A mutation in DNA polymerase γ harbors a shortened lifespan and high sensitivity to mutagens in the filamentous fungus <i>Neurospora crassa</i>

Hyphal elongation is the vegetative growth of filamentous fungi, and many species continuously elongate their hyphal tips over long periods. The details of the mechanisms for maintaining continuous growth are not yet clear. A novel short lifespan mutant of N. crassa that ceases hyphal elongation early was screened and analyzed to better understand the mechanisms for maintaining hyphal elongation in filamentous fungi. The mutant strain also exhibited high sensitivity to mutagens such as hydroxyurea and ultraviolet radiation. Based on these observations, we named the novel mutant "mutagen sensitive and short lifespan 1 (ms1)." The mutation responsible for the short lifespan and mutagen sensitivity in the ms1 strain was identified in DNA polymerase γ (mip-1:NCU00276). This mutation changed the amino acid at position 814 in the polymerase domain from leucine to arginine (MIP-1 L814R). A dosage analysis by next-generation sequencing reads suggested that mitochondrial DNA (mtDNA) sequences are decreased nonuniformly throughout the genome of the ms1 strain. This observation was confirmed by quantitative PCR for 3 representative loci and restriction fragment length polymorphisms in purified mtDNA. Direct repeat-mediated deletions, which had been reported previously, were not detected in the mitochondrial genome by our whole-genome sequencing analysis. These results imply the presence of novel mechanisms to induce the nonuniform decrease in the mitochondrial genome by DNA polymerase γ mutation. Some potential reasons for the nonuniform distribution of the mitochondrial genome are discussed in relation to the molecular functions of DNA polymerase γ.