IONIZING RADIATIONS 433 



(152). Most important, perhaps, is the observation of McElroy (102) 

 that postirradiation treatment with high hydrostatic pressure reduces 

 the recovery of morphological mutants in Nenrospora crassa and Asper- 

 gillus terreus. This observation suggests that the primary action of 

 ultraviolet (and chemical mutagens) is to cause the formation of a 

 semi-stable intermediate which may decay to a new, mutant, state or 

 may revert to its original, normal, state (103). The hypothetical inter- 

 mediate may be the gene itself or a metabolite or metabolic system (143, 

 146). 



Peroxides and unknown products in irradiated media are mutagenic 

 to Neurospora crassa (27, 142). However, results published so far fail 

 to implicate products formed by ultraviolet irradiation of the medium, 

 at the wavelengths employed in mutation studies, as important factors 

 in the induction of mutations. 



Photoreactivation was discovered in Streptomyces griseus (82) and 

 has been observed in several of the true fungi (13, 17a, 52, 81, 109). 

 As usually studied, photoreactivation means that lethal effects of ultra- 

 violet radiation are counteracted in part by subsequent exposure to 

 light of longer wavelength, 330 to 480 m^; however, other changes 

 induced by ultraviolet are also counteracted. The action spectrum of 

 photoreactivation in S. griseus has a pronounced peak at 436 m^, dif- 

 fering significantly from the action spectrum of the same process in 

 Escherichia coli (83). The theories and problems of photoreactivation 

 are reviewed by Dulbecco (35). Wainwright and Nevill (144) present 

 evidence that photoreactivation exerts independent effects on survival 

 and mutagenesis in Streptomyces sp. 



5. IONIZING RADIATIONS 



Ionizing radiations, as the name implies, are radiations whose pas- 

 sage through a material causes the production of ion-pairs. Most 

 studies on fungi have employed X-rays or the equivalent y-rays emitted 

 by radium and other radioactive materials; a few data are available 

 on a-rays (from polonium, but also produced by the cyclotron), /3-rays 

 (from radioactive materials or from an ion source), and cyclotron-pro- 

 duced neutrons. All these types of radiation, we assume, have the same 

 mode of action but differ, of course, in energy, ionization per unit 

 path length, and ability to penetrate (86). Consequently, physically 

 equivalent doses of different types of ionizing radiation are not bio- 

 logically equivalent. However, it is not yet clear how such differences 

 in biological effect are to be explained. Thus, in the killing of spores 

 of Aspergillus terreus, the densely ionizing radiations (protons and 



