Chapter 27 



BACTERIAL EPISOMES AND 

 GENETIC RECOMBINATION 



T: 



(he location and genotype of 

 F determines the kind of male 

 sexuality which occurs in Esch- 

 erichia. In this bacterium, conjugation leads 

 to new combinations of either or both the 

 chromosomal genes and the extrachromoso- 

 mal episomal genes. 



Let us consider the sequence of events 

 in F's genetic recombination. When an Hfr 

 strain reverts to F + , the F particle integrated 

 into the Hfr chromosome is somehow liber- 

 ated from it, or deintegrated. The particle 

 then enters the cytoplasm, replicates, and 

 thereafter is infectious. In some subsequent 

 generation, the F particle may reintegrate 

 into a chromosome, making it Hfr. Before 

 an F particle can integrate, it apparently must 

 synapse with the chromosome — the attrac- 

 tion between F and the chromosome prob- 

 ably being the same as between a segment of 

 donor chromosome and the host chromosome 

 in transformation, conjugation, or transduc- 

 tion. In transformation, the integrating do- 

 nor loci must be homologous to those re- 

 placed in the recipient cell. (Most likely, 

 this homology is also required for the inte- 

 gration of chromosome fragments introduced 

 by conjugation or transduction.) Since F 

 can integrate at a variety of loci, it appears 

 likely that F has segments of DNA homolo- 

 gous to a variety of chromosomal regions. 

 The homologous segments which F presum- 

 ably contains may have been present 'ini- 

 355 



tially," or they may have been obtained at 

 the time of previous deintegrations. (It is 

 not known whether the first F originated as 

 an offshoot of the bacterial chromosome or 

 entered the bacterium from the outside.) If 

 a free F particle sometimes carries an extra 

 segment of chromosomal DNA somehow 

 obtained at the time of deintegration, one 

 should be able to find a type of free F parti- 

 cle which, when introduced into an F - strain, 

 shows a high affinity for a specific chromo- 

 somal region. Recall that temperate phages 

 capable only of restricted transduction show 

 such a restriction, although the wild-type F 

 particles do not. 



Consider next the following results from 

 experiments l concerned with the expecta- 

 tions mentioned. An F^ strain (carrying F 

 extrachromosomally) gave rise to an Hfr 

 strain, P4x, whose chromosomal markers 

 were arranged in the following sequence: O 

 (origin or lead point) -Pro-TL-Thi . . 

 -Gal-Lac-SF (sex factor place of attach- 

 ment). Crosses of P4x by F~ produced F~ 

 progeny, except for Lac recombinants (which 

 are usually Hfr males because of the close 

 linkage of F and Lac). 



From P4x arose a new strain, P4x-1, hav- 

 ing these characteristics: 



1. With respect to chromosomal loci, it 

 was identical to P4x in the order of arrange- 

 ment and in the times of entry (determined 

 by interrupted conjugation). For example, 

 both transferred Pro at about six minutes, 

 TL at about twenty minutes, and Lac last. 



2. Recipients showed a frequency of re- 

 combination for chromosomal loci lower 

 than the frequency of recombination when 

 P4x was the donor. For example. Pro re- 

 combinants were 0.3 to 0.5% with P4x-1 

 as donor and 4.8% with P4x as donor. 



3. Interrupted conjugations revealed that 



1 The following discussion is based primarily upon 

 the work of E. A. Adelberg and S. N. Burns 

 (I960), and F. Jacob and E. A. Adelberg (1959). 



