Chapter *43 



VIRUSES: RECOMBINATION 

 IN BACTERIOPHAGE (II) 



a: 



LTHOUGH most of the work 

 discussed in the last Chapter 

 utilized virulent phages, the 

 concluding portion mentioned that genetic 

 recombination also occurs between temperate 

 phages. It was mentioned specifically that 

 multiple infection of sensitive cells by different 

 mutants of lambda is followed by the occur- 

 rence of genetically recombinant phages among 

 the progeny. From the frequencies of such 

 recombinants it is possible to arrange the 

 mutants in a single linear linkage map, just 

 as was stated can be done for different mu- 

 tants in the virulent T phages. 



The difference between a virulent and a 

 temperate phage lies in the capacity which 

 the latter has to lysogenize its host. Is 

 lysogeny itself dependent upon the phage 

 genotype? When temperate phages infect 

 sensitive bacteria, the plaques produced have 

 a turbid region in their center due to the 

 growth there of bacteria which were ly- 

 sogenized, not lysed. In such a temperate 

 strain, mutants can occur whose capacity for 

 lysogenization is decreased or lost (in the 

 latter case the phage becomes a virulent one), 

 and are detectable because they form less 

 turbid or clear plaques, respectively. This 

 proves that lysogenization is based upon a 

 part of the phage genotype which is expressed 

 in the phenotype of its host. Moreover, 

 "matings," between phages carrying different 

 lysogenization mutants and other markers, 

 show that these lysogeny loci are a regular 

 part of the phage genetic map. 

 390 



While certain mutations affecting lysogeny 

 seem to affect the stability of prophage in the 

 course of bacterial multiplication, the most 

 important ones seem to affect the very process 

 by which phage is converted to prophage. 

 Mutants of the latter type are called c {clear- 

 ing) mutants and these occur in a cluster of 

 loci located within the genetic map of the 

 phage. In lambda, there are three groups of 

 c mutants arranged linearly in segments called 

 Cs, Ci, and Co (Figure 43-1). Mutants in the 

 Ci segment no longer have any measurable 

 capacity to lysogenize, whereas those on 

 either side (being in Cs or C2) have their 

 ability for lysogenization reduced to about 

 .1 to .01 of that of wild-type lambda. 



It has been possible to isolate more than a 

 dozen temperate phages in E. coli, of which 

 some show ultraviolet and zygotic induction 

 and others do not. All seven of the viruses 

 which give rise to inducible prophages were 

 found to be associated with the chromosome 

 at different loci (Lp or ly), all of which are 

 located in a linear order close to and on the 

 same side of Gal (Figure 43-2). These phages 

 are all different, in the respect that a host 

 lysogenic for one of them is immune to 

 subsequent infection by the same phage, but 

 is not immune to subsequent infection by any 

 of the others. 



When multiple infections are made, to 

 cross lambda with any one of the other six 

 phages, some genetic recombinations are 

 found in the progeny phage in each case. 

 However, the markers capable of recombina- 

 tion differ, and this is dependent upon the 

 type of phage with which lambda is crossed. 

 This demonstrates that these six phages differ 

 in the number of loci they contain which are 

 identical or homologous to loci in lambda. 

 The results of crossing lambda and phage 

 0434 are particularly instructive. </)434 

 proves to have a genotype that is completely 

 homologous to that of lambda, with the 

 exception of one region. That is, it is 

 possible to make a "hybrid" phage that is 



