Characterization of a camptothecin-resistant human DNA topoisomerase I in an in vitro system for Simian virus 40 DNA replication

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Abstract
DNA topoisomerase I was required for bidirectional DNA replication in an in vitro system for Simian virus 40 (SV40) DNA replication with purified proteins in which the replication fork moved at the rate of 260 nucleotides/min on average. DNA topoisomerase I purified from camptothecin‐resistant human lymphoblastoid cells, which confers high resistance of cellular DNA replication to camptothecin [Andoh, T., Ishii, K., Suzuki, Y., Ikegami, Y., Kusunoki, Y., Takemoto, Y. & Okada, K. (1987) Proc. Natl Acad. Sci. USA 84, 5565–5569], was characterized using this system. The activity of stimulating bidirectional DNA replication was comparable between two topoisomerase I from parental and resistant cells, i.e. in its dose‐response relationship and in its time course for DNA synthesis. Camptothecin severely inhibited the leading as well as the lagging strand synthesis in the reaction containing the wild type topoisomerase I but not the mutant type topoisomerase I. The mutant type topoisomerase I was over 125‐fold as resistant to camptothecin as the wild type topoisomerase I. These results are in good agreement with those on the sensitivity of cellular DNA synthesis to camptothecin in the resistant cells. These findings suggest that topoisomerase I is involved in cellular DNA replication as a swivelase and the mutation conffering camptothecin‐resistance on the enzyme does not affect its functional efficiency in this system.