410 RADIATION UlULOGY 



• luaiititativc relations to radiation or other mutagens and that some may 

 not respond to radiation or other mutajj;ens at ail. That such diverti;enl 

 results should he obtained with the first few specilic mutations aderiuatcly 

 studied indicates that caution should he exercised in generaliziu};, as 

 regards the mutation-dose relation, and that (Miual caution should he used 

 in interpreting radiation-induced mutation rates which are the sum of the 

 mutation rates of an unknown number of loci. 



PHOTOREACTn VTION OF ULTKAVIOLKT MUTAGENIC EFFECTS 



Although this subject is covered more comi)letely in Chap. 12 by 

 Dulbecco, a brief discussion seems appropriate heie. Since photoreacti- 

 vation indicates an indirect mechanism for a major proportion of the 

 ultraviolet bactericidal efl'ects, it is of interest to know if the same indirect 

 mechanisms are involved in the production of mutations following ultra- 

 violet irradiation. Kelner (1949b) and Novick and Szilard (1949) were 

 the first to study this question and, although Kelner's results were some- 

 what inconclusive, Novick and Szilard observed a reduction in mutagenic 

 effects to a degi-ee corresponding roughly with the reduction in the lethal 

 effects. Similar observations have been reported for Paramecium aurelia 

 (Kimball and (iaither, 1950; Kimball, 1950), Neurospora (Goodgal, 1950: 

 Brown, 1951), PeniciUium chrysoyenum (Roegner, 1951), and for the polar 

 cap cells of Drosophila (Meyer, 1951). Roegner's results are of interest in 

 that the mutation-dosage relation exhibited a maximum, as discussed 

 earlier. When the photoreactivation treatment followed high doses of 

 ultraviolet, i.e., on the descending portion of the curve beyond the maxi- 

 mum, Roegner observed an increase in the fre(iuency of mutation, but at 

 lower ultraviolet doses in the ascending portion of the curve, photoreacti- 

 vation resulted in a decreased proportion of mutations. 



The most extensive studies are those of Xewcombe and Whitehead 

 (1951) with the color-response mutations of E. coli Br on mannitol-tetra- 

 zolium agar. A relatively constant proportion of mutations was observed 

 for ultraviolet doses greater than 1000 ergs/mm-'. Photoreactivation of 

 low doses of radiation resulted in a dose-reduction factor of 5 which is 

 greater than that observed for the bactericidal effects. For very large 

 doses of ultraviolet, no effect of photoreactivation in lowering the muta- 

 tion frofiuency was observed. To explain their results, Newcombe and 

 Whitehead postulate that both a phot()stal)le and photosensitive mutagen 

 poison are produced by the ultraviolet and that the plateau in the muta- 

 tion-dose curve is due to a common limiting step in the reactions by which 

 both of these mutagen poisons cause mutations. Thus, at high doses, a 

 sufficient amount of the photostable mutagen poison is produced to cause 

 a maximum mutation effect, and no photoreactivation is possible. 



In studies of ultraviolet-induced mutations from streptomycin depend- 

 ence to independence in the Sn-4 strain of E. coH, Beckhorn (1951) 



