394 RADIATION BIOLOGY 



reproducible results. They postulate a poison produced intracellularly 

 by photochemical action which selectively inhibits the mechanism of cell 

 division, one molecule bein^ elective. The recovery is explained by 

 simple exponential decay of the poison. However, they emphasize that 

 since no recovery is observed with strain H/r, a (lualitatively different 

 mechanism must be assumed for this strain. Furthermore, since no 

 recovery was observed in cither strain with X rays, they point out that 

 the mechanism of X-ray action is qualitatively different from that of 

 ultraviolet. 



Heat Eeactivation. Anderson (1949, 1951a) and Stein and Meutzner 

 (1950) independently observed that incubating ultraviolet-irradiated cells 

 of E. coli B at temperatures higher than 37°C results in greater survival. 

 The magnitude of the increased surxival is of the same order as that 

 observed when irradiated cells are exposed to visible light (Kelner, 1949b, 

 see section on photoreactivation). The dose-reduction ratios (the ratio of 

 the ultraviolet exposure of cells incubated at 40°C to that for cells incu- 

 bated at 30°C' which produces the same level of survival) for strain B were 

 found by Anderson (1951a) to be a decreasing function of the total dose of 

 radiation, the decrease for heat reactivation being greater than the corre- 

 sponding decrease for photoreactivation. E. coli B/r exhibited only a 

 small heat reactivation, and neither strain show'ed appreciable heat reacti- 

 vation following X irradiation. Among ten E. coli and seven yeast strains 

 tested by Anderson only two E. coli strains were found capable of heat 

 reactivation. It would appear therefore that heat reactivation is not a 

 general phenomenon. Harm and Stein (1952) have shown that the large 

 difference between strain B and its mutant strain B/r in ultraviolet resist- 

 ance, when the irradiation cells are incubated at 37°C\ disappears when 

 the cells are incubated at 44.5°C, equal survival being observed in both 

 strains. Ultraviolet-irradiated cells of strain B remain fully heat reac- 

 tivable for 1 hr at 37°C (Stein and Harm, 1952). If a longer period 

 elapses before incubation at the reactivating temperature, the amount of 

 reactivation decreases rapidly with no reactivation occurring after 3 hr. 



Photoreactivation. Among the more significant recent developments is 

 the discovery of the phenomenon of photoreactivation. No exhaustive 

 discussion will be attempted here since Dulbecco reviews the available 

 data in Chap. 12 of this volume. 



Although Whitaker (1942) presented conclusive data showing that 

 visible light partially counteracted the effects of ultraviolet radiation in 

 Fucus eggs, the present interest in photoreactivation stems largely from 

 ■work by Kelner (1949a, b). Dulbecco (1949) independently observed 

 photoreactivation of ultraviolet-irradiated bacteriophage adsorbed on 

 sensitive host cells. Kelner (1949a) showed that exposure to visil)le light 

 subseriuent to exposure to uiti-aviolet radiation would result in as high as 

 300,000-fold recovery of Strcptomijces griseus conidia. He later (1949b) 



