Chapter 24 



THE EPISOME F 



A' 



re the members of a pair of 

 conjugating bacteria equiva- 

 lent in that DNA from one 

 can go into the other, each bacterium ca- 

 pable of acting either as donor or recipient? 

 Let us take a look at two streptomycin- 

 sensitive and auxotrophically different lines 

 able to conjugate with each other to pro- 

 duce recombinant progeny. If both lines 

 are exposed to streptomycin before — but 

 not after — being mixed and plated, none of 

 the pretreated individuals can divide. In 

 fact, all eventually die, and no recombinant 

 clones are formed. When one of the two 

 parental lines is pretreated with streptomy- 

 cin, again no recombinants are detected. 

 But when the other parental line is pre- 

 treated, prototrophic recombinants do oc- 

 cur. 1 This finding demonstrates that the 

 two parents are not equivalent. The parent 

 giving no recombinants when pretreated 

 acts as the DNA-receiving cell which nor- 

 mally would become the zygote after con- 

 jugation. When this parent is killed by 

 streptomycin, it is impossible to obtain re- 

 combinant clones. The other type of par- 

 ent must always serve as DNA donor in 

 conjugation. After acting as donor, the 

 death of this parent has no effect upon the 

 zygote and subsequent recombination. The 

 parent acting as genetic donor is called F+ 

 (for "fertility'"); the parent acting as genetic 

 recipient is called F~ . These types serve, 

 so to speak, male and female functions, re- 

 spectively. In bacterial conjugation, there- 



!See W. Hayes (1953). 

 317 



fore, the genetic transfer is a one-way 

 process. 



In the discussion on pages 310 through 

 312, the original wild-type strain of E. coli 

 K12 was F + , and an F - variant must have 

 arisen while one of the triply auxotrophic 

 lines was being prepared. F+ by F - crosses 

 are fertile (show recombination); F~ by 

 F - crosses are sterile (show no recombina- 

 tion). F^ by F+ crosses can be fertile be- 

 cause F+ cells can, on occasion, sponta- 

 neously change to F" or because F+ cells — 

 acting as F- phenocopies — can occasionally 

 behave like F~ phenotypically, despite be- 

 ing genetically F+. If one F+ cell is placed 

 in a culture of F~ cells, all the F~ cells will 

 be rapidly converted to F+ type. F~ cells 

 converted to F+ produce F+ progeny. The 

 rapidity of change from F~ to F+ is such 

 that the causative agent of F+ must mul- 

 tiply at least twice as fast as the typical cell 

 (and, therefore, twice as fast as chromo- 

 somal DNA). Consequently, in E. coli, F + 

 male sexuality is an infectious phenomenon 

 due to a factor or particle called F. 

 Several properties of F are known: 



1. F is transferred from male to female 

 only upon contact and cannot be isolated as 

 a cell-free particle retaining sex conversion 

 potency. (Accordingly, it does not give evi- 

 dence of being a typical virus.) 



2. Only one particle of F is transferred 

 per mating. 



3. Matings that transfer F are more fre- 

 quent but less stable than matings involving 

 chromosomal transfer. In fact, the transient 

 conjugations which transfer F do not trans- 

 fer known chromosomal markers. 



4. Exposure of F 4 individuals to the dye 

 acridine orange inhibits the replication of F 

 so that F~ cells appear among the progeny. 

 Acridine dyes also inhibit the synthesis of 

 chromosomal DNA although not as com- 

 pletely as they inhibit F factor replication. 

 Thus, acridine "curing" is really a differen- 



