Bacterial Episomes and Genetic Recombination 



359 



Salmonella or Shigella act as recipients in 

 crosses with F+ or Hfr E. coli, F is trans- 

 ferred but sometimes is unable to act as a sex 

 factor until it is sent back into F~ E. coli. 

 Such results show that promoter functions 

 can be temporarily inhibited or unexpressed, 

 depending upon the host genotype. 



* Resistance Transfer Factors 



A genetic agent — the resistance transfer fac- 

 tor, RTF — has been found in Shigella. 3 

 RTF is a promoter which causes conjugation 

 and the transfer of a series of different, 

 linked, drug-resistance genes; it is, therefore, 

 a sex factor. Under special circumstances, 

 RTF can mobilize chromosomal loci. It can 

 be transferred from Shigella to Escherichia 

 or Salmonella independently of the chromo- 

 some, and such recipients can be cured of 

 RTF by acridine dyes. RTF promotes its 

 own transfer which starts within one minute 

 of mixing the parents, and it replicates au- 

 tonomously. Although RTF remains trans- 

 ferable when introduced into an F + or Hfr 

 cell, F-promoted chromosome transfer in Hfr 

 cells is reduced one hundredfold, and F- 

 promoted merogenote transfer and the trans- 

 fer of free F are completely inhibited by cer- 

 tain RTFs. When RTF is spontaneously 

 lost, these F functions are restored. Other 

 RTF strains have no effect on F function. 



F produces an antigen on the cell surface 

 which must be present for <£f2 to attack 

 males. When RTF and F are both present, 

 a new RTF-antigen replaces the F-antigen. 

 When a cell is infected by RTF its chromo- 

 somal markers are mobilized one hundred 

 times more frequently when its chromosome 

 carries a segment of F than when it does not. 

 These observations suggest that at least a 

 partial genetic homology exists between RTF 

 and F. In view of this and other evidence, 

 it is concluded that RTF has some relation- 

 ship with the chromosome, although it may 

 3 See T. Watanabe (1963). 



not be able to assume a stably integrated 

 state. RTF has been called an episome by 



its discoverers. 



* Colcinogenic Factors ' 



Many strains of enteric bacteria (Escher- 

 ichia, Salmonella, Shigella, for example) pro- 

 duce one or more highly-specific, antibiotic 

 substances called colicins. Colicins are bac- 

 tericidal but not bacteriolytic agents; a thou- 

 sandth of a microgram of colicin can kill a 

 million sensitive E. coli cells. More than a 

 dozen groups of colicins are known; each 

 group is designated by a different capital let- 

 ter; each adsorbs to a different receptor site 

 at the cell surface. Different colicins belong- 

 ing to the same group can be distinguished 

 by other characteristics. Colicins have a 

 high molecular weight; two of them, colicin 

 K and colicin V, have been purified and iden- 

 tified as lipocarbohydrate-proteins. These 

 purified colicins seem to be the same as the 

 O antigen of the bacteria." 1 The antigen 

 molecule can be separated into a lipocar- 

 bohydrate and a protein fraction, the latter 

 having all the colicidal activity. 



A cell able to produce a colicin is colicino- 

 genic; one without this ability is noncolicino- 

 genic. Colicinogeny is stable and can be trans- 

 mitted through thousands of cell generations. 

 Although it can be lost spontaneously, 

 spontaneous acquisition of colicinogeny has 

 never been observed. Consequently, it is 

 reasonable to suppose that colicinogenic 

 bacteria possess genetic material — colicino- 

 genic (col) factors — that govern the syn- 

 thesis of different colicins. Not only are 

 these factors transmitted to progeny via vege- 

 tative reproduction, but new strains can be- 

 come colicinogenic through bacterial conju- 

 gation or phage-mediated transduction. 



Only a small fraction of the bacteria in a 



4 This section follows the work of P. Fredericq 

 (1963). See also W. Hayes (1964). 

 "See W. F. Goebel (1962). 



