456 INACTIVATION OF BACTERIOPHAGES 



actually depolymerizes highly radioactive DNA molecules with an efficiency 

 similar to a. It is known, however, that x-rays and other ionizing radiations do 

 break down DNA to random fragments of progressively smaller molecular 

 weight at doses comparable to those necessary for the "direct" inactivation of 

 bacteriophages (Taylor, Greenstein, and Hollaender, 1948; Conway, Gilbert, 

 and Butler, 1950). Hence it is not unlikely that the lethal effect of x-ray ioniza- 

 tions inside the phage particle is also one of cutting DNA molecules, similar 

 to that postulated above for P^'- decay. Two sets of facts would appear to make 

 this comparison useful : 



(c) The efficiency of killing per x-ray ionization inside the volume of the 

 phage particle is only of the order of 0.05 in the bacteriophage strains studied 

 here (Watson, 1950); i.e., similar in magnitude to a. The energy released by 

 each x-ray ionization is thought to be 32 ev., i.e. similar in magnitude to that 

 of the P^^' recoil, and to be confined to a radius of a few Angstrom units (Lea, 

 1947). (The average energy available locally may actually be either more or 

 less than 32 ev. because, on one hand, the ionizations tend to occur in clusters 

 but, on the other hand, their energy has been determined only in air and not 

 in a condensed phase.) Since the two polynucleotide chains of the DNA macro- 

 molecules are separated by at least 10 A (Watson and Crick, 1953), it would 

 appear possible that many of the ionizations, like many of the P^' disintegra- 

 tions, damage only one of the strands without causing a complete rupture of 

 the double helix. 



(b) The x-ray sensitivity of Tl depends on temperature very much like 

 a. At temperatures below freezing, the rate of inactivation by x-rays is only 

 65 per cent of that just above freezing (Bachofer et al., 1953). At higher tem- 

 peratures, the sensitivity first remains relatively constant and then increases 

 sharply above 50°C., reaching a sixfold greater value at 60°C. (Adams and 

 Pollard, 1952). These observations had already suggested to Adams and 

 Pollard that the weakening of secondary, interchain bonds by heat at the 

 moment of the x-ray ionization might be responsible for increasing the chance 

 of causing lethal damage at higher temperatures. As in the case of P'*- decay, 

 it is apparent that the greater the extent to which the hydrogen bonds of the 

 DNA macromolecule are dissociated, the more likely will a cut of the double 

 helix result from an energetic rupture of a single polynucleotide strand. 



SUMMARY 



The inactivation of the phages Tl, T2, T3, T5, T7, and X by decay of in- 

 corporated P^- has been studied. It was found that these phages fall into two 

 classes of sensitivity to P^- decay: at the same specific activity of P^- in their 

 deoxyribonucleic acid (DNA), T2 and T5 are inactivated three times as rapidly 

 as Tl, T3, T7, and X. Since the strains of the first class were found to contain 



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