Neurospora mutants sensitive both to mutagens and to histidine

Previous work in this and other laboratories showed that histidine strongly inhibits growth of mutants at ten out of 20 known mutagen-sensitivity loci in Neurospora, and that nine of the histidine-sensitive mutants disturb meiosis when homozygous. These and other results suggested that histidine affects recombination or DNA repair. Current work with the histidine-sensitive mutant uvs-6 shows that it is also inhibited by several other metabolites but none of them is as effective as histidine. On minimal medium without histidine or other inhibitors, uvs-6 first grows normally, then slows drastically and begins stop-start growth. Conidia from stop-start uvs-6 mycelia produce rejuvenated cultures. The stop-start growth, UV-sensitivity, histidine-sensitivity, and recessive meiotic characters of uvs-6 segregated together in crosses, and reverted together. In tests on other mutagen-sensitive mutants, sensitivity to histidine was strongly correlated with stop-start growth and with sensitivity to other metabolites. Histidine induces premature stop-start growth in at least two mutants. Several possible explanations for the histidine-sensitivity have been excluded.