Alteration of the cytochrome c oxidase subunit 2 gene in the [exn-5] mutant of Neurospora crassa.
The maternally inherited [exn-5] mutant of Neurospora crassa is characterized by its slow-growth rate and deficiency of cytochrome aa3 relative to wild-type strains. We have determined the DNA sequence of the COXI and COXII genes of the mutant, which encode subunits 1 and 2 of cytochrome c oxidase, respectively. No changes in the DNA sequence of the COXI gene relative to the corresponding wild-type gene were found. In the region of the COXII gene we found two alterations, one a C to T transition eight base pairs upstream of the coding sequence and the second within the coding sequence for subunit 2 affecting amino acid 27 of the precursor polypeptide (amino acid 15 of the mature polypeptide). The altered codon in [exn-5] specifies an isoleucine residue rather than the wild-type threonine residue. The corresponding position in subunit 2 sequences of all other organisms examined is conserved either as a threonine or a serine residue. Thus, we consider it likely that the mutation directly affecting the coding sequence of the polypeptide is responsible for the [exn-5] phenotype. Analysis of serially passaged heterokaryons constructed between wild-type and [exn-5] shows that both mutations segregate with the [exn-5] phenotype. Examination of mitochondrial translation products in [exn-5] revealed a deficiency of subunit 2, as well as the presence of a polypeptide that corresponds to a previously described precursor of subunit 1 that accumulates in a COXI mutant of N. crassa, [mi-3]. We propose possible relationships between [exn-5], [mi-3], and the nuclear su-1[mi-3] allele, which suppresses both mutations.
Lemire
E G
EG
Department of Genetics, University of Alberta, Edmonton, Canada.
Percy
J A
JA
Correia
J M
JM
Crowther
B M
BM
Nargang
F E
FE
D000595:Amino Acid Sequence; D001483:Base Sequence; D003001:Cloning, Molecular; D004271:DNA, Fungal; D003576:Electron Transport Complex IV / Q000235:genetics* / Q000378:metabolism; D005091:Exons; D008928:Mitochondria / Q000201:enzymology; D008969:Molecular Sequence Data; D009154:Mutation; D009492:Neurospora crassa / Q000201:enzymology / Q000235:genetics*; D014176:Protein Biosynthesis; D012689:Sequence Homology, Nucleic Acid; D013057:Spectrum Analysis
D004271:DNA, Fungal; D003576:Electron Transport Complex IV