Genetically determined conidial longevity is positively correlated with superoxide dismutase, catalase, glutathione peroxidase, cytochrome c peroxidase, and ascorbate free radical reductase activities in Neurospora crassa

Abstract

Aging of post-mitotic cells, the conidia, of Neurospora crassa is defined as the time-dependent loss of viability under a constant laboratory environment which probably resembles the organism's tropical habitat; namely, at 30°C, 85–100% relative humidity under white light. Median lifespan is defined as the age at which survival of a conidial population has declined to 50% of that of a fully viable population at birth. A collection of short ( age −) and long-lived ( age +) mutants were previously selected from the wild-type whose median lifespan is 22 days. Thus, five groups of strains with distinct lifespans of 7, 22, 36, 50 and 60 days were defined. The purposes of the present investigation were to determine if the activities of anti-oxygenic enzymes are correlated with lifespan and to elucidate the function of the cellular longevity determinant genes. The activities of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPX) were highly-correlated with lifespan; whereas glutathione reductase and non-specific peroxidase activities were not correlated. The short-lived mutants were also deficient in cytochrome c peroxidase (CPX) and ascorbate free radical reductase (AFR), but not deficient in dehydroascorbate reductase. (These latter three enzymes were not examined in age + mutants.) By isoelectric focusing analysis, the deficiencies of SOD, CAT, and GPX activities of age − mutants were defined in terms of specific isozymes. The mutants were specifically deficient in a cyanide-resistant mitochondrial isozyme of SOD. Sixteen age − gens, called the age-1 complex, were previously mapped on one arm of the seven chromosomes. On the basis of mapping and complementation data, it was inferred that the genes are spatially and functionally redundant. The hypothesis of functional redundancy is also supported by the enzyme data. Of seven mutants examined, representing seven of the age - genes, all were deficient in SOD, CAT and CPX, and six were deficient in AFR . Of four mutants examined, representing four of the genes, all were deficient in GPX. The results indicate a molecular basis for the previously observed photosentivity of the mutants. Deficiency of the antioxygenic enzymes may render the cells more susceptible to lethal macromolecular damage by photochemically generated free radicals and peroxides. A model is proposed in which age - and age + mutations occur at genes whose function is the regulation of the synthesis of the five antioxygenic enzymes. Thus, age - and age +mutations, respectively, repress and depress the amount of synthesis relative to the constitutive level of wild-type age °. Other observations pertaining to the biochemical and developmental regulation of these in Neurospora are discussed. We speculate that the large number of genes in the age-1 complex may provide a genetic mechanism for the amplication of regulatory signals (proteins?) in response to an increase in flux of free radicals and peroxides.