The Episome F 



323 



ticlc. Since the fertility of Hfr is unaffected 

 by exposure to acridine orange, the latent 

 F particle is probably not located extrachro- 

 mosomally. This contention is supported 

 by the fact that maleness is not infective; 

 that is, maleness is not transmitted to F~ 

 cells after short mating intervals with Hfr 

 males. Consequently, the latent F particle 

 in Hfr must be located chromosomally. 

 Since, at this point, F is the only known 

 factor essential for maleness, the chromo- 

 somal locus assigned to the Hfr must be 

 that of chromosomal F. Once F enters the 

 chromosome, as mentioned, replication of 

 any remaining cytoplasmic F particles nor- 

 mally is prevented or repressed. 



What happens in those few cells of an F + 

 clone which transfer chromosomal material, 

 causing the F + clone as a whole to give a 

 low frequency of recombination? Suppose 

 that for an F+ cell to transfer its chromo- 

 some, an F particle must attach to the chro- 

 mosome, making it an Hfr chromosome. 

 This hypothesis can be tested as follows. 7 

 After mixing suitably marked F+ and F~ 

 and plating them on a complete medium, 

 appropriate replica plates are made to de- 

 tect the positions where recombination has 

 taken place. A search is then made for Hfr 

 strains among the cells on the master plate. 

 Although new Hfr strains rarely occur, they 

 are found most frequently on the master 

 plate at positions where replicas show that 

 recombination has taken place. Moreover, 

 the Hfr strains discovered often produce a 

 high frequency of recombination of the same 

 markers that show recombination in the cor- 

 responding positions in the replicas. In 

 other words, it seems valid to believe that to 

 transfer chromosomal material, an F+ indi- 

 vidual must first change to an Hfr condition. 

 It is the Hfr which produces the recombina- 

 tion detected on the replica, its clonal mem- 



7 Based upon work of F. Jacob and E. L. Woll- 

 man. 



bers on the master plate yielding the same 

 type of Hfr. 



Some evidence exists, however, that not 

 all recombinants in F + by F _ crosses result 

 from the formation of stable Hfr cells. For 

 example, UV radiation enhances the ability 

 of an F^ culture to give chromosomal re- 

 combinants by 30- to 50-fold but this en- 

 hancement is not the result of stable integra- 

 tion of F into the chromosome since it only 

 persists for one or two generations. Fur- 

 thermore, all attempts to isolate stable Hfr 

 from other bacteria harboring other types of 

 fertility factors have been unsuccessful. Ap- 

 parently, some sort of temporary or "abor- 

 tive" Hfr state is sometimes — if not always 

 — produced in F+ cells which transfer chro- 

 mosomal markers. 



Since F+ males rarely become Hfr, we 

 conclude that extrachromosomal F has a low 

 affinity for t/ie chromosome; and, since F+ 

 males produce Hfr strains having any one of 

 various O points, we conclude that F has no 

 preferential site of attachment. F is com- 

 posed of DNA (F transferred to Serratia 

 gives a satellite DNA band in CsCl not 

 found in Serratia alone) and contains 

 roughly 2.5 times 10 5 base pairs, or about 

 3.7% of the amount present in a nuclear 

 body. Although F has a considerable num- 

 ber of sites at which it can integrate, this 

 number is by no means unlimited in view of 

 the repeated isolation of strains with Hfr 

 mapping in the same region. In any case, 

 if one accepts the idea that naturally-occur- 

 ring F possesses regions which are homol- 

 ogous to certain chromosomal regions and 

 that integration is in fact the result of a 

 recombinational event between F and the 

 chromosome, then it is reasonable that F, 

 because of its size, could carry genetic re- 

 gions homologous to only a small fraction 

 of all chromosomal sites. 



The bacterial chromosome always repli- 

 cates in a polarized manner, starting at one 

 point and continuing to the end. Although 



