Sequential cold-sensitive mutations in Aspergillus fumigatus

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Abstract
Mutants of the thermotolerant fungus Aspergillus fumigatus I-21 (ATCC 32722) unable to grow at 37 °C were sought. Cold-sensitive mutants were enriched from progeny spores of γ-irradiated conidia by two or more incubations at various nonpermissive temperatures alternating with filtrations through cheesecloth. The approximate minimum, optimum, and maximum growth temperatures of the parent were 12, 40, and 50 °C, respectively. Mutants unable to grow at 37 °C were not successfully isolated directly from the wild type. A mutant unable to grow at 25 °C was isolated and mutations further increasing the cold sensitivity by increments of 3–5 °C were found to occur. Mutants completely unable to grow at 37 °C were obtained by five sequential mutations. All mutants grew as fast as the wild-type parent at 45 °C and higher. Each mutant produced revenants able to grow not only at the nonpermissive temperature used for its isolation but also at lower temperatures.