Nuclear mutants of Neurospora crassa temperature-sensitive for the synthesis of cytochrome aa3. I. Isolation and preliminary characterization.

Three nuclear mutants of Neurospora crassa, temperature-sensitive for the synthesis of cytochrome aa3 have been isolated. When grown at 41 degrees C the mutants have large amounts of KCN-insensitive respiration, reduced amounts of cytochrome aa3 and cytochrome c oxidase activity, and grow more slowly than wild-type cultures grown at the same temperature. When the mutants are grown at 23 degrees C, they are virtually indistinguishable from wild-type strains. The mutants were selected on the basis of their slow growth at 41 degrees C in medium containing salicylhydroxamic acid, and by their inability to reduce 2,3,5-triphenyltetrazolium chloride at 41 degrees c. The selecttion technique was designed to eliminate mutants that did not carry thermolabile electron transport chain components. However, studies on the thermolability of the cytochrome oxidase activity in isolated mitochondria indicate that the enzyme of the mutants is no more susceptible to heat denaturation than is the enzyme in wild-type mitochondria. This suggests that the synthesis or assembly of cytochrome aa3 may be altered in the mutants at the restrictive temperature.