Bacteria: Recombination {III) 



357 



some kind of bridge-formation between male 

 and female cell, over which F' is passed to 

 the F~ cell; it must also be the cause of the 

 formation of a receptor, at the surface of the 

 cell, for a virus that attacks males only.^ 

 Finally, the /ow/requency of recombination, 

 Lfr, of chromosomal markers following the 

 mating of F+ and F~ must also be a property 

 attributable to F^ 



So far, we have found that E. coli has two 

 mating types, F" and F+ (Lfr). Another 

 mating type arose from F+ cells. This type 

 produces a /?igh /requency of recombination 

 of chromosomal genes, and is hence called 

 Hfr. Since the fertility of Hfr cells is un- 

 affected by pretreatment with streptomycin, 

 Hfr cells are donors. Hfr can mate with F~ 

 cells, and, with low fertility, with F+ (Lfr 

 which have probably spontaneously reverted 

 to F-). Crosses of Hfr X F" produce 100- 

 20,000 times as many recombinants as does 

 the Lfr X F~ cross. Since the progeny of 

 the Hfr X F^ are typically F~, and rarely 

 Hfr, Hfr does not carry infective F^ particles. 

 However, Hfr can revert to Lfr strains which 

 show all the characteristics of F+, including 

 infective F^ Since Hfr can only come from, 

 and revert to, F+, it is concluded that F' must 

 be retained in masked or bound form in Hfr 

 strains, in which condition the presence of 

 extrachromosomally located infective F' is 

 prohibited. 



The occurrence of Hfr strains makes a 

 cytological search for conjugating pairs more 

 likely to be successful. This is found to be 

 the case. Figure 39-1 is an electron micro- 

 graph showing conjugation between an F^ 

 cell and an Hfr cell. The Hfr cell has 

 ultraviolet-killed bacterial virus particles 

 (tadpole-shaped objects) adsorbed to its sur- 

 face; the F" cell does not since it is genetical- 

 ly resistant to this virus. The cytoplasmic 

 bridge between the conjugants is obvious. 

 When exconjugants of such visibly marked 



2 See reference to T. Loeb and N. D. Zinder (1961) 

 on p. 305. 



pairs of Hfr and F~ cells are isolated by 

 micromanipulation and cultured, only the 

 clones from the F" partner yield recombin- 

 ants. We may note, in passing, that these 

 findings conclude another demonstration, of 

 numerous onesalready cited, of the mutual aid 

 genetics and cytology have provided in the ad- 

 vancement of both branches of investigation. 

 Using Lfr strains, it is commonly found that 

 most of the unselected markers in recombin- 

 ant progeny are those derived from the F~ 

 parent. This can be explained either by the 

 transfer of the entire genome of the male into 

 the female followed by the integration of only 

 a portion of it, or by the transfer of only a 

 portion of the male genome and its integra- 

 tion in toto, or a combination of these two 

 possibilities. Experiments can be designed 

 to test one or more of these explanations.^ 



^ The following discussion is based principally upon 

 work of E. L. Wollman and F. Jacob (see references 

 at end of this Chapter). 



FIGURE 39-1. Conjugation in E. coli. (Courtesy 

 of T. F. Anderson.) 



