title: Intramolecular recombination and deletions in mitochondrial DNA of senescent, a nuclear-gene mutant of Neurospora crassa exhibiting "death" phenotype.
abstract: In Neurospora crassa, a nuclear-gene mutant, senescent, derived from a phenotypically normal wild isolate of Neurospora intermedia exhibits a 'death' phenotype. Regardless of the composition of the culture medium, the mycelium ceases to grow in 2-6 subcultures at 26 degrees C and 1 or 2 subcultures at 34 degrees C. Senescence of vegetative mycelium is associated with deficiencies in cytochromes aa3 and b and reduced oxygen uptake. The restriction fragment analysis of mitochondrial DNA from senescing mycelia showed deletions and gross sequence rearrangements. Analysis of mitochondrial DNA of (sen + sen+) heterokaryons constructed with "excess" sen cytoplasm suggested correlation between mtDNA deletions and senescence. Three novel sen-specific EcoRI fragments of sizes 3.6, 3.9, and 4.4 kb were cloned, sequenced, and analyzed. Nucleotide sequences of the sen-specific EcoRI fragments suggested that deletions were a consequence of intramolecular recombination between EcoRI-5 and -10 and/or between EcoRI-8 and -10. The recombination junctions were close to stretches of GC-rich-PstI palindromic sequences that potentially form stable hairpin structures and might facilitate recombination between homologous repeats as short as 6-10 bp. These observations suggest that the wild-type (sen+) allele encodes a factor that protects the mitochondrial genome from undergoing intramolecular recombination and deletions. In this respect sen+ (linkage group V) has a function similar to nd+ (linkage group I) and the two gene products probably have mutually exclusive roles in suppressing cruciform-associated and homologous recombination, respectively, thus safeguarding mitochondrial genome integrity. The sen+ allele most likely codes for a factor involved in recombination, repair or replication of the mitochondrial genome, or a transcription factor that regulates the expression of genes affiliated with mitochondrial DNA metabolism.
authors.affiliation: Department of Biochemistry, Indian Institute of Science, Bangalore 560 012, India.
authors.forename: Anthony D
authors.initials: AD
authors.lastname: D'Souza
authors.forename: Helmut
authors.initials: H
authors.lastname: Bertrand
authors.forename: Ramesh
authors.initials: R
authors.lastname: Maheshwari
mesh_terms: D001483:Base Sequence; D002467:Cell Nucleus / Q000235:genetics*; D003434:Crossing Over, Genetic; D045566:DNA, Cruciform / Q000235:genetics / Q000378:metabolism; D004271:DNA, Fungal / Q000235:genetics / Q000378:metabolism; D004272:DNA, Mitochondrial / Q000235:genetics* / Q000378:metabolism; D005800:Genes, Fungal; D005804:Genes, Lethal / Q000235:genetics*; D008969:Molecular Sequence Data; D025282:Mycelium / Q000235:genetics / Q000378:metabolism; D009492:Neurospora crassa / Q000235:genetics*; D010641:Phenotype; D011995:Recombination, Genetic; D015183:Restriction Mapping; D017384:Sequence Deletion
chemical_list: D045566:DNA, Cruciform; D004271:DNA, Fungal; D004272:DNA, Mitochondrial