Chapter *39 



BACTERIA: RECOMBINATION 

 (III. The Episome F) 



Ai 



RE THE members of a pair of 

 conjugating bacteria equiva- 

 -lent? In other words, does 

 DNA from either one go into the other, so 

 that both bacteria can act either as donor or 

 recipient? Suppose two auxotrophically 

 different, streptomycin-sensitive Hues can 

 normally conjugate and give recombinant 

 progeny. If both lines are exposed to strepto- 

 mycin before, but not after being mixed, none 

 of the pretreated individuals can divide, and 

 since all eventually die, no recombinant 

 clones are formed. When one of the two 

 parental lines is given such a pretreatment 

 with streptomycin, it is found ^ that no re- 

 combinants are detected, whereas when the 

 other parental line is pretreated prototrophic 

 recombinants do occur. This demonstrates 

 that the two parents are not equivalent. The 

 former type of parent (giving no recombin- 

 ants when pretreated) must always act as the 

 DNA-receiving cell, which, after conjugation, 

 normally becomes the zygote. So, when this 

 parent is killed by streptomycin, it is impos- 

 sible to obtain recombinant clones. The 

 latter type of parent must always serve in 

 conjugation as DNA donor, its death, after 

 acting as donor, having no effect upon the 

 zygote and subsequent recombination. The 

 latter type which acts as genetic donor is 

 called F+ (for "fertility"), whereas the former 

 type, which acts as genetic recipient, is called 

 F; these types serve, so to speak, male and 

 female functions, respectively. In bacterial 

 conjugation, therefore, the genetic transfer 

 ' Following the work of W. Hayes (1953). 

 356 



which takes place is a one-way process. 



The original wild-type strain of E. coli K12 

 is F+, and in the period during which one of 

 the auxotrophic lines was being prepared 

 (cf. pp. 349-351), an F~ variant must have 

 arisen. F+ X F~ crosses are fertile; F~ X F~ 

 crosses are sterile (show no recombination); 

 F+ X F+ crosses are fertile only because F+ 

 cells may on occasion spontaneously change 

 to F~. If one F+ cell is placed in a culture of 

 F~ cells, all the F~ cells are rapidly converted 

 to F+ type! The rapidity of the change from 

 F to F+ is such that the causative agent must 

 multiply at least twice as fast as the typical 

 cell (and, therefore, twice as fast as chromo- 

 somal DNA), sex conversion occurring with 

 an efiiciency about 10'^ times higher than that 

 of recombination. Moreover, the new F+ 

 cells transmit this trait to their progeny. We 

 conclude, that in E. coli, F+ male sexuality is 

 an infectious phenomenon due to a factor or 

 particle which we can call EK 



Several properties are known regarding 

 F^ It is transferred from male to female 

 only upon contact, and it cannot be isolated 

 as a cell-free particle retaining sex conversion 

 potency (accordingly, it does not give evi- 

 dence of being a typical virus). The matings 

 that transfer F^ are more frequent but less 

 stable than matings involving chromosomal 

 transfer. Exposure of F+ individuals to the 

 dye, acridine orange, inhibits the replication 

 of F^ so that F+ cells are converted to F . 

 However, this dye has no apparent effect on 

 chromosomal genes. This fact, plus a divi- 

 sion rate which is faster than that of the 

 chromosome, is sufficient evidence for con- 

 cluding that E^ is an extra-chromosomal 

 particle. 



The F' particle modifies the cell harboring 

 it, in several ways. Not only does F^ make 

 a cell a potential male, but it has several 

 effects upon the cell surface. F' must change 

 the cell surface of a male, so that a male cell 

 can recognize, and react with, a female cell 

 which it contacts; it must be the cause of 



