The absence of DNA photoreactivation enzyme in yeast mitochondria.

Mitochondria isolated from haploid yeast cells by spheroplast lysis were purified by flotation on renografin gradients. Electron micrographs and respiratory control ratios revealed that the purified mitochondria were still intact and functional. Assays for photoreactivation enzyme using as substrate [3H]-thymine-labeled Escherichia coli DNA were performed on crude and purified mitochondrial preparations. While the crude preparation contained high amounts of photoreactivation enzyme, it appeared to be associated with contaminating nuclei. The purified mitochondria lacked any photoreactivation enzyme activity. We suggest that yeast mitochondria do not normally contain photoreactivation enzyme.