Some properties of a mitochondrial endonuclease from yeast

A mitochondrial endonuclease from Saccharomyces cerevisiae was previously shown to cut both strands of native DNA at opposite or nearby sites. The present studied demonstrate that the endonuclease activity is dependent on the strength of the hydrogen bonds between the DNA strands; the activity was measured at different ionic strengths, with substrates of different base compositions and also with DNA in which the double helix has been locally destabilized by ultraviolet irradiation, by depurination, and by single-stranded nicks. The activity is 30% greater with mitochondrial DNA (mt-DNA) than with nuclear DNA. At 0.08 ionic strength, the relative activities with double-stranded polydeoxyribonucleotides are 2.4:1:0.6 for poly(dA).poly(dU) : poly(dA).poly(dT) : poly(dG). poly(dC). Increasing ionic strength decreases similarly the activity with poly(dA).poly(dU) and poly(dA).poly(dT), but has little effect with poly(dG).poly(dC). The greater activity with poly(dA).poly(dU) than with poly(dA).poly(dT) was confirmed with nick-translated mt-DNA and with DNA synthesized in isolated mitochondria using [3H]TTP and [3H]dUTP in both cases. The endonuclease cuts modified DNA preferentially in the thymine dimer regions, at the apurinic sites, and at sites opposite to nicks. The possible involvement of this endonuclease in the degradation of mitochondrial DNA during "petite" induction is discussed.