344 



K. DULBECCO 



[vol. 59 



The maximum absorption of this band is located at a shorter wave length 

 than the maximum of the action spectrum of PHTR; this difference, however, 

 is not such as to exclude the band from belonging to the photosensitive pigment, 

 because the location of the band may be shifted toward longer wave lengths if 

 the pigment is bound with some bacterial constituent after adsorption of the 

 inactive phage on bacteria. 



Influence of the Metabolic Conditions of Bacteria in PHTR of Phages 



PHTR is not appreciably different whether the phage is adsorbed by bacteria 

 in broth or in a synthetic medium; the rate of PHTR is somewhat lower with 

 resting bacteria than with bacteria suspended in nutrient media. 



The influence of oxygen on PHTR was determined. Resting bacteria and UVP 

 were placed in separate compartments of a Thunberg tube, nitrogen was bubbled 

 for about 20 minutes through the bacterial suspension, the tube was then evacu- 

 ated by a pump, and air was replaced with nitrogen, the operation being re- 



TABLE 6 

 PHTR rate and Qio at different temperatures 

 The Qio was determined from the ratio of Qd of the rates at two successive temperatures, 

 using the formula: Qio = (Qd)'"''', where d is the temperature interval between two ob- 

 servations. Phage T2r irradiated for 20 seconds with the germicidal lamp, illuminated in 

 liquid. 



peated 4 times. Phage and bacteria were mixed and the tube was exposed to 

 light. The control consisted of an open tube from which oxygen had not been 

 removed. The same amount of PHTR was observed in both tubes. Oxygen is 

 therefore not necessary for PHTR, at least not for the initial photochemical re- 

 action. We also found that cyanide in 10"^ m concentration does not affect PHTR. 



The Effect of Temperalure on PHTR 



PHTR can occur at temperatures too low to allow growth of active phage 

 (+1 C). To obtain a measurable amount of PHTR at this temperature, one 

 must mix UVP and bacteria in the dark at 37 C to allow enough adsorption, 

 then chill the mixture to 1 C and expose it to light. 



Determinations of PHTR rate with constant illumination were made at 3, 11, 

 16, 24, and 37 C, using phage T2 irradiated for 20 minutes with the germicidal 

 lamp. The Qio was determined for each interval, and the results are reproduced 

 in table 6. 



243 



