The mitochondrial DNA of Neurospora crassa: deletion by intramolecular recombination and the expression of mitochondrial genes

Unstable mitochondrial DNA in natural-death nuclear mutants of Neurospora crassa

The natural-death mutant of Neurospora crassa has an accelerated senescence phenotype caused by a recessive mutation, nd, in a nuclear gene that is located in linkage group I. An examination of mitochondrial functions, however, revealed that the mutant has phenotypic and molecular defects similar to those commonly associated with maternally transmitted fungal senescence syndromes, including (i) deficiencies in cytochromes aa3 and b; (ii) a deficit in small subunits of mitochondrial ribosomes, and hence defective mitochondrial protein synthesis; and (iii) accumulation of gross rearrangements, including large deletions, in the mitochondrial chromosome of vegetatively propagated cells. These traits indicate that the nd+ allele codes for a function that is essential for stable maintenance of the mitochondrial chromosome, possibly a protein involved in replication, repair, or recombination.

Unstable mitochondrial DNA in natural-death nuclear mutants of Neurospora crassa

The natural-death mutant of Neurospora crassa has an accelerated senescence phenotype caused by a recessive mutation, nd, in a nuclear gene that is located in linkage group I. An examination of mitochondrial functions, however, revealed that the mutant has phenotypic and molecular defects similar to those commonly associated with maternally transmitted fungal senescence syndromes, including (i) deficiencies in cytochromes aa3 and b; (ii) a deficit in small subunits of mitochondrial ribosomes, and hence defective mitochondrial protein synthesis; and (iii) accumulation of gross rearrangements, including large deletions, in the mitochondrial chromosome of vegetatively propagated cells. These traits indicate that the nd+ allele codes for a function that is essential for stable maintenance of the mitochondrial chromosome, possibly a protein involved in replication, repair, or recombination.

Duplication of the tRNA(MMet) and tRNA(Cys) genes and of fragments of a gene encoding a subunit of the NADH dehydrogenase complex in Neurospora grassa mitochondrial DNA

Neurospora crassa mitochondrial DNA (mtDNA) contains duplications of the tRNA(MMet) gene upstream of a gene (ND2) encoding a subunit of the NADH dehydrogenase complex and of the tRNA(Cys) gene which is found downstream of the apocytochrome b gene. Both duplicated genes are located upstream of the small rRNA gene. The duplications are extended to flanking sequences. In the case of the tRNA(MMet) duplication, two fragments of the ND2 gene are also duplicated. These two fragments, which are not contiguous in the ND2 gene, are connected to each other by a palindromic sequence of 37 bp and together they constitute an open reading frame. The possible involvement of this palindromic sequence in the processes of gene duplication and transfer is discussed. Two overlapping reading frames are present between the tRNA(MMet) and tRNA(Cys) copies. All information of the ND2 duplication and the two overlapping reading frames are present on a polycistronic transcript.