A EUKARYOTIC ENZYME THAT CAN DISJOIN DEAD-END COVALENT COMPLEXES BETWEEN DNA AND TYPE-I TOPOISOMERASES

The covalent joining of topoisomerases to DNA is normally a transient step in the reaction cycle of these important enzymes. However, under a variety of circumstances, the covalent complex is converted to a lon g-lived or dead-end product that can result in chromosome breakage and cell death. We have discovered and partially purified an enzyme that specifically cleaves the chemical bond that joins the active site tyro sine of topoisomerases to the 3' end of DNA. The reaction products mad e by the purified enzyme on a variety of model substrates indicate tha t the enzyme cleanly hydrolyzes the tyrosine-DNA phosphodiester linkag e, thereby liberating a DNA terminated with a 3' phosphate. The wide d istribution of this phosphodiesterase in eukaryotes and its specificit y for tyrosine linked to the 3' end but not the 5' end of DNA suggest that it plays a role in the repair of DNA trapped in complexes involvi ng eukaryotic topoisomerase I.