LETHAL ACTION OF X-RAYS IN K. coll Ivl2 179 



4. Cross between irradiated Tlfr and normal F', plated on non- 



selective medium 



If a letlial nuitation was induced on the Hfr chromosome near the 

 origm "O"" it would express itself, in the cross, by the killing of the F" 

 hacteria ])lated on broth medium. The ex]ieriment which has been ]ier- 

 formed consisted of mating Hfr and F" cells in the proportion of 10 to 1, 

 so that the non-conjugated F" would be very rare. The dose given to 

 the Hfr was such that the probability was low for a mating between an 

 F- and an Hfr with an unharmed Hfr chromosome. As the frequency of 

 recombination for the selected markers in this experiment was 30 per 

 cent one would expect at least a 30 per cent decrease in the number of 

 the colonies formed by the F" crossed with the irradiated Hfr as com- 

 pared to the control. No detectable loss in the number of colonies has 

 l)een observed. 



5. Cross between irradiated F bacteria, and normal or irradiated Hfr 



If F~ bacteria are irradiated and crossed with a non-irradiated Hfr 

 it would be expected that the survival curve of the recombinants and 

 of the F~ would coincide, on the basis of the hypothesis of a dominant 

 lethal being induced in the F~. Many experiments have been done to 

 examine this point. Although for unknown reasons the results lack re- 

 producibility, some restoration of the irradiated F" by the Hfr is always 

 observed. 



If the cross is made between two irradiated bacteria, Hfr and F~, the 

 survival of the recombinants is what might be expected on the assump- 

 tion that the radiation damages to the Hfr and to the F"" would act 

 independently in the zygote to reduce the number of the recombinants. 

 This result eliminates the possibility that the exponential inactivation 

 curve for X-rayed cells is the expression of the interaction between a 

 radiation chromosomal event with a radiation cytoplasmic one. 



DISCUSSION 



The experiments reported here have not provided any evidence for 

 the induction of some kind of lethal dominant mutation on the chromo- 

 somal material of the Hfr strain. With selected markers, they have 

 shown that a rescue by the F" cells may be exerted on the characters 

 of the irradiated male. It seems that the X-ray damage Avould be, not 

 on the genetic information itself, but mainly on the transfer mechanism 

 of the selected markers to the recombinants. 32p decay experiments 

 have yielded the same conclusion (Fuerst et al., 1956). Since there exist 



