The 3645-base pair human topoisomerase I complementary DNA (cDNA) clone isolated by D'Arpa et al. (Proc. Natl. Acad. Sci. USA, 85: 2543–2547, 1988) and a mutated version of the cDNA encoding a protein with phenylalanine instead of tyrosine at position 723 have been overexpressed 2- to 5-fold in stably transfected baby hamster kidney cells. The overexpressed proteins are the same size as the topoisomerase I present in Hela cells, indicating that the cDNA clone contains the complete topoisomerase I coding sequence. Some human colon carcinoma cells have increased levels of topoisomerase I and are hypersensitive to the drug camptothecin. The overexpressed wild-type topoisomerase I does not affect the cell growth or morphology of the baby hamster kidney cells, suggesting that elevated levels of topoisomerase I alone are not sufficient to cause cell transformation. However, the overexpressed wild-type protein is active, as shown by the hypersensitivity of clonal cell lines to camptothecin. The mutant form of topoisomerase I is enzymatically inactive by two criteria. First, extracts of Escherichia coli cells carrying the mutant cDNA contain no activity capable of relaxing superhelical DNA under conditions where activity is easily detectable in extracts from cells containing the wild-type cDNA. Second, baby hamster kidney cells stably transfected by the mutant cDNA are no more sensitive to camptothecin than control untransfected cells. These results indicate that tyrosine 723 is essential for enzyme activity and are consistent with predictions based on homology comparisons with the yeast enzymes, that this is the active-site tyrosine in the human topoisomerase I.

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Supported by Grant DMB-8917877 from the National Science Foundation.

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