- 

 sheimer, 1956). T2 plates with very low efficiency on certain 

 stocks of E. coli K12 whereas T4 plates with high efficiency. T4 

 suppresses the growth of T2 in mixed infections, whereas T2 does 

 not suppress the growth of T4. These three characters will be 

 called bar characters in the remainder of this discussion. They 

 seem to have a common genetic determination or, at any rate, 

 fail to segregate in crosses. 



If a bacterium is infected with both T2 and T4 phages, it 

 liberates many particles with the host range genotype of T4 but 

 also some with the host range genotype of T2. Similarly, if the 

 T2 parent carried an r marker, some few of the offspring particles 

 will be r. When the offspring particles are examined for the 

 bar genotype, however, all of them prove to resemble T4. 

 Moreover, when one of the off'spring particles with the host 



