LETHAL ACTION OF X-RAYS IN E. coli K12 175 



where N is the number of chromosomes, t the probability that the postu- 

 lated dominant lethal should be transmitted to the zygote, and / the 

 probability that it would be expressed in the recombinants. According 

 to equation (2), the induction of a dominant lethal mutation in the 

 Hfr chromosomes will allow some predictions which may be experi- 

 mentally tested: 



1. The survival curve of the recombinants will be exponential and 

 have a smaller slope than the survival curve of the Hfr. 



2. If the cross is interrupted, the factor / would decrease (lower 

 probability of transfer of the lethal mutation) and the slope of the 

 recombinants survival curve would be the more diminished, the earlier 

 the interruption. 



3. Equation (2) may apply to any kind of selected marker, regardless 

 of the absolute frequency of transfer from the Hfr to the F~ bacteria. 

 It is also valid for the case of non-selected plating ; if the irradiation 

 induces, in the Hfr chromosomes, dominant lethal genes, which kill the 

 selected recombinants to wdiich they are transmitted, they will also 

 express themselves in a decrease of the number of the viable F~ cells 

 when plated, after a cross, on complete agar. 



MATERIALS AND TECHNIQUES 

 Bacterial strains 



These have kindly been supplied by Drs. Jacob and Wollman. They 

 are: E. coli K12 Hf^H (Bi-, Ss) ; ^. coli C600B34-5 (T-, L-, B-, Gal-, 

 Sr); and E. coli PA309 (T-, L-, Try-, H-, Ar-, B-, Lac-, Gal-, Man-, 

 Xyl- Mai- T+, Sr) 



The selected marker in the crosses utilized in this work are T+ L+ Sr. 

 In our experimental conditions, the bacterial cells have two to three 

 Feulgen-positive bodies, the more frequent number being two. 



Media 



The growth medium is 0-3 per cent Difco Nutrient Broth, 0-5 per 

 cent Bacto Tryptone and 0-5 per cent NaCl. The synthetic medium 

 utilized has been described by Vogel and Bonner. For solid medium 

 Difco Bacto Agar is added to these media at a concentration of 1-5 

 jjer cent. 



The bacteria are plated using the soft agar (0-8 per cent) technique. 

 Vitamine Bi, and streptomycin are added in the synthetic soft agar 

 tubes before plating. 



EXPERIMENTAL PROCEDURE 

 The bacteria are grown in broth up to 2-108 cells/ml and irradiated 



