EFFECTS OF RADIATION ON BACTERIA "ill 



observed a decrease in induced mutation rates following photoreactivation 

 at all ultraviolet doses. In this case, however, there is no maximum or 

 plateau in the mutation-dose curve. 



ULTRAVIOLET ACTION SPECTRUM OF MUTATION INDUCTION 



\ction spectra of mutation induction with maxima in the neighborhood 

 of 2G00-2650 A have been reported by Knapp et al. (^^^Q) for .Sp/.aero- 

 carvus donnelli sperm; Hollaender and Emmons (1939a, b, 1941 and 

 Emmons and Hollaender (1939) for Trichophyton mentagrophytes ;hiad[ev 

 and Uber (1942) for maize pollen grains; Hollaender . . . Demerec 

 (1945) for Neurospora; Hollaender and Zimmer (1945) for P. notatum; and 

 Hollaender, Raper, and Coghill (1945) for A. terreus. The similarity 

 between the action spectrum for mutation production and the absorption 

 spectrum for nucleic acid is commonly interpreted to indicate that the 

 nucleic acid portion of the gene acts as the chromophore for mutation 

 production. Changes in viscosity, stream birefringence, and colloid 

 osmotic pressure of deoxyribonucleate following ultraviolet irradiation 

 have been studied by Hollaender ./ al (1941). The doses required to 

 produce a detectable effect in vitro are large compared to those required 

 for the bactericidal or mutagenic effects. 



Noethling and Stubbe (1934; Stubbe and Noethling, 1937) found 2967 

 \ to be the most efficient wave length of four tested in producing muta- 

 tions when pollen of Antirrhinum majus was irradiated. Differential 

 absorption of the irradiated pollen grains and the small number of tests 

 cast doubt on the significance of this observation. 



McAulay and his associates (McAulay and Ford, 1947; McAulay et al, 

 1949- Ford and Kirwan, 1949) have published some unusual results 

 obtained in studies of mutation induction with the fungus, Chaeto7mum^ 

 globosuni. The most efficient ultraviolet wave length is 2804 A and 

 mutations are reported following irradiation with wave lengths of 33d4, 

 3654 and 4047 A. They report that the shorter wave lengths, i.e., 2654, 

 9804' and 2967 A, selectively induce the so-called "K mutant," whereas 

 very'few K mutants are found following the near-ultraviolet and short- 

 visible treatment. The statistical validity of this conclusion seems 

 questionable since the sizes of samples observed are quite small. No Iv 

 mutants were observed following X irradiation, whereas, with large doses 

 of 2804 A ultraviolet, as high as 62 per cent of K mutants were observed. 

 The mutation-dose curves for "lethal mutations" (mutations resulting m 

 cessation of growth while the colony is still very small) indicate that, 

 although two quantum hits of 2804 A ultraviolet will induce the mutation, 

 five quantum hits of X rays are necessary for lethality (Ford and Kirwan, 

 1949) These results are difficult to evaluate since, except lor the lethal- 

 mutation studies, sample sizes are very small owing to the characteristics 

 of the fungus which produces huge colonies. That the lethal mutations 



