A "Stopper" mutant of Neurospora crassa containing two populations of aberrant mitochondrial DNA.

[E35], an extranuclear mutant of Neurospora crassa has all the phenotypic characteristics of the "stopper" mutants (De Vries et al. 1980). In the present work, the mitochondrial DNA as well as the mitochondrial translation products are characterized further. The primary mutational event appears to have been the deletion of about 4 kbp from the wild-type genome. Moreover, after prolonged vegetative growth the mutant accumulates an 8-μm circular mtDNA, which was demonstrated both by electronmicroscopy and by restriction enzyme analysis. Hence, the mutant contains two populations of aberrant mitochondrial DNA, the smaller of which is an amplification of the rRNA-tRNA part of the larger. We propose that the primary deletion has generated a signal in the larger DNA which can cause premature termination of replication at the deletion site, and subsequent circularization of the unfinished daughter molecule. Finally, the deleted part may contain a determinant for synthesis of a protein of 11 kDal. The function of this protein, which is not a subunit of the F0 ATPase, is not yet known.