Temperature-sensitive nuclear mutants of Neurospora crassa deficient in mitochondrial ribosomes

We have isolated two non-allelic nuclear mutants of Neurospora crassa that are temperature-sensitive for the production of cytochromes aa 3 and b. When grown at 23 °C the mutants are virtually indistinguishable from the parent wild-type strains. When grown at 41 °C the mutants have large amounts of KCN-insensitive respiration and lack cytochromes aa 3 and b. Further examination of the mutants revealed that they were extremely deficient in their capacity for mitochondrial protein synthesis when grown at 41 °C. This protein synthesis deficiency appears to be related to a virtual absence of both small and large mitochondrial ribosomal subunits following growth at 41 °C. Examination of the mitochondrial RNAs of the mutants suggests that mitochondrial rRNAs are synthesized in greatly reduced amounts or that they are misprocessed when these mutants are grown at the non-permissive temperature.