The activity of a yeast mitochondrial endonuclease extracted from mutants (sas1) with increased sensitivity to petite-inducing treatments was compared to that from wild-type cells. The specificity of the endonuclease was altered in haploids carrying a single mutant nuclear gene that conferred increased sensitivity to petite induction by ultraviolet light, by growth at an elevated temperature and by growth in the presence of aminopterin and sulfanilamide. At high ionic strengths the endonuclease from the mutants digested double stranded DNA much faster than did that from the wild-type strain. Also the mutant enzyme was less selective for poly(dA) poly(dU) in comparison to poly(dA) · poly(dT); it had less preference for reduced hydrogen bond strength between the strands of double stranded DNA. Results indicate that this endonuclease, as a degrading enzyme, is involved in the initial step in repair of damaged mitochondrial DNA.
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