EFFECTS OF RADIATION ON BACTERIA 407 



greater. This observation has important implications with respect to 

 the mechanism of the lethal action of radiation. On the basis of energy 

 absorbed per bacterium, X rays were found to be approximately 200 times 

 as efficient in producing inactivation and about ten times as efficient in 

 the production of mutations. 



In later studies, Demerec (1949, 1951) observed a linear increase in 

 reverse mutations of the SD-4 streptomycin-dependent mutant of strain 

 B/r (Bertani, 1951) with both X and ultraviolet irradiation. With this 

 mutation system, no zero-point mutations occur since no mutations are 

 expressed until at least one division has occurred. 



In this connection, Crigg (1952) has shown that, in quantitative studies 

 of reverse mutations of biochemical mutations in Neurospora conidia, 

 precautions must be taken to ensure that the observed reversions are 

 actually mutations and are not already present in the irradiated culture. 

 He found that large numbers of mutant conidia which refjuire a particular 

 growth factor inhibit the growth of a known smaller number of wild-type 

 cells when plated on minimal medium. He suggested that, in induced- 

 reverse-mutation experiments, the radiation merely kills enough of the 

 growth-factor-requiring cells to remove the inhibition, thus permitting 

 spontaneous reverse mutations already present in the culture to develop. 

 This was demonstrated in tests in which the apparent proportion of wild- 

 type nuclei increased tenfold when the culture was plated at low dilutions. 

 Somewhat similar observations have been reported by Bryson (1950) 

 with streptomycin-resistant mutants of E. colt. 



Still another type of mutation-dose relation has been observed by 

 Newcombe and Whitehead (1951) in studies of color-response mutants of 

 E. coll B/r on mannitol-tetrazolium agar. With this system the frequency 

 of mutation rises very rapidly with ultraviolet doses below 500 ergs/mm- 

 and is nearly constant for doses of 1000 5000 ergs/mm^. No decline in 

 the percentage of mutations is observed at the higher doses. 



Extensive quantitative studies of reverse mutations of the same SD-4 

 mutant of strain B/r used by Demerec (1949, 1951) and of a purineless 

 mutant of the same strain were made by Anderson (1951b). The chief 

 results of his experiments, w^hich were designed to test whether the oxygen 

 concentration at the time of X irradiation had a parallel influence on 

 mutagenesis and inactivation, are shown in Figs. 10-12 and 13. Survival 

 curves for the two mutant strains were nearly identical. However, 

 greatly different response curves were obtained for the two reverse muta- 

 tions. Thus Fig. 10-12 shows that, for the streptomycin-dependent 

 strain, a linear relation exists between induced mutations and dose and 

 that only a slightly higher rate of induced mutation was observed when 

 the organisms were irradiated in the presence of oxygen. For the purine- 

 less strain, an approximately exponcMitial increase in mutation rate was 

 observed with increasing dose, the frequency of mutations observed for a 



