Cytochrome c is not essential for viability of the fungus Aspergillus nidulans

Abstract.

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.