EADIOBIOLOGY OF BACTERIOPHAGE 361 



in a formal sense like Luria imits. The total sensitivity of these units ("vnl- 

 nerable centers," Barricelli, 1956) determines the final slope; their number 

 determines the zero-dose extrapolate. 



This theory is obviously similar to the original Luria hypothesis but adds 

 one parameter for the fraction of phage lethal hits which affect vulnerable 

 centers (the formal equivalent of Luria units). Such a two-parameter theory 

 gives a satisfactory fit to the T4 data (Harm, 1956) (see Fig. 4). Within the 

 terms of this notion, the data indicate that 40 % of the total T4 target is 

 composed of three vulnerable centers; the remaining 60 % is reactivated 

 with an efficiency so high as to have no appreciable influence on the curves. 

 [Barricelli (personal communication) has pointed out that the slight decrease 

 in final slope with increasing multiplicity, whieh, whether significant or not, 

 is a feature of Harm's curves, can be explained on the assumption that the 

 highly reactivable 60 % of the phage is less than perfectly reactivated at 

 high doses.] 



The challenge of multiplicity reactivation experiments with T4 becomes 

 that of identifying the three vulnerable centers and the nature of the re- 

 maining highly reactivable portion of the radiosensitive target. We proceed 

 with the story having in mind the following conjecture: the three centers 

 represent regions of the phage genetic structure which must perform func- 

 tions essential for multiplication and recombination of the vegetative phage; 

 the remaining 60 % of the damages occur in regions which need not function 

 until after genetic recombination has occurred. Recombination, in those 

 cells where it can occur, is (almost) always able to reconstitute a genetic 

 structure free of damages. This is essentially the hypothesis put forth by 

 BarriceUi (1956). 



3. Cross Reactivation 



a. Definition. We define the phenomenon and pose the problems related 

 to it by describing the following (not completely hypothetical) experiment: 

 Genetically marked phages are irradiated to various survival levels. Some 

 phages from each irradiated sample are then tested for their ability to con- 

 tribute the genetic marker (or markers) to live progeny phages when they 

 are adsorbed to bacteria along with nonirradiated phages of complementary 

 genotype. For a given marker, the "ability to contribute" is characterized 

 by two measurements: (1) the fraction of the irradiated phage which succeeds 

 in transmitting the marker into at least one particle in the progeny; and 

 (2) for those particles which do make a contribution, the number of progeny 

 particles which possess the gene derived from the irradiated parent. In such 

 an experiment, the ability to contribute in both senses decreases vnth 

 increasing dose (Luria, 1952; Doermann et al., 1955). 



