IMMUNITY AND PROPHAGE LOCALIZATION 519 



DISCUSSION 



In crosses between X and any one of the other phages studied, 434, 82, 

 and 21, genetic recombination observed in the ms and mi regions indi- 

 cates clearly that genetic homology exists between the linkage maps of 

 these phages, at least in their outer parts. That the degree of homology 

 may vary according to the pair of phages is shown by the fact that in 

 crosses with X, the regions of the map within which recombination can 

 occur is different for 434, 82, and 21. 



The most interesting of these three phages is 434, which appears to 

 exhibit complete homology with X except for a small region of its linkage 

 map, the Cj region. The Ci region of the genetic material of phage X has 

 been defined as that segment of the linkage map within which are lo- 

 cated all mutations (22 out of 22 studied) that suppress the ability of the 

 phage to lysogenize or, at least, reduce the lysogenization frequency to 

 less than 10"^ (Kaiser, 1957). Mixed infection with different pairs of 

 Ci mutants have indicated that the Ci region is probably a single cistron 

 or functional unit as defined by Benzer (1957). 



The failure to find intra-Ci recombinants in a X X 434 cross may be 

 explained by two kinds of hypotheses. Either the Ci regions of X and 434 

 fail to pair, or recombination occurs but the recombinants are inviable 

 because of the hybrid character of their Ci region. In both models, struc- 

 tural dissimilarity of the Ci regions of X and 434 would seem to be implied 

 by the absence of recombination there. 



The main result obtained by comparing the properties of X and 434hy 

 is that these two phages, which have in common the whole linkage map 

 except the Ci region, differ both by their immunity pattern and by their 

 specific location as prophages on the bacterial chromosome. The conclu- 

 sion, therefore, seems inescapable that the C\ region controls not only a 

 reaction involved in the lysogenization process, but also the immunity 

 pattern and the specificity of prophage location. The question arises, 

 therefore, whether these properties are but different expressions of the 

 same function. 



To understand what this function might be it is necessary to consider 

 the nature of prophage. Prophage is defined as that element in a lysogenic 

 bacterium which is responsible for the production of a particular bac- 

 teriophage (Lwoff, 1953). Bacterial crosses between K12 strains lysogenic 

 for different mutants of X (Appleyard, 1953) as well as transduction of 

 prophage (Lennox, 1955; Jacob, 1955) demonstrate that prophage is the 



363 



