Chapler *40 



BACTERIA: RECOMBINATION 



(IV. Episomes and Nucleotide-Sharing) 



G 



ENETic recombination, by the 

 .sexual process of conjugation, 

 is known to occur in bacteria 

 like Pseudomonas and Salmonella, as well as 

 Escherichia. In Escherichia, male sexuality 

 is attributed to the presence of an F particle, 

 the particular male type being determined 

 by the location and genotype of F. We have 

 found that bacterial conjugation leads to new 

 combinations of either or both the chromo- 

 somal genes and the extrachromosomal, 

 episomal, genes. 



Let us consider the sequence of events in- 

 volved in the genetic recombination of the F 

 particles themselves. What happens when an 

 Hfr or Vhf strain reverts to F+ (Lfr)? The 

 F particle which is integrated into the Hfr or 

 Vhf chromosome is somehow deintegraled or 

 liberated from it, enters the cytoplasm, repli- 

 cates, and is infectious thereafter. Subse- 

 quently, in some future generation, the F+ 

 particle may reintegrate into a chromosome 

 making it Hfr or Vhf. What would we expect 

 to be the property of an F particle responsible 

 for its integration into a chromosome? It is 

 reasonable to require that the F particle be 

 attracted to the chromosome. Could the 

 cause of the attraction between F and chro- 

 mosome be the same as that between a trans- 

 forming segment of chromosome (or a piece 

 of chromosome transmitted by conjugation) 

 and the recipient chromosome? If so, the 

 attraction aspect might be explained merely 

 by supposing that the F particle is composed 

 of DNA. But this assumption, alone, is not 

 366 



sufficient to explain the integration of F, since 

 in transformation the donor loci which inte- 

 grate must be homologous to those replaced 

 in the recipient cell (and this homology is 

 probably also required for the integration of 

 chromosome fragments introduced by con- 

 jugation). We would have to suppose, in 

 addition, that an integrating F particle con- 

 tains a piece of DNA homologous to a seg- 

 ment of DNA already present in the chromo- 

 some. What might be the source of the ho- 

 mologous segment F presumably contains? 

 It might have been present "initially" or it 

 might have been obtained on the last occasion 

 that F deintegrated from the bacterial chro- 

 mosome. 



If a free F particle sometimes carries a 

 particular segment of chromosomal DNA, 

 which it obtained at the time of deintegration, 

 one should be able to find free F particles 

 which have an affinity for a given chromo- 

 some region when introduced into a normal 

 F~ strain. This is indeed the case for the F- 

 particle (see p. 363), which has a high affinity 

 for a particular locus near Lac. On the other 

 hand, the fact that F^ has a low affinity for 

 the chromosome may mean that it has a 

 smaller amount of chromosomal DNA at- 

 tached to it than has F-. 



Using the F- particle as a particular ex- 

 ample, it is possible to visualize several differ- 

 ent genetic segments which might be liberated 

 from the chromosome by deintegration. 

 What may be liberated is the complete F- 

 particle with, or without, normal chromo- 

 somal material still attached, or a defective 

 F- particle with, or without, normal chro- 

 mosomal material still attached. Liberated, 

 defective F- particles may or may not be 

 capable of replication. (By being defective 

 or incomplete, we mean that such particles 

 would have lost their F characteristics, and 

 would be undetected phenotypically. If, in 

 fact, deintegration produces defective F- 

 particles that cannot replicate, these particles 

 would be lost at some future time, furnishing 



