Abstract A strain with an elevated level of spontaneous mutations and an especially high rate of reversion at a specific locus ( pab − ) was identified. The mutator trait is recessive. UV sensitivity and the absence of a UV-specific endonucleolytic activity were associated with the enhancement of the mutation rate in mutator strains. The endonuclease associated with the regulation of the mutation rate also acted on single-stranded DNA. The molecular weight of this enzyme is about 38 000 daltons.