BACTERIOPHAGE GENETICS 361 



second kind of damage involves so-called "vulnerable centers" 

 whose probability of inactivation is relatively great. The 

 number of vulnerable centers is small and multiplicity reactiva- 

 tion can occur only if at least one copy of each kind of vulnerable 

 center in a bacterium is undamaged. The quantitative for- 

 mulation of this theory is rather complex but at higher doses 

 of radiation simplifies to 



log IV„ = rlogm - 0.43 (\V + m\oL"'~^ F'«) 



where W^™ is the probability of multiplicity reactivation; r 

 is the number of vulnerable centers per phage particle; m 

 is the average multiplicity of infection; X is the average num- 

 ber of inactivations in vulnerable centers per unit V; \ is 

 the average number of inactivations in ordinary genes per unit 

 V; L' is the average length of ordinary chromosome damages 

 relative to the total length of ordinary genetic material taken as 

 unity; L is L' \; V is the dosage of irradiation in lethal hits 

 under conditions of maximum photoreactivation. 



This equation gives an excellent fit to the data of Dulbecco 

 (1952a) both in the dark and under conditions of maximum 

 photoreactivation. The values of the constants for phage T2 

 are: 



r L' Xo X Xo + X 



In darkness 3 0.0049 1.83 0.47 2.3 



Photoreactivation 1 0.0038 0.895 0.105 1.0 



It will be noticed that two of the three vulnerable centers 

 are always photoreactivated. The fact that this formulation fits 

 the experimental data does not prove that the theory is correct 

 and further modifications of the theory are likely to be advanced. 

 However, it does suggest that modifications of the original theory 

 of Luria (1947) can be reconciled with the facts and that genetic 

 recombination may play a role in multiplicity reactivation. 

 A possible clue to the meaning of the vulnerable centers is 

 offered in the work of Krieg (1957), which is not discussed in 

 this book. 



