176 



H. MARCOVICH 



in the same medium. O-l ml of the Hfr strain is then mixed with 0-9 ml 

 of the F~ strain, and the mixture slowly agitated at 37°C. Unless other- 

 wise sj)ecified, the bacteria are alloANed to stand for 1 lir before dilution 

 and plating. 



The plates are incubated at 37°C for 48 hr before counting. Irradiation 

 is performed with a Machlett tube AEC' 50 with a tungsten target 

 operating at 40 kV. The dose is estimated by comparison with the same 

 lethal effect on the strains observed with a ^OQo source. 



RESULTS 

 1. HfrH survival curve and T+ L+ Sr recombinants 



In our experimental conditions the frequency of T+ L+ Sr recom- 

 binants formed by the unirradiated controls varies from 20 to nearly 

 50 per cent. 



The survival curves of HfrH and of T+ L+ Sr recombinants are 

 exponential ; the slope of the latter is less than the slope of the Hfr and 

 depends on the F~ strain (Fig. 1). Yet the same damage to the HfrH 

 may be quantitatively expressed differently according to the recipient 



100 

 X 10^ rods 



Fig. l.—E. coli HfrH and E. coU C600B34 5 are grown in broth up to 2-108 bacteria/ml. 

 The Hfr strain is exposed to increasing doses of X-rays and mated to the F-sti'ain: 0-1 ml 

 of the former is added to 0-!> ml of the latter. The suspensions are agitated slowly for 1 hr 

 and then ])hited on synthetic agar medium, supplemented witli all the required growtli 

 factoi's except T and L. The survival curve of Hfr is determined on agar sohdifietl 

 complete medium; the slope is the same if synthetic medium is used. 



