Epistatic Grouping of Repair-Deficient Mutants in Neurospora: Comparative Analysis of Two uvs-3 Alleles, uvs-6 and Their mus Double Mutant Strains.

The nuclease halo mutant, nuh-4, of Neurospora crassa was identified conclusively as an allele of uvs-3, a gene involved in error-prone DNA repair. Like uvs-3, nuh-4 showed spontaneous mutator effects, and any previous contradictory findings were found to be due to newly arisen mutants. In normal strains the two alleles are noncomplementing and indistinguishable for sensitivity to UV and methyl methanesulfonate (MMS). Like uvs-3, nuh-4 lacked secretion of the extracellular enzyme, DNase A, a Ca(++)-dependent strand-nonspecific endonuclease which was found to be phosphate repressible. However, nuh-4 differed from uvs-3 in showing much higher conidial viability and lower sensitivity to ionizing radiation and mitomycin C.--Epistatic relationships of the two uvs-3 alleles with seven other MMS-sensitive mutants were determined and compared with those of the highly X-ray-sensitive mutant, uvs-6. Three epistatic groups were found, based on survival of double mutant strains relative to that of their component single mutant strains after treatment with MMS. Both, uvs-3 and nuh-4, were epistatic to mus-9 which also is a mutator. None of the three produced viable double mutants in crosses to uvs-6. On the other hand, uvs-6, but not the uvs-3 alleles, was found to be epistatic to mus-7 and mus-10. The excision-defective uvs-2 and mus-8 both showed synergism with the uvs-3 alleles and with uvs-6, forming a third, separate epistatic group.