The Episome F 



321 



Hfr locus at the end of the linkage map. 

 These results suggest " that: 



1. The linkage group of E. coli is nor- 

 mally circular. 



2. The Hfr-causing factor can locate itself 

 at various places on the chromosome during 

 the genetic event by which Hfr strains are 

 formed. 



3. At the time of chromosome transfer. 

 the linkage group is open adjacent to the 

 point of Hfr attachment so that the Hfr locus 

 is at the end opposite the O point. 



Studies of other Hfr strains ' confirm all 

 these assumptions, including various posi- 

 tions for Hfr — which consequently determine 

 different O points and sequences of entry. 



When DNA synthesis occurs in the pres- 

 ence of radioactive thymidine, autoradio- 

 graphs of replicated DNA strongly suggest 

 the presence of a single, circular, double- 

 helix chromosome in E. coli. The single 

 chromosome may be. in fact, a single DNA 

 molecule or it may be composed of a series 

 of DNA molecules held end-to-end by non- 

 nucleic acid links. There is no definitive 

 evidence which allows us to decide between 

 these two alternatives. It has been found, 

 however, that the chromosome tends to frag- 

 ment at certain predetermined points during 

 the growth of particular bacterial viruses in 

 infected cells. If the chromosome is actually 

 composed of an assemblage of DNA mole- 

 cules and if breakage does occur at all non- 

 nucleic acid links, then the DNA molecules 

 would have a molecular weight of 10" or 

 contain about 16.000 nucleotide pairs. 

 Other evidence suggests that the linkage 

 group of Hfr is usually circular, though the 

 linkage group transferred during conjugation 

 is open at the Hfr locus. 



Since an Hfr male strain always has the 

 same chromosomal marker leadine the others 



" Following F. Jacob and E. L. Wollman. 



6 See A. L. Taylor and E. A. Adelberg < 1961). 



T L 



Ser/g|y 



figure 2-! — X. Linear chromosomes of three 

 Hfr strains. Arrows show direction of chro- 

 mosome penetration during conjugation. \ 

 A. L. Taylor and E. A. Adelberg. 1960. See 

 References.) 



in transfer. Hfr apparently causes the ring 

 chromosome to open at only one of the two 

 regions immediately adjacent to it. That 

 the entry sequence is different in different 

 strains (the chromosome of AB-312 enters 

 in the reverse direction from that of AB-3 1 1 

 or AB-3 13. as can be seen in Figures 24—3 

 and 2-1 — I ) may be due to an inversion of 

 Hfr when it locates itself at different chro- 

 mosomal positions. 



Since the ring chromosome of E. coli is 

 opened when the Hfr chromosome is trans- 

 ferred, one might question whether these two 

 new ends are able to join in restitution. 

 WTien considering whether the two new ends 

 rejoin, one must distinguish between what 

 goes into the recipient cell and what is left 

 behind in the donor cell. There is little 

 evidence about how much chromosome is 

 left behind — whether, in fact, there is or is 

 not a complete chromosome — in the donor 



