32 L. H. GRAY 



experimental conditions which have been investigated so far, the log 

 survival curve is linearly related to dose, except near the origin, where 

 a slight curvature is generally observed when the cells to be irradiated 

 are suspended in a phosphate buffer. In the case of aerobic irradiation 

 at room temperature, the log surviving fraction is linearly related to 

 dose over almost ten powers of ten. Do = 1'7 krad, and the extrapola- 

 tion number is 14:. These vegetative bacteria are thus 25/1*7 (= 15) 

 times as sensitive as spoi*es of B. megaterium irradiated in the dry state 

 under conditions that permit the maximum participation of oxygen. 

 When oxygen is rigidly excluded at the time of irradiation, the sensi- 

 tivity oi Serratia marcescens is reduced by a factor of 4. The sensitivity 

 rises, however, very rapidly with increasing O2 concentration. Assuming 

 the equation projDOsed by Howard-Flanders and Ali)er (1957) to be 

 applicable, viz: 



S Oo 



= l = (m-l)- (1) 



Sn O2 + A 



where S is the sensitivity, measured by the slope of the log survival 

 curve, at a concentration of oxygen in the medium equal to O2 /iuioles/l, 

 and Sn the sensitivity when oxygen is rigidly excluded, then m = 4 and 

 K = 4/xmoles/l. Both constants are, to a first approximation, inde- 

 pendent of temperature over the range to 30°C. This very marked 

 influence of oxygen on radiosensitivity is almost universally observed 

 with proliferating cells, and the question naturally arises as to whether 

 this influence is mediated through the cytochrome system. A number of 

 circumstances make this improbable. For example, Moustacchi (1958) 

 showed that the sensitivity of several cytochrome-deficient mutants of 

 yeast showed the same oxygen dependence as wild type organisms. In 

 the case of Serratia marcescens, an involvement of the cytochrome 

 system can be definitely excluded by the observation of Dewey and 

 Longmuir (unpublished) that the oxygen concentrations at which 

 respiration begins to decline at room temperature is very much lower 

 than 4/xmoles/l. At temperatures near 0°C it is less than O.l/x mole/1. 



Sensitivity varies by nearly a factor of 4 over a range of oxygen con- 

 centration within which the. respiration of the cells is virtually inde- 

 pendent of oxygen concentration. 



The fact that Q02 is maintained approximately constant to much 

 lower oxygen concentrations than 4 /anoles/1 also shows that the intra- 

 cellular concentration of oxygen cannot differ from that of the surround- 

 ing medium by more than a small fraction of the value of K. We may, 

 therefore, safely conclude that, unless the nuclear membrane offers 

 substantial impedance to the movement of oxygen, equation (1) is an 



