Bacteriophage: Recombination and Genetic Maps 



345 



/•// to r//+ can be determined readily by 

 plating T4/7/ on strain K12, since only mu- 

 tants to /•+ will form plaques (r+ is "se- 

 lected" on strain K12). A large number of 

 rll mutants that have a low mutation fre- 

 quency (sometimes as low as one per 10 s 

 phages) can be obtained. T4/7/ mutants can 

 be divided into two classes, A and B, on the 

 basis of their behavior after mixed infection 

 of strain K12. When K12 is infected with 

 an rll phage from each class, growth of the 

 phage and lysis of the host occurs. This 

 behavior suggests that the rll region is com- 

 posed of two subregions, A and B, and the 

 products of both are required to produce the 

 normal r+ phenotype. Mutants defective 

 only in the A subregion presumably can still 

 make normal B product, and vice versa. In 

 a bacterium multiply-infected with one phage 

 mutant in A and another in B, the B and A 

 products produced by the mutants can co- 

 operate — that is, show complementation — to 

 produce the r+ phenotype (Figure 26-6). 

 If the two different rll mutants in a mul- 



tiple infection of strain K12 are located in 

 the same subregion — region A, for example 

 — they will be unable to produce the r + 

 phenotype by complementation since neither 

 phage can produce normal A product. In 

 such cases the phage cannot grow and the 

 host will not lyse unless the infecting mu- 

 tants have lesions far enough apart in region 

 A to permit wild-type progeny to result from 

 recombination between them. 



Two mutants, rl and r2, arising independ- 

 ently in the same subregion. may fail to re- 

 combine with each other; however, rl may 

 recombine with a third independent mutant, 

 r3, even though mutant r2 does not. These 

 results suggest that mutant r2 has a long de- 

 ficiency or deletion that includes all or part 

 of the region defective in rl and r3. Such 

 deletion mutants are never found to revert to 

 r+. Other mutants which do revert and 

 which give no evidence of having long de- 

 ficiencies are considered to be point mutants. 

 Of the more than 1500 spontaneously-occur- 

 ring rll mutants which have been typed, 



FUNCTIONAL COMPLEMENTATION 



A B 



x 



x y 



r X r 



» 



NO COMPLEMENTATION 



figure 26—6. The occurrence 

 or nonoccurrence of comple- 

 mentation between different 

 rll mutants. 



X z 



r X r 



