title: DNA ligation catalyzed by human topoisomerase II alpha.
abstract: The DNA ligation reaction of topoisomerase II is essential for genomic integrity. However, it has been impossible to examine many fundamental aspects of this reaction because ligation assays historically required the enzyme to cleave a DNA substrate before sealing the nucleic acid break. Recently, a cleavage-independent DNA ligation assay was developed for human topoisomerase IIalpha [Bromberg, K. D., Hendricks, C., Burgin, A. B., and Osheroff, N. (2002) J. Biol. Chem. 277, 31201-31206]. This assay overcomes the requirement for DNA cleavage by monitoring the ability of the enzyme to ligate a nicked oligonucleotide in which the 5'-terminal phosphate at the nick has been activated by covalent attachment to the tyrosine mimic, p-nitrophenol. The cleavage-independent ligation assay was used to more fully characterize the DNA ligation activity of human topoisomerase IIalpha. Results suggest that the active site tyrosine contributes little to the catalysis of DNA ligation beyond its primary role as an activating/leaving group. Although arginine 804 (the residue immediately N-terminal to the active site tyrosine) has been proposed to help anchor the 5'-DNA terminus during cleavage, conversion of this residue to alanine had only a modest effect on DNA ligation. Thus, it appears that arginine 804 does not play an essential role in DNA strand joining. In contrast, disruption of base pairing at the 5'-DNA terminus abrogated DNA ligation in the absence of a covalent enzyme-DNA bond. Therefore, it is proposed that base pairing represents a secondary mechanism for aligning the 5'-DNA termini for ligation. Finally, the human enzyme appears to ligate the two scissile bonds of a cleavage site in a nonconcerted fashion.
authors.affiliation: Department of Biochemistry, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0146, USA.
authors.forename: Kenneth D
authors.initials: KD
authors.lastname: Bromberg
authors.forename: Renier
authors.initials: R
authors.lastname: Vélez-Cruz
authors.forename: Alex B
authors.initials: AB
authors.lastname: Burgin
authors.forename: Neil
authors.initials: N
authors.lastname: Osheroff
mesh_terms: D000951:Antigens, Neoplasm; D020029:Base Pairing; D002384:Catalysis; D004249:DNA Damage; D004260:DNA Repair; D004250:DNA Topoisomerases, Type II / Q000378:metabolism*; D004278:DNA, Superhelical / Q000378:metabolism; D004268:DNA-Binding Proteins; D006801:Humans; D006868:Hydrolysis; D009841:Oligonucleotides / Q000378:metabolism; D010957:Plasmids; D013379:Substrate Specificity
chemical_list: D000951:Antigens, Neoplasm; D004278:DNA, Superhelical; D004268:DNA-Binding Proteins; D009841:Oligonucleotides; D004250:DNA Topoisomerases, Type II