318 



i HAPTER 24 



tial phenomenon. In stable F cultures, F 

 multiplication proceeds al exact!) the same 

 rate as does chromosomal multiplication. 

 This means that there is precise regulation 



of the number of F particles per chromo- 

 some and it is onl\ when this regulation is 

 inoperative — when a population of F cells 

 is being converted to F b\ a tew F+ cells, 

 lor example — that one sees the explosive 

 multiplication of F. 



These evidences are sufficient to conclude 

 that / is an extrachromosomal panicle. F 

 not only makes a cell a potentially fertile 

 male — that is. potentially capable of acting 

 as chromosomal donor — but it affects the 

 cell harboring it in several ways: 



1. It changes the surface of a male cell 

 so that on contact, it can recognize and react 

 with a female cell. 



2. It must be the cause of some kind of 

 bridge between the male and female cell 

 through which F or the chromosome is 

 passed to the F~ cell. 



3. It must also cause the formation of a 

 receptor at the surface of the cell for viruses 

 that attack males only.- 



4. Finally, F has the ability to initiate, at 

 a low frequency, the transfer of random 

 chromosomal markers during F + by F~ 

 crosses. This phenomenon has been termed 

 Lfr (low frequency of recombination). 



So far we have found E. coli to have two 

 mating types, F~ and F . Another mating 

 type can arise from F+ cells. This type 

 produces a rather /ngh frequency of recom- 

 bination of chromosomal genes; hence its 

 name. Hfr. Since the fertility of Hfr cells 

 is unaffected by pretreatment with strepto- 

 mycin. Hfr cells are donors. These cells 

 can mate with F~ cells, and — although fer- 

 tility is low — with cells in F cultures 

 (which have either changed spontaneously 

 to F - or act as F phenocopies). Crosses 



-According to T. Loeb and N. D. Zinder (1961). 



oi Hfr with F ; produce 100 to 20,000 times 

 as main recombinants as F+ by F crosses. 

 Since the progenj o\ Hfr by F arc typically 

 F~ and rarely Hfr. Hfr does not usuall) 

 carry infective F particles. Hfr can. how- 

 ever, revert to F strains apparently show- 

 ing all the characteristics of F + , including 

 infective F. Since Hfr can only come from 

 F+ and can only revert to F . it is con- 

 cluded that F must be retained in masked 

 or bound form in Hfr strains. When F is 

 present in this masked condition, the replica- 

 tion of any extrachromosomally located F 

 capable of infection is suppressed. 



Using an Hfr strain, the cytological search 

 for conjugating pairs is successful. Figure 

 24-1 is an electron micrograph showing 

 conjugation between an F^ cell and an Hfr 

 cell. The Hfr cell has ultraviolet-killed bac- 



figure 24-1. Conjugation in E. coli. {Cour- 

 tesy of T. F. Anderson.) 



