Bacteriophage: Recombination and Genetic Maps 



343 



netically, phages can differ according to the 

 hosts they are able to infect, and mutants 

 occur which change the range of hosts at- 

 tacked. Therefore, phage mutants affect- 

 ing host range can also be detected and 

 maintained. 



One can obtain a strain of intemperate T 

 phage that is mutant both for host range, 

 h, and plaque type, r. Sensitive bacteria 

 are infected by a single phage particle con- 

 taining both markers to determine the muta- 

 tion frequencies to the wild-type alleles (h + 



and r+); wild-type phages are used to de- 

 termine the mutation rates to each of the 

 two kinds of mutant alleles. When the sen- 

 sitive bacterial strain is exposed to a highly 

 concentrated mixture of the double mutant 

 (h r) and wild-type (h + r+) phages, so 

 that some of the multiply-infected cells carry 

 both phage types, not only do the parental 

 types (h r and h + r+ ) occur among the 

 progeny, but the recombinant types {h r 

 and h r+ ) occur in frequency too high to 

 result from mutation (Figure 26-3). Con- 



figure 26-4. A recombination map of T4D. Filled-in areas represent min- 

 imal lengths for genes. The symbols for phage components represent the 

 typical morphological products present in lysates of mutant-infected E. coli. 

 (Courtesy of R. S. Edgar; see F. W. Stahl, et al.. Genetics, 50:539-552, 1964.) 



