THE INITIATION OF BACTERIOPHAGE INFECTION 221 



The procedure described above, of first isolating a phage-resistant cell 

 mutant and subsequently isolating a host-range mutant of the phage that 

 can infect the resistant cell, has been extended with T2 and E. coli B, as 

 shown in Table II (Luria 1945a,b; Baylor et al., 1957). The resistant cells 



TABLE II 



Host-range Mutants of T2 and Corresponding T2-Resistant Mutants of 



E. coli B 

 (From Baylor et al., 1957) 



Key to symbols: 



+ Indicates plaque formation. 



Indicates failure of plaques to appear. 



B/2, B/2//i, and B/2//i, h' were obtained by plating, respectively, B with an 

 excess of T2/i'*' and B/2 with an excess of T2^, and selecting surviving colonies. 

 The phage mutants h' and h" were obtained from plaques formed on plating, 

 respectively, Jv^ with B/2/h and hf with 'Qf^jh, h' . It is evident from these 

 results that both a phage and a cell possess the genetic capacity for 

 modifying their surface structure in many ways. 



Host-range mutants of phage have not been found with aU systems. For 

 example, attempts to isolate a TIA mutant that can infect B/1, 5 have failed, 

 although h mutants that infect B/1 are readily found (Luria, 1945a,b). Simi- 

 larly, T4A mutants are known only for Kr2/4 but not for B/4 (Benzer, per- 

 sonal communication). 



Another type of host-range mutant, called lit, has been detected in k^ 

 strains of T2 (Hershey and Davidson, 1951; Baylor et al., 1957). Mutants of 

 this type form turbid plaques on B/2 (plaques from h are clear). A strain of 

 ht attaches to B/2 at a slower rate than h (probably accounting for the 

 plaque turbidity), but both strains attach at about the same rate to sensitive 

 B ceUs (Hershey and Davidson, 1951). An ht mutation, therefore, partially 

 restores the h phenotype to an Ti^ strain. Independent ht mutations in an h^ 

 strain occur at many separate positions scattered over almost the entire 

 phage chromosome, and in this way differ from h mutations, which can occur 



