1:;l' 



P H Y S I C A L AGENTS 



(5, 109, 110) suggest instead that the shape of the dose-response curve 

 is a function of the number of nuclei per cell: exponential for uninu- 

 cleate spores, sigmoidal for bi- and multinucleate spores. Death in a 

 multinucleate spore requires that several targets must absorb radiation; 

 the sigmoidal curve follows from this requirement. 



Two mechanisms of lethality have been proposed (109): induction 

 of recessive lethal mutations and non-genetic damage to nuclei. Ob- 

 viously, the first of these cannot be of much significance in the death of 

 conidia having more than one nucleus. 



The induction of morphological and biochemical mutations by 

 ultraviolet radiation responds rather differently as dosage is increased. 

 As shown in Figure 6, mutation frequency increases with dosage only 

 to a point, then levels off or declines at higher dosage. Dose-response 

 curves of this general type have been reported from several studies in 

 which all morphologically visible mutations are scored (63, 66, 67, 73, 

 100, 135). However, in one instance in which a known mutation at a 

 single locus was scored, the response curve was strikingly different (51), 

 and the mutation frequency in Streptomyces flaveolus does not drop 

 at high dosage (80). 



Ultraviolet-induced mutagenesis is affected by several modifying 

 environmental factors. Pretreatment before irradiation with infrared 

 radiation, 2,4-dinitrophenol, or nitrogen mustard increases mutagenesis 

 (133, 134, 135, 136). Survival and mutation rate in Streptomyces sp. 

 are modified by treatments applied after irradiation (143, 145, 146). 

 Respiratory poisons and anaerobiosis decrease both the lethal and the 

 mutagenic effects of ultraviolet radiation on Penicillium chrysogenum 



Figure 6. Morphological muta- 

 tions in Penicillium notatum as 

 a function of ultraviolet dosage. 

 Plotted from data of Hollaender 

 (63). 



30 60 



Dosage, ergs per spore 



