Cytochrome c is not essential for viability of the fungus Aspergillus nidulans.
The filamentous fungus Aspergillus nidulans is an obligate aerobe, which is capable of anaerobic survival, but not anaerobic growth. Since cytochrome c forms an essential part of the oxidative respiratory pathway it was expected that mutants lacking this component would be non-viable. Gene replacement of one homologue of the cycA (cytochrome c) gene was carried out in a diploid strain. Benomyl-induced haploidisation of this diploid yielded all cycA+ haploid colonies, initially suggesting that loss of cycA was indeed lethal. However, use of an alternative unbiased method to recover haploids yielded viable, but slow-growing, cycA- mutants. Replacement of the cycA locus in the cycA- mutants was verified by Southern blotting. Spectral analysis confirmed the absence of detectable levels of cytochrome c, and respiratory insensitivity to cyanide suggested the absence of cytochrome c-dependent respiration. Growth parameters were consistent with those expected of a CycA- mutant. Compared to the wild type, the mutants grew slowly on fermentable carbon sources, did not grow on non-fermentable carbon sources, and produced higher levels of ethanol. To our knowledge, this is the first report of a filamentous fungus that remains viable after complete elimination of a functional cytochrome c gene. We propose that the mutants are viable due to their ability to ferment and to use alternative respiratory pathways.
Bradshaw
R E
RE
Institute of Molecular BioSciences, Massey University, Palmerston North, New Zealand. r.e.bradshaw@massey.ac.nz
Bird
D M
DM
Brown
S
S
Gardiner
R E
RE
Hirst
P
P
D001233:Aspergillus nidulans / Q000201:enzymology / Q000235:genetics / Q000254:growth & development* / Q000378:metabolism; D003574:Cytochrome c Group / Q000378:metabolism*; D004171:Diploidy; D000431:Ethanol / Q000378:metabolism; D005285:Fermentation; D005800:Genes, Fungal; D009154:Mutation; D010100:Oxygen / Q000378:metabolism; D013172:Spores, Fungal
D003574:Cytochrome c Group; D000431:Ethanol; D010100:Oxygen