Intracellular DNA exists in a number of topological forms, some of which are very complexed and difficult to investigate.  However, to fully understand the functions of DNA and DNA transactions it is imperative to characterize these unusual DNA structures.  One of such examples for intracellular DNA structures is a novel form called hemicatenane.  A hemicatenane conjoins two DNA duplexes through a single strand interlock.  It has been proposed to be an important intermediate for DNA replication, repair, and recombination.  However, the biochemical analysis of hemicatenanes is hampered by the relative inaccessibility of such a structure.  We report in this paper that a DNA topoisomerase III from a hyperthermophilic archaeum bacterium can carry out synthesis and dissolution of hemicatenanes.  We also show that the complex formed by human topoisomerase 3, Bloom syndrome helicase and their accessory proteins Rmi1/Rmi2 has a biochemical function of reversing this hemicatenation.  In addition, our results also demonstrate that type IA topoisomerases can regulate the formation of hemicatenane structures.  This research will help further unravel the mystery of the relationship between DNA structure and its function.

　　This research was carried out by a research team under the direction of Professor Tao-shih Hsieh in ICOB and led by researchers including Shun-Hsiao Lee and Grace Ee Lu Siaw, first and second author of this paper.

　　Synthesis and dissolution of hemicatenanes by type IA DNA topoisomerases.

　　Shun-Hsiao Lee, Grace Ee-Lu Siaw, Smaranda Willcox, Jack D. Griffith, and Tao-Shih Hsieh.

　　PNAS 2013 ; published ahead of print September 3, 2013, doi:10.1073/pnas.1304103110

　　The title, the complete list of authors, and the weblink for the paper is: http://www.pnas.org/content/early/2013/08/28/1304103110.abstract