Kannan Balasubramanian, Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany. Electronic address: b.kannan@fkf.mpg.de.
A high sensitivity assay has been developed for the detection of human Topoisomerase I with single molecule resolution. The method uses magnetic sepharose beads to concentrate rolling circle products, produced by the amplification of DNA molecules circularized by Topoisomerase I and detectable at a confocal microscope as single and discrete dots, once reacted with fluorescent probes. Each dot, corresponding to a single cleavage-religation event mediated by the enzyme, can be counted due to its high signal/noise ratio allowing detection of 0.3 pM of enzyme and representing a valid method to detect the enzyme activity in highly diluted samples.