Evidence for a nucleotide-dependent topoisomerase activity from yeast mitochondria

Yeast mitochondria were found to contain a novel topoisomerase-like activity which required nucleoside di- or tri-phosphates as a cofactor. ADP supported activity as effectively as ATP and the optimal concentration for each was approximately 20 microM. None of the other standard ribo- or deoxyrib-onucleotides could fully substitute for either ADP or ATP. The non-hydrolyzable ATP analogs, adenosine-5'-0-(3-thiotriphosphate) (ATP-gamma-S), adenylyl (beta,gamma-methylene) (AMP-PCP), and andenyl-imidodiphosphate (AMP-PNP) also supported activity suggesting that the nucleotide cofactor regulated topoisomerase activity rather than serving as an energy donor in the reaction. The mitochondrial topoisomerase activity relaxed both positively and negatively supercoiled DNA. It was not inhibited by concentrations of ethidium bromide up to 2 micrograms/ml nor by either nalidixic or oxolinic acids; novobiocin, coumermycin, and berenil inhibited the activity. Genetic and biochemical analysis of the mitochondrial topoisomerase activity indicated that it was not encoded by the nuclear TOP1, TOP2, and TOP3 genes.