RECOMBINATION IN VIRUSES AND BACTERIA 



145 



transduce bacterial genes. An Hfr strain which carries an F factor at the 

 end of its chromosome hnked to, say, a lac^ or a pro'^ gene, may revert 

 to the F"*" condition in which the F factor becomes extrachromosomal. 

 When this F factor infects F~ bacteria, many become F"*" but an un- 

 usually large proportion become Hfr. Two interesting facts emerge: first, 

 these Hfr all transfer genes in the same order and, second, they are 

 heterogenotes for the lac'^ or the pro^ factor. All results are consistent 

 with the hypothesis that, in the original Hfr, the chromosomal F factor 

 incorporated a neighboring gene which it carried into the next cell it in- 

 fected. This incorporated material is homologous with a region of the 

 chromosome in the new host, to which site it is attracted; this explains the 

 "memory" of the F factor for the site it used to occupy. The situation 

 is quite comparable to a X phage which may pick up a gal'^ gene from a 

 site neighboring its point of attachment. In the case of the F factor, the 

 exchange with the bacterial chromosome seems reciprocal. The cell from 

 which the transducing F factor has been obtained may be cured of it by 



Hfr 



cell 



F factor 



xyl 



~21 



mal 



424 



thr leu azi Tl lac T6 gal X 381 21 424 str mal xyl mtl met thi Hfr 



An Hfr chromosome, with F factor in reduced condition at distal end 



FIGURE 5.17. Speculation on the structure of the different mating types of E. coli 

 (after Jacob and Wollman, 1958, in: The Biological Replicafion of Macromolecules, 

 New York, Academic Press). 



