RADIATION AND VIRUSES 345 



accurate. For phage T2, the quantum yield (2537 A) is about IQ-" 

 (M. R. Zelle, personal communication), assuming a one-hit mechanism in 

 spite of the deviation from the logarithmic inactivation rate. These 

 (juantum yields are much lower than those reported for several chemical 

 changes in simple organic compounds, including nucleic acid constituents 

 (0.01-0.1). Most absorption takes place in the nucleic acid component of 

 the virus. Unless absorption in this component happens to be much less 

 effective than that in some other components (which is not supported by 

 our knowledge of the action spectra), we are led to suppose that a virus 

 can withstand an appreciable amount of chemical change in its nucleic 

 acid moiety without being inactivated. This is in agreement with the 

 results of experiments on phage inactivation following radioactive decay 

 of its P^' atoms (Hershey et al, 1951; see Sect. 2-1), and the same con- 

 siderations apply to both instances. 



The ultraviolet sensitivity of several viruses of a certain group, such as 

 bacteriophages, roughly parallels the particle size when the doses are 

 measured in incident energy (Luria and Dulbecco, 1949). This probably 

 reflects in part the greater cross section of larger viruses and suggests that 

 the quantum yields for inactivation may be of the same order. Bacterio- 

 phages T2, T4, and T6 have equal size and morphology, yet T4 is twice as 

 resistant to ultraviolet (2537 A) as T2 or TG. It is not yet clear whether 

 this difference is due to a low^er nucleic acid content or to a lower quantum 

 yield. The radiation resistance of T4 becomes associated with some of 

 the distinctive characteristics of T2 or TO in type-hybrid phages produced 

 by mixed infection (Luria, 1949). This makes it possible to investigate 

 the determination of the ultraviolet sensitivity of a group of viruses by 

 genetic means. 



2-3. VISIBLE LIGHT 



Wahl and collaborators (Wahl, 1946; Wahl and Latarjet, 1947) found 

 that several bacteriophages are inactivated at an appreciable rate when 

 exposed to visible light. The action spectrum has a maximum in the 

 near-ultraviolet and violet regions and a limit of effectiveness in the green 

 region of the spectrum. Yellow and red light are ineffective. This might 

 indicate that the viruses contain a pigment with a maximum of absorption 

 or of photochemical yield for the long ultraviolet radiation. It is 

 unknown whether this pigment plays any role in the photoreactivation 

 phenomenon (see Chap. 12 of this volume). It has also been suggested 

 that the inactivation may be due to a photodynamic action mediated by 

 components of the medium (Dulbecco, personal communication). 



3. DIFFERENTIAL EFFECT OF RADIATIONS ON VARIOUS PROPERTIES 



OF VIRUSES 



The loss of ability to reproduce is only one of the alterations that may 

 be produced in a virus particle. Since inactivation results from suppres- 



