Bacteriophage: Recombination and Genetic Maps 



349 



esis that the smallest recombinational unit in 



phage equals one nucleotide.'" 



The entire rll region contains about 2000 

 linearly-arranged nucleotides. Consider the 

 functional characteristics of this region. If 

 only the production of the r and r + pheno- 

 types were considered, the rll region would 

 behave as a single functional unit. The rll 

 region, however, is composed of two sub- 

 regions, A and B, which show complementa- 

 tion. Such a division suggests that A and B 

 are independent, separate units at the func- 

 tional level. 



When E. coli strain K12 is doubly-infected 

 with wild-type ( + + ) and T4 doubly-mutant 

 (a x a- 2 ) in the A (or B) region, the r+ pheno- 

 type is produced. In this case, the mutants 

 are present in the same DNA double helix, 

 being in the cis position (Figure 26-8). 

 When the strain K12 bacterium is doubly- 

 infected and each virus particle carries one 

 of these mutants (a x -\-, +a 2 ), the mutants 

 are in the trans position. No complementa- 

 tion occurs, and no plaque is produced. 

 When such a cis-trans test gives this result, 

 the two mutants failing to complement in the 

 trans position are said to belong to the 

 same functional (A or B) unit, or cistron. 

 The closest mutational sites between the A 

 and B cistrons (Figure 26-6) are no more 

 than 0.4 map units apart, indicating that the 

 two cistrons are not separated by a large 

 amount of DNA within which recombination 

 can occur. 



Temperate Phages 



Genetic recombination also occurs between 

 temperate phages. From the frequencies 

 of genetically-recombinant phages occurring 

 among the progeny of sensitive cells multiply 

 infected with different mutants of lambda, 

 it is possible to arrange the mutants on a 

 single linkage map, just as is done for dif- 

 ferent mutants in the virulent T phages. The 



9 Support for this view is found in work by D. R. 

 Helinski and C. Yanofsky (1962). 



map for A, however, is (like T5) not circular 

 as is the map for T4. 



The difference between a temperate and 

 an intemperate phage is that the former can 

 lysogcnize its host. When temperate phages 

 infect sensitive bacteria, the plaques pro- 

 duced have a turbid center caused by the 

 growth of the bacteria that were lysogenized 

 — not lysed. In such a temperate strain, 

 mutants occur whose capacity for lysogeniza- 

 tion is either decreased or lost (in which case 

 the phage become virulent) and are detect- 

 able because they form less turbid or clear 

 plaques. Matings between phages that carry 

 mutants with different degrees of virulence 

 and other markers show that the loci con- 

 trolling ability to lysogenize are a regular 

 part of the phage genetic map. 



Some mutants seem to affect the very 

 process by which phage is converted to pro- 

 phage. Mutants of the latter type are called 

 c {clear) mutants and occur in a cluster of 

 loci within the phage genetic map. In 

 lambda, three groups of c mutants occur in 

 the sequence of c 3 , Ci, c- 2 (Figure 26-9). 

 Mutants in the Ci segment no longer have 

 any measurable ability to lysogenize and, 

 therefore, cause completely-clear plaques, 

 whereas those in c 3 and c 2 have about . 1 to 

 .01 of the lysogenization ability of wild-type 

 lambda. It has been demonstrated, further- 

 more, that these three loci act at different 

 times after infection. 



More than a dozen temperate phages in 



a, a 



1 "2 



+ + 



CIS 



+ a. 



TRANS 



figure 26-8. Cis and trans positions for two 

 mutants in cistron rll A. 



