Topoisomerase I is in situ associated with DNaseI hypersensitive sites located in the promotor and terminator regions of the extrachromosomal rDNA in Tetrahymena ther5hila at sites with sequences fitting the motif AGACTTAGA0.AAATTT . Reconstitution experiments with purified topoisomerase I and cloned fragments of rDNA demonstrate that the enzyme exhibits the same binding and cleavage properties on naked DNA. These observations are striking as topoisomerase I previously has been found to exhibit low sequence specificity. The specific binding of the enzyme has an absolute requirement for divalent cations with a preference for Ca2+. The strong binding to the hexadecamer has been characterized by competition experiments, and it has been used to determine the molecular weight of the enzyme.
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