Bacterial Episomes and Genetic Recombination 



357 



Using Lac F cells and a Lac + strain of 

 Hfr with F integrated very close to Lac 

 rare recombinants are obtained from inter- 

 rupted conjugations which receive Lac + too 

 early. Certain of these recombinants have 

 the following properties: 



1. They receive only F and Lac + . 



2. They are unstable and, occasionally, 

 give rise to Lac~F~ individuals; hence, the 

 original recombinant must be a merozygote 

 carrying both Lac + and Lac~ alleles. 



3. When crossed to Lac~F~ cells, they 

 simultaneously transfer both F and Lac + 

 with 50% or higher frequency. This trans- 

 fer starts soon after conjugation begins, just 

 as in the case of free F (or F'), and is 

 unlinked to other chromosomal markers. 

 Thus, F-Lac+ behaves as a free single unit. 



4. The recombinant transfers its chromo- 

 some in the same sequence as, but with a 

 lower frequency (%o) than, the original Hfr 

 line. These frequencies are exactly those 

 found in comparing P4x-1 with P4x. 



5. The F-Lac + element can be trans- 

 mitted in a series of successive conjugations, 

 each recipient possessing the properties of 

 the original recombinant. 



All these characteristics are most simply 

 explained by an F particle which carries a 

 chromosomal piece bearing Lac + deintegrat- 

 ing in the original Hfr strain. The attached 

 Lac + piece is known, moreover, to contain 

 three cistrons governing the synthesis of /?- 

 galactosidase, /3-galactoside permease, and 

 the repressor for this system. From subse- 

 quent integrations and deintegrations, one 

 can also obtain F-Lac~ particles — composed 

 of F-Lac with a Lac~ point mutation. 

 Finally, another Hfr, with F integrated close 

 to Pro, is found to produce an F-Pro particle 

 whose properties are analogous to those of 

 the F-Lac particle. If, however, an F-Lac + 

 particle enters a cell containing a deletion in 



the Lac region, the particle, besides transfer- 

 ring Lac ' , behaves like F in that it transfers 

 random chromosomal markers with the low 

 frequencies characteristic of ordinary F by 

 F~ crosses. 



Since F can integrate at a variety of loci, 

 these results suggest (and it turns out to be 

 true) that upon deintegration any one of a 

 variety of normal chromosomal loci can be- 

 come a part of the genotype of cytoplasmic F 

 and can replicate and function in the extra- 

 chromosomal state. 



Particles like F' and F-Lac — substituted 

 sex factors — represent a third type of male 

 sex factor characterized by serving as inter- 

 mediate donors of the usual chromosomal 

 markers. It can be hypothesized that F, F' 

 and other substituted sex factors are nor- 

 mally small, double-helix, DNA ring chro- 

 mosomes. If such small ring chromosomes 

 were integrated in the E. coli chromosome 

 by a single crossing over, the product would 

 be a bigger (Hfr) ring. The now-integrated 

 F particle would cause the enlarged ring 

 chromosome to be open, usually at one end 

 of F, and would mobilize the chromosome 

 during conjugation. If the chromosome 

 were opened at some internal position in F, 

 part of F would be at the O point and part 

 at the opposite terminus. Some evidence 

 suggests that openings of this type sometimes 

 occur. F may deintegrate by an internal 

 crossing over, producing free ring F and the 

 ring E. coli chromosome. It should be em- 

 phasized that the suggested circular model of 

 F is based upon no evidence and is nothing 

 more than speculation at present. 



When F is not integrated, only one or a 

 few F particles are present per E. coli chro- 

 mosome — at least in cells that have carried 

 F for some time. (This case is similar to the 

 situation in which organisms with more than 

 one chromosome are regulated by some 

 mechanism in the cell which permits each 

 chromosome to replicate only once a gen- 



