354 F. W. STAHL 



phage with, three t}Toes of agents — (1) ultraviolet light; (2) decay of incor- 

 porated radiophosphorus ("suicide"); and (3) ionizing radiations (usually 

 X-rays). At times it may appear that the reviewer has forgotten that the 

 primary aim in employing radiation in the study of phage is to elucidate the 

 normal state of affairs. However, almost all experiments involving the 

 irradiation of phage have raised far more questions than they have answered. 

 This has resulted in the situation that there now exists, a "radiobiology of 

 bacteriophage," a collection of observations and hypotheses arising from 

 irradiation experiments, leading no one knows where, but selfishly demanding 

 an explanation. This review will confine itself primarily to those issues 

 which fit this definition of "phage radiobiology."^ In so doing, it will not 

 only avoid treading upon the areas covered by other chapters in this volume, 

 but wiU permit the detailed treatment of the subject demanded by its in- 

 herent complexity. 



Each of the radio-agents inactivates phage in the sense that a fraction of 

 the particles in a population exposed to the agent is rendered incapable of 

 multiplying. It is likely that each of these agents exerts its lethal effect in 

 large part by damaging the deoxyribose nucleic acid (DNA) of the phage 

 particle. This conclusion is more fuUy justified in another chapter (Chapter V, 

 Volume I); here we simply indicate why one might expect this to be so. 



Most of the ultraviolet light experiments to be discussed were performed 

 with a low-pressure mercury vapor lamp. Such equipment emits wave- 

 lengths which are efficiently absorbed by DNA and to which protein is 

 relatively transparent. 



Justification for the tentative conclusion that X-rays exert part of their 

 lethal effect on DNA comes simply from the observation that DNA com- 

 prises about one-half of the phage particle, coupled with the weU-known 

 lack of specificity of X-ray effects on biological molecules. A consideration 

 of phage structure in more detail makes it seem likely that a major part of 

 the lethal effects of X-rays is due to damage to DNA. Work of Herriott 

 (1951) and of Hershey and Chase (1952) leads to the picture that the protein 

 (which comprises the other half of the mass of the phage particle) serves as 

 an "overcoat" for the DNA. Its primary function seems to be to encase the 

 DNA and to facihtate its penetration into the bacterial ceU. The first function, 

 while perhaps vital, might be supposed to be easily fulfilled; the second 

 function seems to be dependent on only a small part of the protein coat 

 comprising the "tail" of the phage particle. 



It would be surprising indeed if the transmutation of phosphorus to sulfur 

 within the phage DNA with its attendant release of energy did not exert a 

 lethal effect on the DNA. Indeed, we shall see later that most, if not all, of 

 the lethal effect of the decay of incorporated P^^ is due to damage in DNA. 



^ The facts and problems of phage radiobiology as they appeared in 1951 have been 

 ■well stated by Luria (1955). 



