SUMMARY: The killing of conidia of by ultraviolet irradiation followed first-order kinetics; about 0.5 % of the conidia which survived a 97 % lethal dose were nutritionally deficient mutants. This percentage of mutants among survivors was constant for different wild-type isolates for the first-, second- and third-order irradiations. This regularity implies that most conidia exposed at the first irradiations contained a single haploid nucleus. Adenine-requiring mutants formed pionnotes when grown on an agar medium containing low concentrations of adenine, and the extent of pionnotal formation depended on the position at which adenine synthesis had been blocked by the irradiation. 4-Aminoimidazole-5-carboxamide apparently did not act as an extracellular intermediary in adenine synthesis. An acriflavine-resistant mutant arose spontaneously in one of the mutants selected for nutritional deficiency, but mutants resistant to certain other growth inhibitors were not found.


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