ACTIVE FROM INACTIVATED BACTERIOPHAGE 117 



units between the infecting particles bringing together into one particle all the 

 active units before multiplication begins. Evidently, this particle should be 

 either T2 or T2r, and could not give rise to active particles of both types. This 

 conclusion will be analyzed further in the discussion. Quantitative analysis of 

 the number and contents of mixed yields from mixed infection with T2 and 

 T2r shall be the subject of future publications. 



Phages inactivated by X-rays or nitrogen mustard 



In a preliminary article (Luria 1947) it was stated that no reactivation had 

 been detected for T-even phages inactivated by hard X-rays, using the same 

 technique employed for ultraviolet. The same was found in our laboratory by 

 Miss M. E. Willis for phages T2 and T6 inactivated by a nitrogen mustard 

 (methyl bis (/3-chloroethyl) amine hydrochloride). 



Experiments by Mr. J. Watson, still in progress in our laboratory, have 

 recently shown, however, that phage T2 inactivated by hard X-rays can take 

 part in reactivation, but this reactivation occurs with such a low probability 

 that special techniques are required for its detection. In part, the low probabil- 

 ity of reactivation of X-ray inactivated phage is due to reduced rate of adsorp- 

 tion. This work will be reported by Mr. Watson in a future publication. 



It seems possible that phages Tl and T7, for which no reactivation was 

 detected after ultraviolet inactivation, may also be found by similar techniques 

 to give some reactivation. It is clear that the probability of reactivation will be 

 low if the number of transferable units is small, or if each lethal mutation in- 

 volves several units. Its detection will be difficult whenever the number of bac- 

 teria in which reactivation occurs is small in comparison with the number of 

 bacteria that receive residual active phage. 4 



DISCUSSION 



The experiments described above have given results consistent with the 

 hypothesis that inactivation of several and possibly all bacteriophages by ul- 

 traviolet light is to be attributed to lethal mutations in discrete units of genetic 

 material. A genetic basis for inactivation of viruses and bacteria by radiation 

 has often been postulated either on statistical grounds or by analogy (see Rahn 

 1929; Lea 1947). Our results bring new support to this view, and suggest that 

 most, if not all, the inactivating effect of ultraviolet light (2537 A) on certain 

 phages is due to the production of localized lethal mutations. 



The hypothesis of inactivation by lethal mutations and reactivation by 

 transfer of genetic material following multiple infection, as developed in this 

 paper, has been useful in suggesting a quantitative analysis of the reactivation 

 phenomena and has led to fairly accurate predictions of the experimental re- 

 sults. The following discussion assumes the correctness of this working hy- 

 pothesis. 



4 While this paper was in press, the senior author found that some reactivation by multiple 

 infection takes place with phage Tl inactivated by ultraviolet light. For equal multiplicity of in- 

 fection and equal number of hits, the frequency of reactivation is much lower with Tl than with 

 any of the T-even phages and with T5. Assuming that the type of analysis presented in this paper 

 applies to the results with Tl, a value of n smaller than 5 would be obtained. 



290 



