SITE-SPECIFIC RIBONUCLEASE-ACTIVITY OF EUKARYOTIC DNA TOPOISOMERASE-I

Type I topoisomerases alter DNA topology by cleaving and rejoining one strand of duplex DNA through a covalent protein-DNA intermediate. Her e we show that vaccinia topoisomerase, a eukaryotic type IB enzyme, ca talyzes site-specific endoribonucleolytic cleavage of an RNA-containin g strand. The RNase reaction occurs via transesterification at the sci ssile ribonucleotide to form a covalent RNA-3'-phosphoryl-enzyme inter mediate, which is then attacked by the vicinal 2' OH of the ribose sug ar to yield a free 2', 3' cyclic phosphate product. Introduction of a single ribonucleoside at the scissile phosphate of an otherwise all-DN A substrate suffices to convert the topoisomerase into an endonuclease . Human topoisomerase I also has endoribonuclease activity. These find ings suggest potential roles for topoisomerases in RNA processing.