Deletion mutants of Neurospora crassa mitochondrial DNA and their relationship to the "stop-start" growth phenotype.

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Abstract
Stoppers are a class of N. crassa extranuclear mutants characterized by gross deficiencies of cytochromes b and aa3 and an unusual growth phenotype which involves irregular periods of growth and nongrowth. Mitochondrial [mt]DNA from all 4 stopper mutants contain deletions or insertions detectable by restriction enzyme analysis. [stp] mtDNA consists predominantly of defective molecules which retain a 16 Mdalton segment (EcoRI-1, -4 and -6) of wild-type mtDNA (40 Mdaltons). The other stopper mutants show smaller alterations: [stp A18t]-618, a 0.35 kbase deletion in EcoRI-7b; [stp B2]-651, a 4 kbase insertion in EcoRI-2; and [stp A]-574, a 5 kbase deletion in EcoRI-2 and -10. Stop-start growth results from competition between certain defective mtDNA which have a tendency to predominate, and low concentrations of less defective mtDNA species which must be retained to sustain growth. Three additional extranuclear mutants (nonstoppers) also contain deletions in mtDNA. Remarkably, the defective mtDNA species in 2 of these mutants ([poky] H1-10 and [SG-3]-551) retain different sizes (18 and 13 Mdaltons, respectively) of the same region retained in [stp] mtDNA (i.e., EcoRI-1, -4 and -6). The findings suggest that production of defective mtDNA in Neurospora is apparently nonrandom with a preferred mechanism leading to retention of this segment. It may be significant that the retained segment contains both mitochondrial rRNA genes and most mitochondrial tRNA genes. These deletion mutants may provide a tool for genetic mapping of Neurospora mtDNA.