A MODEL FOR THE MECHANISM OF HUMAN TOPOISOMERASE-I

The three-dimensional structure of a 70-kilodalton amino terminally tr uncated form of human topoisomerase I in complex with a 22-base pair d uplex oligonucleotide, determined to a resolution of 2.8 angstroms, re veals all of the structural elements of the enzyme that contact DNA. T he linker region that connects the central core of the enzyme to the c arboxyl-terminal domain assumes a coiled-coil configuration and protru des away from the remainder of the enzyme, The positively charged DNA- proximal surface of the linker makes only a few contacts with the DNA downstream of the cleavage site. In combination with the crystal struc tures of the reconstituted human topoisomerase before and after DNA cl eavage, this information suggests which amino acid residues are involv ed in catalyzing phosphodiester bond breakage and religation. The stru ctures also lead to the proposal that the topoisomerization step occur s by a mechanism termed ''controlled rotation.''