174 H. MARCOVICH 



to the strain and the physiological conditions. Since one event is 

 sufficient to kill E. coli cells (permanent division inhibition) the process 

 must either involve a singly existing organelle of the cell or induce some 

 dominant lethal damage in one of many similar entities. If the affected 

 site were one of the nuclei, the lesion might be considered some kind of 

 dominant lethal mutation. 



The experiments that will be reported in this paper aim to examine 

 the second alternative, e.g. a dominant lethal mutation in the genetic 

 material. 



PRINCIPLE OF THE EXPERIMENTS 



The material and the genetic methods utilized are those described 

 in the monograph by Wollman and Jacob (1960). The properties of 

 sexual recombination in E. coli K12 which will be used in this work are 

 the following : 



1 . The characters are transmitted from the Hfr donor Ijacteria to the 

 F~ recipient cells with a frequency directly proportional to their dis- 

 tance from the origin ''0'" of the chromosome. Their sequence is always 

 the same for a given Hfr strain. 



2. No cytoplasmic material has yet been detected to pass from the 

 Hfr to the F"^ bacteria. 



3. During conjugation, the parent cells may be separated by shaking 

 the suspension vigorously ; the transfer of the markers still remaining 

 in the Hfr is then interru]ited. 



Let us call a the probal)ility, per unit of dose, for the induction 

 of a dominant lethal mutation anywhere in any one of the Hfr chromo- 

 somes. The equation of the survival curve to X-rays would be 



n 



— = e-^D (1) 



where w/wq is flie proportion of surviving bacteria to D rads. Let us 

 assume that the probability for a given chromosome to be involved in a 

 cross does not depend u})on \^'hether or not it carries a dominant X-ray 

 induced lethal mutation. If we assume, in addition, that the transfer 

 to the zygote of this hypothetical mutation will, sooner or later, kill all 

 the recombinants, the expected dose-effect relationshi]) for the survival 

 of recombinants derived from a mating between an irradiated Hfr 

 bacterium and a normal F~ one is 



__ = ^-tl/N aD (2) 



"0 



