274 



Table 1 



Phenotype Plaque Morphology of T4 



Wild and rll Mutant Plated on 



Various Hosts 



T4 wild type 

 T4 rll mutant 



Host Strain n 



E. colt E. colt E. coli 



B K12S KUSiX) 



WUd Wild Wild 



r Type Wild 



produces r-type plaques on strain B, 

 wild-type plaques on strain K12S 

 (nonlysogenic strain sensitive to >^), 

 and no plaques on K12S (^) (derived 

 from K12S by Ivsogenization with 'k). 

 The wild-type phage produces similar 

 plaques on all three strains. In the case 

 of T4, with which we shall be con- 

 cerned in this paper, the efficiencies of 



BENZER 



plating are approximately equal on the 

 three strains, except, of course, for rll 

 on K12S (^). The three bacterial 

 strains will be here designated as "B," 

 "S," and "K." 



Approximately two-thirds of the in- 

 dependently arising r mutants isolated 

 on B are of the rll type. This group in- 

 cludes the "cluster" of r mutants of T2 

 described by Hershey and Rotman and 

 the r47 and r51 mutants described by 

 Doermann and Hill in the correspond- 

 ing map region of T4 but does not in- 

 clude r mutants located outside that 

 region. Similarly, all newly isolated 

 mutants showing the rll character 

 have turned out to fall within the 

 same region, as indicated in Figure 1. 



The properties of the rll group are 



/o =10% 



m 



42 



r r 



47 51 



tu 



41 



rl region 



Fig. 1. Partial linkage map of T4 (Doermann), indicating the location of the rll 

 region, m and tu designate "minute plaque" and "turbid plaque" mutations. The cir- 

 cular inset shows, diagrammatically, the corresponding dimensions of the DNA 

 chain magnified 1,000 diameters. 



especially favorable for detailed gene- 

 tic study. An rll mutant has three dif- 

 ferent phenotypes on the three host 

 strains (Table 1): (1) altered plaque 

 morphology on B, (2) indistinguish- 

 able from wild type on S, and (3) un- 

 able to produce plaques on K. These 

 properties are all useful. By virtue of 

 their altered plaque type on B, r mu- 

 tants 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 

 rates, S can be used as a nondiscrimi- 

 nating host. The failure of rll mutants 

 to plate on K enables one to detect 



very small proportions of wild-type 

 particles due to reversion or due to re- 

 combination between different rll mu- 

 tants. 



Fate of rll Mutants in K.— Wild- 

 type and rll mutants adsorb equally 

 well to strains S and K. Whereas the 

 wild type provokes lysis and libera- 

 tion of a burst of progeny on both 

 strains, the rll mutant grows normally 

 only on S. Infection of K with an rll 

 mutant provokes very little (and/or 

 very late) lysis, although all infected 

 cells are killed. The block in grow th of 

 rll mutant is associated with the pres- 

 ence of the carried phage ^. The rea- 

 son for this association is unknown. 



