title: Human DNA topoisomerase I: relaxation, roles, and damage control.
abstract: Human DNA topoisomerase I is an essential enzyme involved in resolving the torsional stress associated with DNA replication, transcription, and chromatin condensation. The catalytic cycle of the enzyme consists of DNA cleavage to form a covalent enzyme-DNA intermediate, DNA relaxation, and finally, re-ligation of the phosphate backbone to restore the continuity of the DNA. Structure/function studies have elucidated a flexible enzyme that relaxes DNA through coordinated, controlled movements of distinct enzyme domains. The cellular roles of topoisomerase I are apparent throughout the nucleus, but the concentration of processes acting on ribosomal DNA results in topoisomerase I accumulation in the nucleolus. Although the activity of topoisomerase I is required in these processes, the enzyme can also have a deleterious effect on cells. In the event that the final re-ligation step of the reaction cycle is prevented, the covalent topoisomerase I-DNA intermediate becomes a toxic DNA lesion that must be repaired. The complexities of the relaxation reaction, the cellular roles, and the pathways that must exist to repair topoisomerase I-mediated DNA damage highlight the importance of continued study of this essential enzyme.
authors.affiliation: Department of Microbiology, School of Medicine, University of Washington, P.O. Box 357242, 1959 N.E. Pacific St., Seattle, WA 98195-7242, USA.
authors.forename: John B
authors.initials: JB
authors.lastname: Leppard
authors.forename: James J
authors.initials: JJ
authors.lastname: Champoux
mesh_terms: D004249:DNA Damage; D004261:DNA Replication; D004264:DNA Topoisomerases, Type I / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008954:Models, Biological; D008958:Models, Molecular; D011487:Protein Conformation
chemical_list: D004264:DNA Topoisomerases, Type I