EFFECT OF RADIATIONS ON PHAGE PARTICLES 67 



and the megaterium phage M5 (Fluke and Pollard, 1949; 

 Zelle and Hollaender, 1954; Franklin, Friedman, and Sedow, 

 1953) and of the ultraviolet absorption spectra of suspensions of 

 purified T6 (Putnam, KozlofT, and Neil, 1949), T2 (Cohen and 

 Arbogast, 1950b), and T7 (Putnam, Miller, Palm, and Evans, 

 1952), showed that in all these cases both action and absorption 

 spectra are very similar to the typical absorption spectrum of 

 purified nucleic acids, which displays a maximum absorption 

 near the wavelength of 2,600 A. It seems probable, therefore, 

 that the inactivating photons of ultraviolet light are absorbed by 

 the phage nucleic acid. The quantum yield, i.e., the number of 

 phage particles inactivated per quantum absorbed, has been 

 estimated for Tl and T2 at various wavelengths by Zelle and 

 Hollaender (1954). These authors found that the yield for 

 either phage is only about 3 X lO^^ throughout the range of 

 wavelengths from 2,200 to 3,000 A. 



Campbell-Renton (1937) compared the efl^ects of ultraviolet 

 irradiation on five strains of phage and found the inactivation to 

 be an exponential function of dose. The ultraviolet sensitivity of 

 these strains, measured as the slope of the exponential survival 

 curve, varied over a ten-fold range. Latarjet and Wahl (1945) 

 compared the ultraviolet inactivation of phages CI 6 and SI 3 and 

 found that while the survival curves for both types are exponen- 

 tial, phage SI 3 has only ^/g the ultraviolet sensitivity of phage 

 CI 6, although SI 3 contains only about V30 as much nucleic acid 

 as CI 6 (Sinsheimer, 1957). The nucleic acid of SI 3, therefore, 

 appears to be intrinsically more sensitive to ultraviolet light than 

 that of CI 6. It is possible that the same difference in chemical 

 composition which is responsible for the very different sensi- 

 tivities of these two phage strains to visible light is also respon- 

 sible for the greater intrinsic ultraviolet sensitivity of SI 3. In 

 this connection it may be noted that although T4 is indistin- 

 guishable from T2 and T6 in size and morphology, it has only 

 one half the ultraviolet light sensitivity of its two relatives. 

 Streisinger (1956a) traced the difference in ultraviolet sensi- 

 tivity of these three strains to a single genetic locus. Phage 



