KEMPNER AND POLLARD 133 



The slopes at the origin (dashed for proline, solid for methionine) are cal- 

 culated from the sensitive volumes found with y radiation. The points near 

 400 ev/100 A are from the deuteron experiments as described above. The LET 

 for a particles in our experimental arrangement was 1500 ev/100 A, and the 

 cross sections found by this method are shown at that LET value. 



DISCUSSION 



The analysis of the methionine-cold-TCA-insoluble fraction data indicates a 

 spherical target. The radius calculated from the inactivation volume is 130 A; 

 the radius calculated from the cross-sectional area is 127 A. The agreement is 

 well within the uncertainties of this method of study. 



The proline data are somewhat more complex. The upward concavity near 

 the origin indicates a rather complex response to radiation. It can be explained 

 most simply by supposing that closely grouped ionizations are more effective 

 than single ionizations, and the rapid change of slope suggests that this multi- 

 plicity is small, probably a double ionization requirement [5]. There is no 

 agreement between the area and volume determinations if a spherical target 

 is assumed, no matter which ionization requirement (one, two, three, or four) 

 is used. The only simple model that fits the data is a long, thin rod [10]. For 

 a double ionization requirement, the volume and area determinations lead to 

 a model of a rod of 11 A radius and 2.2 u length. The analysis based upon 

 the model of a thin plate leads to substantially the same result, an extremely 

 long plate of small cross-sectional area. 



Preliminary experiments with deuteron-bombarded E. coli cells indicate that 

 the incorporation of glucose into the TCA-insoluble portion of these cells re- 

 quires the integrity of a sphere of approximately 80 A radius. Since, in these 

 experiments, glucose was the single carbon source available to the organisms, 

 it seems reasonable to conclude that amino acids are produced in the cells in 

 spheres of this size. Since the doses required to stop glucose incorporation into 

 the cold-TCA-insoluble fraction are large compared with the doses required to 

 stop exogenous methionine from being incorporated into the same fraction, it 

 would appear that the formation of proteins from amino acids is not directly 

 related to the incorporation of glucose. Further work on this very interesting 

 result is clearly necessary. 



If the two radiation-sensitive spheres found for glucose and methionine in- 

 corporation were assumed to have a density of 1.3, and to be sedimenting in a 

 solution of viscosity 1.5 centipoises, then from 



d 2 = lSv)sV/(l-Vp) 



it can be calculated that the spheres associated with glucose incorporation 

 (160 A diameter) should have a sedimentation constant of the order of 25 S, 

 while the larger, methionine-associated sphere, 260 A in diameter, should have 

 a sedimentation constant of about 77 S. Since particles of such size have been 



