The Episome F 



327 



the starting point of replication in F~ is 

 possibly random, some evidence indicates 

 that replication starts at the F-containing 

 end in Hfr strains/ Chromosome transfer 

 in Hfr lines may require initiation of a new 

 replication of the chromosome." 



The chemical composition, genetic alter- 

 natives, 10 capacity for autonomous self-du- 

 plication, and "suicide" due to incorporated 

 P :i - after transfer to F~, demonstrate that 

 the male fertility factor, F, of E. coli must 

 be composed of genetic material. Since F 

 is probably neither lytic nor otherwise rap- 

 idly lethal to F~ cells, and since F has a 

 stable relationship with the F+ cell, it can 

 be considered a "normal" cellular compo- 

 nent, when present. Because it can func- 

 tion and reproduce in an autonomous man- 

 ner when located extrachromosomally, F 

 furnishes the first example so far presented 

 of "normal," extrachromosomal genetic ma- 

 terial. When extrachromosomal F is lost — 

 either spontaneously or after treatment with 

 acridine orange — it represents genetic ma- 

 terial not conserved for future generations. 

 Since F is genetic material, its transmission 



8 See J. Cairns (1964), T. Nagata (1964), and 

 N. Sueoka and H. Yoshikawa (1964). 



9 See J. Roeser and W. A. Konetzka (1964). 



10 See E. A. Adelberg and S. N. Burns (1960). 



from F+ to F - is an example of genetic 

 recombination. 



F also has the ability to assume a regular 

 locus on a chromosome. When stably at- 

 tached to or integrated in the chromosome, 

 F functions and replicates just as any other 

 ordinary chromosomal locus. In regular 

 vegetative reproduction, chromosomal F is 

 transmitted to all progeny; that is, F is con- 

 served. In conjugation, however, chromo- 

 somal F may not be conserved, for inte- 

 grated F is not transmitted to the zygote 

 with appreciable frequency except in the 

 case of certain Hfr strains, and — even when 

 transmitted — may fail to be integrated. In 

 such cases, the nonconservation of F in F~ 

 cells is no different from the nonconserva- 

 tion of other chromosomal loci. 



The only type of genetic elements dis- 

 cussed in detail prior to this chapter were 

 those restricted to the chromosome. To 

 these can now be added the male fertility 

 factor, F, which may or may not be present 

 in the cell, and, when present, can be either 

 autonomous extrachromosomally or inte- 

 grated in the chromosome. Such genes 

 which can participate in the cell either as 

 extrachromosomal or as chromosomal ele- 

 ments are called episomes. 11 



11 See F. Jacob and E. L. Wollman (1958). 



SUMMARY AND CONCLUSIONS 



E. coli has only one female mating type (genetic recipient) — F~ — but two male mating 

 types (genetic donors) — F+ and Hfr. Male mating type depends upon the presence 

 and location of F. 



F, an episome, is infective when present extrachromosomally. Thus, F+ males 

 donate F only. F can assume a chromosomal locus and produce the Hfr male. When 

 the Hfr male conjugates, the ring chromosome of E. coli is open near the locus of F. 

 The end opposite the F locus proceeds into the recipient conjugant first, in the linear 

 transfer of part — or sometimes all — of the opened ring chromosome. Thus, Hfr males 

 mobilize and donate chromosomal loci. 



