362 



CHAPTER 39 



is stopped by Blendor treatment, we cannot 

 decide whether the treatment breaks the 

 chromosome, or the cytoplasmic bridge, or 

 both. However, since conjugants, which have 

 been visibly joined for two or more hours, 

 may show only limited amounts of recombi- 

 nation, it is likely that a chromosomal defect, 

 and not bridge rupture, is responsible for 

 spontaneously halting transfer and/or inte- 

 gration. 



Taking into account the changes in rate of 

 penetration and in integration efficiency, it 

 is possible to construct, from interruption ex- 

 periments using Vhf or Hfr males, a genetic 

 map of E. coli markers whose relative dis- 

 tances are expressed in minutes. This is 

 shown in Figure 39-5. This map is essentially 

 identical to the one derived from the relative 

 frequencies of recombinants from Lfr X F^ 

 crosses (p. 354). 



Let us now return to a consideration of F+ 

 (Lfr) strains. One can perform an interrup- 

 tion experiment to determine when the F^ 

 particle in Lfr males is transferred in mating. 

 It is found that F^ is first transferred about 

 five minutes after mixing F+ and F . This 

 is several minutes earlier than any marker on 

 the chromosome is transferred. Moreover, 

 there is no linkage of F^ with any marker on 

 such a chromosomal segment. Accordingly, 

 this is additional support for the extrachro- 

 mosomal nature of F^ 



We have mentioned already that Hfr 

 strains are always derived from F+ strains, 

 as were the Vhf strains. We have also noted 

 that Hfr strains may revert to F+. (So can 

 Vhf.) We have taken this to mean that Hfr 

 (and doubtless Vhf too) harbors a latent F^ 

 particle. Since the fertility of Hfr is unaffected 

 by exposure to acridine orange, the latent F' 

 particle cannot be located extrachromo- 

 somally, or it would disappear. Accordingly, 

 the latent F^ particle in Hfr (or Vhf) must be 

 located chromosomally. Since F^ is the only 

 known factor essential for maleness, the locus 

 we have assigned to the Hfr and Vhf genes 



must be the locus oj chromosomal FK Once 

 F' enters the chromosome all remaining cyto- 

 plasmic F' particles are normally lost. 



In the light of this information, what can 

 we reason concerning what happens in F+ 

 cells when, on rare occasions, they do transfer 

 chromosomal material (so that, as a whole, 

 the F+ clone gives a low frequency of recom- 

 bination)? A number of hypotheses can be 

 suggested, but we will consider a particular 

 one. We can suppose that in order for chro- 

 mosomal transfer to take place in F+ cells, 

 an F' particle must attach to the chromosome, 

 making it a typical Hfr chromosome. This 

 can be tested as follows.^ After mixing suit- 

 ably marked F+ and F^, replica plates are 

 made to show the places where recombination 

 has taken place. A search can then be made 

 for Hfr strains among cells growing on the 

 master plate. Although new Hfr strains 

 occur rarely, they are obtained much more 

 frequently from positions on the master plate 

 where recombination has taken place than 

 where it has not. Moreover, it is often found 

 that the Hfr strain obtained produces a high 

 frequency of recombination of the same 

 marker whose recombination on the replica 

 aided in the detection of the Hfr strain. In 

 other words, it seems valid to believe that 

 before an F"*" individual transfers chromo- 

 somal material, it first changes to a particular 

 Hfr (or Vhf) condition. This Hfr produces the 

 recombination detected on the replica, while 

 its clonal members on the master plate yield 

 the same type of Hfr. This interpretation 

 also receives support from other experiments. 



Some additional properties of F^ may now 

 be listed. F^ is characterized by having a low 

 affinity for the chromosome; it has no prefer- 

 ential site of attachment, and once it attaches, 

 the extrachromosomal multiplication of F^ 

 ceases. A genetic variant is now known,^ 

 F^, which has a high affinity for the chromo- 

 some (so that it frequently produces an Hfr 



7 Based upon work of F. Jacob and E. L. Wollman. 



8 See E. A. Adelberg and S. N. Bums (1960). 



