346 GENETICS: S. BENZER Proc. N. A. S. 



r Mutants. — The wild-type phages T2, T4, and T6 produce small plaques with 

 rough edges when plated on strain B of Escherichia coli. From sectors of clearing 

 in these plaques, mutants can be readily isolated which produce large, sharp-edged 

 plaques (Hershey^). These mutants have been designated "r" for rapid lysis; 

 they differ from the wild type by a failure to cause "lysis inhibition" on strain 

 B (Doermann^"). The wild type has a selective advantage over r mutants when the 

 two types grow together on B. The genetics of r mutants was studied by Hershey 

 and Rotman,^^ who found three regions in the linkage map of T2 in which various 

 mutations causing the r phenotype were located, including one large "cluster" of 

 mutants which were shown to be genetically distinct from one another. The 

 genetic study of T4 by Doermann and HilF showed r regions corresponding to two 

 of those in T2. T6 also has at least two such r regions. 



The rll Group. — For all three phages, T2, T4, and T6, the r mutants can be sep- 

 arated into groups on the basis of their behavior on strains other than B. This 

 paper will be concerned only with one group, which will be called the "rll group." 

 Mutants of the rll group are distinguished from those of other groups, and from 

 wild type, by a failure to produce plaques on certain lysogenic strains^^ of E. coli 

 which carry phage X. As shown in Table 1, a mutant of the rll group produces 



TABLE 1 



Phenotypes (Plaque Morphology) of T4 Wild and rll 

 Mutant Plated on Various Hosts 



'— ■ — ■ • Host Strain . 



E. coli E. coli E. coli 



B K12S K12S (X) 



T4 wild type Wild Wild Wild 



T4 rll mutant r Type Wild 



r-type plaques on strain B, wild-type plaques on strain K12S (nonlysogenic strain 

 sensitive to X), and no plaques on K12S (X) (derived from K12S by lysogenization 

 with X). The wild-type phage produces similar plaques on all three strains. In 

 the case of T4, with which we shall be concerned in this paper, the efficiencies of 

 plating are approximately equal on the three strains, except, of course, for rll on 

 K12S (X). The three bacterial strains will be here designated as "B," "S," and 

 "K." 



Approximately two-thirds of the independently arising r mutants isolated on 

 B are of the rll type. This group includes the "cluster" of r mutants of T2 de- 

 scribed by Hershey and Rotman and the r47 and r51 mutants described by Doer- 

 mann and Hill in the corresponding map region of T4 but does not include r mu- 

 tants located outside that region. Similarly, all newly isolated mutants showing 

 the rH character have turned out to fall within the same region, as indicated in 

 Figure 1. 



The properties of the rll group are especially favorable for detailed genetic 

 study. An rll mutant has three different phenotypes on the three host strains 

 (Table 1): (1) altered plaque morphology on B, (2) indistinguishable from wild 

 type on S, and (3) unable to produce plaques on K. These properties are all useful. 

 By virtue of their altered plaque type on B, r mutants are readily isolated, and 

 those of the rll group are identified by testing on K. Where it is desired to avoid 

 a selective disadvantage compared with wild type, e.g., in measuring mutation 



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