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Bioengineering Seminar Series: Keir Neuman
Wednesday, September 5, 2007
11:00 a.m.
2108 Chemical and Nuclear Engineering Building
For More Information:
Professor Joonil Seog
jseog@umd.edu

Enzymatic symmetry breaking: How does Topoisomerase IV distinguish left from right?

Presented by Keir Neuman
Laboratory of Molecular Biophysics
National Heart, Lung, and Blood Institute
National Institutes of Health

Topoisomerases are essential enzymes that perform the crucial task of maintaining DNA topology and unlinking catenated or knotted DNA. Topoisomerase IV (Topo IV) is a bacterial topoisomerase that transports one segment of DNA through a transient double-strand break in a second segment of DNA. In contrast with homologous topoisomerases from higher organisms, Topo IV demonstrates remarkable selectivity in its strand passage activity. The plectonemes formed in over-wound DNA (positive supercoils) are relaxed ~20-fold faster by Topo IV than those formed in under-wound DNA (negative supercoils). How can an enzyme that acts locally distinguish the global topological state of DNA? What mechanisms underlie the asymmetric relaxation of positive versus negative supercoils by Topo IV? To address these questions, we developed a single-DNA crossing decatenation assay. This assay allows us to measure, in real time, individual strand passage events by Topo IV. Our results demonstrate that symmetry breaking by Topo IV is achieved through an unusual processivity mechanism. Furthermore, these results reconcile seemingly contradictory aspects of previous findings and resolve a longstanding paradox concerning the in vivo activity of Topo IV.

This Event is For: Graduate • Undergraduate • Post-Docs

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