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Nucleic Acids Research, 1978, Vol. 5, No. 10 3811-3820
© 1978

Articles

Escherichia coli DNA topoisomerase I catalyzed linking of single-stranded rings of complementary base sequences

Karla Kirkegaard and James C. Wang

Biochemistry and Molecular Biology Department, Harvard University Cambridge, MA 02138, USA

Received July 27, 1978. Eco DNA topoisomerase I (E. coli {omega} protein) has been observed to catalyze the formation of double-stranded, covalently closed DNA from complementary single-stranded DNA rings, a novel reaction which is topologically forbidden without the enzyme-catalyzed breakage and rejoining of DNA backbone bonds. Incubation of a mixture of single-stranded PM2 DNA rings of complementary base sequences with {omega} yields a species with a sedimentation coefficient in an alkaline medium characteristic of a covalently closed circular double-stranded DNA. Buoyant density measurements in CsC1 at alkaline pH also identify the product as a covalently closed duplex ring. If the {omega}-catalyzed reaction is stopped short of completion, highly negatively supercoiled molecules are formed which sediment more slowly in an alkaline medium than the final duplex product. As the reaction proceeds the mean sedimentation rate of the intermediates in creases. This is in agreement with the expectation that the linking number between the two complementary rings increases gradually during the course of the reaction from zero to that of a relaxed covalently closed circular DNA duplex. The possible role of DNA topoisomerases in genetic recombination is discussed.


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