Bacteria: Recombination {IV) 



367 



an example of nonconserved chromosomal 

 DNA.) 



What would be the reciprocal product if a 

 defective F- particle is deintegrated? In that 

 event, the chromosome would retain a por- 

 tion of the F- genotype as a "memory." 

 Thus, just as complete or incomplete F- might 

 carry a piece of chromosome which serves as 

 "memory," a chromosome may retain a part 

 of the F- genotype as "memory." Should a 

 complete F- with chromosomal memory infect 

 a normal F" cell, it would be expected to inte- 

 grate with relatively high frequency at a pref- 

 erential position in the chromosome. And, 

 reciprocally, should a chromosome with F- 

 memory be exposed to complete F-, the epi- 

 some also would be expected to integrate 

 with relatively high frequency at a preferential 

 locus. 



We have just expressed certain expecta- 

 tions, regarding the frequency and locus of 

 F integration, on the basis of certain assump- 

 tions. Let us now see how experiments ^ 

 (part of whose conclusions were already pre- 

 sented in the last Chapter) bear upon these 

 expectations. An F+ strain (carrying F^ 

 extrachromosomally) gave rise to an Hfr 

 strain, P4x, whose chromosome is transferred 

 with the following orientation: O (origin or 

 lead po'mt)-Pro-TL-Thi- . . . -Gal-Lac-SF (place 

 of attachment of the sex factor). P4x X F" 

 gives F~ progeny except for Lac recombin- 

 ants which are in turn Hfr males, because 

 of the close linkage of F' and Lac. 



A new strain, P4x-1, derived from P4x, has 

 the following characteristics: (1) From in- 

 terrupted conjugations it is established that 

 P4x-1 is identical to P4x with respect to order 

 and times of entry of chromosomal markers. 

 Thus, for example, both transfer Pro at about 

 six minutes, TL at about 20 minutes, and Lac 

 last. However, the frequency of recombina- 

 tion for Pro is reduced in P4x-1, being only 



^ The preceding and following discussion is based 

 largely upon the work of E. A. Adelberg and S. N. 

 Burns (1960), and F. Jacob and E. A. Adelberg (1959). 



.3-. 5% of donor cells as compared to 4.8% 

 of donor cells for P4x. (2) Moreover, in 

 interrupted conjugation experiments with 

 P4x-1 many of the recombinants for Pro or 

 TL behave as males. It is also found that 

 the male factor in P4x-1 is linked neither to 

 these loci nor to any other chromosomal 

 marker showing recombination, and that, 

 like free F\ it enters the F^ cell about five 

 minutes after conjugation begins. In brief, 

 the results prove that the male sex factor in 

 P4x-1 is located extrachromosomally. 



This cytoplasmic sex factor is called F- 

 because it shows certain differences from F^ 

 (cf. p. 362). Whereas F' can attach at any 

 one of a number of different sites, giving Hfr 

 and Vhf chromosomes which differ in O point 

 position and in direction of transfer, F- 

 attaches at a particular locus near Lac to give 

 rise to an Hfr which always transfers its chro- 

 mosomal loci in the same order and direction. 



The fact that P4x-1 transfers its chromo- 

 some more frequently than does the typical F+ 

 (F'-containing) male may mean that the F- 

 particle has a chance for integrating at its re- 

 stricted locus near Lac which is greater than 

 the total chance that F^ has of integrating at 

 any one of a number of different loci. On 

 the other hand, P4x transfers more frequently 

 than P4x-1. This suggests the possibility 

 that F is already integrated in P4x, but is 

 usually only paired with a homologous region 

 in P4x-1, at which position it has the poten- 

 tiality of being integrated. This suggested 

 difference could explain the observation that 

 P4x has no free F while P4x-1 has, repression 

 of F reproduction cytoplasmically being a 

 characteristic only of F when integrated into 

 the chromosome. The experimental data are 

 consistent with the view that, in P4x-1, inte- 

 gration of the F- particle takes place only 

 after conjugation is initiated. 



In those cases where F- is known to be 

 transferred as an extrachromosomal particle 

 to F~ cells, these cells are converted to males 

 who transfer their chromosome relatively 



