title: Characterization of a potent catenation activity of HeLa cell nuclei.
abstract: Using an assay which measures catenation of a supercoiled DNA template, we have characterized and quantitated a potent activity identified in crude fractions of HeLa cell nuclei. Catenation requires Mg-ATP and a DNA-condensing agent, polyvinyl alcohol. A filter-binding or agarose gel assay can be used to quantitate activity. In this reaction, DNA topoisomerase I relaxes the input supercoiled DNA to provide DNA topoisomerase II, a strongly favored template for catenation. DNA topoisomerase II preferentially catenates relaxed DNA over supercoiled DNA by a factor of 100. One molecule of DNA topoisomerase II is able to catenate about 20 circles of relaxed DNA/min at 30 degrees C but only 0.16 circle of supercoiled DNA/min at 30 degrees C. The purified HeLa topoisomerase I can also catenate DNA under these assay conditions, yet in an ATP-independent fashion. It is much less efficient than topoisomerase II; one molecule of topoisomerase I catenates only about 3.8 X 10(-3) molecules of supercoiled DNA/min at 30 degrees C with a DNA template containing 5% nicked circles. This remarkable difference between the two enzymes allows quantitation of DNA topoisomerase II activity seen in the presence of excess topoisomerase I. Unlike Escherichia coli topoisomerase I (omega), catenation by the HeLa topoisomerase I is not stimulated by gapped circles.
mesh_terms: D002467:Cell Nucleus / Q000201:enzymology*; D004264:DNA Topoisomerases, Type I / Q000378:metabolism*; D004250:DNA Topoisomerases, Type II / Q000378:metabolism*; D004273:DNA, Neoplasm / Q000302:isolation & purification* / Q000378:metabolism; D004926:Escherichia coli / Q000201:enzymology; D006367:HeLa Cells / Q000201:enzymology; D006801:Humans; D007700:Kinetics; D046911:Macromolecular Substances
chemical_list: D004273:DNA, Neoplasm; D046911:Macromolecular Substances; D004264:DNA Topoisomerases, Type I; D004250:DNA Topoisomerases, Type II