RADIOBIOLOGY OF BACTERIOPHAGE 



367 



The conclusion from the high- and low-dose cross-reactivation experiments 

 are in eminent accord with our working hypothesis for the basis of multi- 

 plicity reactivation. 



4. Inactivation of Genie Funetion 



Mutants of T4, called rll, cannot grow in Eseherichia eoli K12 (A), 

 (Benzer, 1955). Wild-type T4 grows well in K12(A). The rll mutants 

 fall into two groups, called A and B; a simultaneous infection of K12(A) 

 by a mutant of group A and one of group B leads to the apparently 

 normal production of phage; a mixed infection in which both mutant phages 

 come from the same group fails to yield phage. Mutants of group A all occur 

 within one region of the genome about 5 map miits long; those in group B 

 all occur within a region about 3 map units long. The two regions, called 



0-01 



0-001 



00001 



Phage 

 survival 







30 



6 12 18 24 

 UV dose (min) 

 Fig. 7. The survival of the function of rll cistrons in T4 following ultraviolet irradia- 

 tion. The slopes of the survival ciirves indicate that the functions of the A and B cistrons 

 are inactivated by 0.1 and 0.05 of the phage-lethal hits, respectively. The figure is 

 adapted from Krieg (1957). 



"cistrons" (Benzer, 1957), are adjacent on the genetic map. We may define 

 the function of the wild-type state of an rll cistron as its power in mixed 

 infection to permit particles which are mutant at that cistron to grow in 

 K12(A). Kjrieg (1958) has studied the inactivation by ultraviolet hght of this 

 genie function. He found that 0.1 of the phage lethal hits inactivate the 

 function of the A cistron; the B cistron is inactivated by 0.05 of the total 

 phage-lethal hits. The survival curves for each cistron suggest strongly that 

 one hit is sufficient to inactivate the function of a cistron (Fig. 7). With 



