EFFECTS OF RADIATION ON BACTERIA 417 



the cells. Beckhorn demonstrated that this unusuul effect of ultraviolet 

 was photoreactivable to about the same degree as are the bactericidal and 

 mutagenic effects. Lederberg et al. have shown that X rays produce a 

 similar effect and have found that, on continued incubation, most of the 

 apparent haploid colonies develop spots of growth which are diploid. 

 Thus, although diploid cells surviving the irradiation are temporarily- 

 altered so as to produce many haploid segregates, they retain the ability 

 to transmit the diploid condition. These interesting results indicate that 

 recessive lethal-mutation induction is not an important mechanism in 

 killing by irradiation in these strains. If such were the case, marked 

 differences in sensitivity should be observed between haploid and diploid 

 cells and segregation of diploids to haploids should be decreased rather 

 than stimulated since recessive lethals, although masked in the diploid, 

 are expressed in the haploid cells. 



That the nuclear constitution of microbial cells does influence the sensi- 

 tivity to the lethal effects of radiation is indicated by the results of 

 Latarjet and Ephrussi (1949) in which haploid and diploid cells of the 

 same yeast strain were irradiated with X rays. They obtained exponen- 

 tial survival curves for the haploid cells and sigmoidal survival curves for 

 the diploid cells. The sigmoidal curve corresponded approximately to a 

 two-hit curve, as had been observed by other workers in studies of diploid 

 yeast. Similar results have been obtained by Tobias (1952) who used 

 haploid, diploid, and presumed tetraploid yeasts in tests of the diffusion 

 model of the biological effects of high-energy radiations. 



Atwood (1952) has devised a technique employing heterokaryotic 

 conidia of Neurospora which permits, in addition to determination of the 

 surviving fraction of cells, the determination of the surviving fraction of 

 nuclei, the fraction of nuclei containing at least one recessive lethal muta- 

 tion, the frequency with which separately induced recessive lethal muta- 

 tions are homologous, and the degree to which all these effects are 

 independent of one another. Although these studies are still in the 

 preliminary stage, it seems warranted to conclude that killing of Neuro- 

 spora conidia by X rays is primarily, if not entirely, a nuclear phenomenon 

 and that recessive lethal mutations, although more important in uni- 

 nucleate conidia, have but a minor effect in the inactivation of hetero- 

 karyotic nuclei since homologous recessive lethal mutations must be 

 induced for their expression. Utilizing similar techniques, Norman 

 (1951) has published extensive experiments on the ultraviolet inactivation 

 of Neurospora conidia. He concludes that inactivation of conidia is a 

 consequence of the inactivation of nuclei and that nuclei exhibit first- 

 order kinetics. Two kinds of inactivation processes are postulated: 

 recessive lethal mutations and a nongenetic effect on the nucleus. The 

 two mechanisms are intimately related, however, since the same action 

 spectrum is obtained with monochromatic ultraviolet radiation and each 



