Viruses: Recombination in Bacteriophage {II) 



393 



GENOTYPE 



PLAQUES FORMED 

 ON HOST STRAIN 



FIGURE 43-3. Be/mvior of r mutants of T-even 

 phages in t/ie B and K strains ofE. coli. 



duce large, sharp-edged plaques. When the 

 r mutants are mapped they are found to 

 occupy three distinct regions of the phage 

 genetic map, rl, rll, and rlll. The r mutants 

 in all three regions can be detected and 

 harvested using E. coli strain B. However, 

 the mutants in the rll region are unique in 

 having an additional attribute, namely, that 

 they cannot form plaques when their host is 

 strain K of E. coli (which is lysogenic for 

 lambda) although the rl and rlll mutants and 

 r+ phages can do so (Figure 43-3). Thus, 

 mutants in the rll region show a restriction in 

 host range as compared with r+ or r mutants 

 in other regions. We shall restrict our atten- 

 tion henceforth to the mutants of the rll 

 group in phage T4, 



The host range restriction of rll mutants is 

 useful not only for their identification, but 

 for the study of rates of mutation and of 

 genetic recombination. After identifying a 

 mutant as being in the rll region, its mutation 

 rate to /•+ can be determined readily by plat- 

 ings on strain K, since only r+ mutants will 

 form plaques there. From a large number of 

 rll mutants, those which were stable and had 

 a low mutation frequency (sometimes as low 

 as 1 per 10** phages) were selected for further 

 study. Using high multiplicities of phage 

 infection, the results with these showed that 

 mutants in the rll region fall into one of two 

 groups. If a bacterium is infected by one 

 mutant from each of the two groups, both 

 viruses can reproduce and lyse the cell. (In 

 this case almost all the multiply infected K 

 bacteria on a plate would lyse, clearing the 

 entire plate in about half an hour.) This 

 behavior can be explained by considering 

 that the rll region is composed of two sub- 

 regions, A and B. Each subregion in normal 

 phage independently forms a phenotypic 

 product, the products of both subregions 

 being required to cause the r+ phenotype. So, 

 a mutant defective only in the A subregion 

 still makes proper B product, and vice versa. 

 In a bacterium multiply infected with one 

 phage mutant in A and another mutant in B, 



FIGURE 43-4. T/ie occurrence or non-occurrence of complementation between 

 different rll mutants. 



X y 



r X r 



FUNCTIONAL 

 COMPLEMENTATION 



r'^X r^ 



NO 

 COMPLEMENTATION 



