366 S. GARD AND O, MAAL0E 



1952; Jacob and Wollman, 1956), therefore points to these structures as the 

 most likely target material. In an attempt to test this hypothesis, Epstein 

 (1953), has compared estimates of target volume and nucleic acid volume for 

 about 10 different viruses; he finds rather close agreement except for vaccinia 

 virus, where the target volume is less than 10 % of the nucleic acid volume. 

 Buzzell et al. (1956) have shown that, except for vaccinia virus, the agreement 

 is improved by introducing more recent analytical data. Phage Tl, which was 

 not considered by Epstein or by BuzzeU et al., seems to be another excep- 

 tion: according to PoUard and Forro (1951), its target volume is about 3 % 

 of the particle volume, as contrasted with 15-30 % for the DNA volume 

 (Mennigmann and Schaechter, unpublished data). 



The assumption that a virus particle is inactivated by the first primary 

 ionization to occur within the nucleic acid volume would thus seem to be valid 

 in many cases, but not in all. For larger biological units this assumption may 

 be far from correct; in cells of E. coli, the target volume (calculated by Lea, 

 1947) is only about 1-2 % of the DNA volume (assuming between 5 X 10~^^ 

 and 10~^^ gm. DNA/cell). These considerations show that, in some cases, 

 only one out of a rather large number of primary ionizations within the 

 "genetic target" would seem to cause damage that results in irreversible 

 inactivation. 



In this connection, it is interesting to recall that phage particles carrying 

 radiophosphorus (P^-) in their DNA undergo exponential inactivation, but 

 that only one decay out of about ten constitutes an effective hit (Hershey et al., 

 1951; Stent, 1953). From what was said above, it would seem that, at least 

 in certain DNA structures, a primary ionization is not more effective in 

 causing inactivation than is the decay of a P^^ atom. 



A low efficiency of inactivation could mean that "weak spots" existed 

 within the genetic structure and that ionizations (or P^^ decays) were effective 

 only if they occurred in these spots (Stent and Fuerst, 1955), If so, we should 

 be dealing with "radiation targets" distributed within a chemically and 

 genetically defined structure, and the target volume would mean something 

 very different from what it is commonly thought to mean. 



In the case of inactivation by ultraviolet light (UV), the situation is quite 

 different. First, the efficiency with which the absorption of a quantum of UV 

 causes inactivation is low; the "quantum-yield", ^, being of the order of 

 10~^ — 10~*. Second, the UV energy is absorbed by characteristic chemical 

 compounds; and, finally, UV causes inactivation by direct effects only (see 

 Sections II, B, 2 and 3). 



2. Ionizing Radiation 



a. X-Eays. The introductory remarks about irradiation of viruses dealt 

 with the direct effect of ionizations occurring in the target material. It has 



