144 CELL HEREDITY 



TABLE 5.8 

 Orders of Genes in a Variety of Hfr Mutants 



(From Jacob and Wollmaii, 1958, in The Biological Replication of 

 Macromolecules. New York. Academic Press, p. 75) 



Type < 



H thr leu azi Tl lac T6 ga/ X 



1 leu thr B 1 met mtl xyl mal str 



2 Tl azi leu thr Bl rtwt mtl xyl mal str 



3 T6 lac n azi leu thr Bl met mtl xyl mal str 



4 Bl met mtl xyl mal str X gal 



5 met 81 thr leu azi Tl lac T6 gal 



F~ in a linear sequence. Therefore we must suppose that in each Hfr 

 the chromosome is not a circle, but rather that it is a rod. 



All these facts can be resolved if we assume, as in Figure 5.17, that in 

 the F~ cells the chromosome is a circular structure. In F^ cells there 

 must also be the contagious F factor, probably not in the linkage group 

 because it has not been found to be linked with any other factor, and 

 because it can be irreversibly lost. Unless the F factors are divided by 

 a regular mechanism, there must be a fair number of them present be- 

 cause random distribution during cell division very rarely gives rise to 

 an F~ daughter and, moreover, they can be transferred to F~ cells on 

 contact. We may assume that the mutation from F"*" to Hfr represents a 

 reduction of an F particle to a site on the chromosome where it induces 

 a break. We know that an invading phage DNA molecule may reduce to 

 a prophage on the bacterial chromosome. If the F factor can insert at 

 any site, or at least at many different sites, on the chromosome, then the 

 different classes of Hfr mutants can be explained. Just as in the case 

 of prophage, the reduced F factor causes an immunity so that Hfr cells 

 cannot be infected by F particles. Likewise, the unreduced particles 

 would eventually be lost if in the presence of reduced F their multiplica- 

 tion was prevented, just as prophage prevents the multiplication of in- 

 vading phage DNA. The F particle must be at the rear end of the 

 chromosome because it so seldom gets into the F~ mate. This picture, 

 speculative as it is, accounts for many facts that are not consistent with 

 any other hypothesis yet proposed. It invokes the concept of an episome, 

 a dispensable genetic unit which may multiply independently of the 

 chromosome or as an addition to it. 



The resemblance between the F factor and the temperate phage, both 

 episomes, is obvious; it is strengthened by the fact that the F factor may 



