Influence of elsamicin A on the activity of mammalian topoisomerase I

The strong effect of elsamicin A on the mobility of DNA in agarose gels has been characterized. This antibiotic forms tight complexes that are resistant to an electrophoretic field, though they are not covalent and can be removed by phenol or 1-butanol extraction. In the presence of mammalian topoisomerase I, elsamicin A behaves as an intercalating agent in unwinding experiments performed with either phi X174 rf I (double-stranded, covalently closed DNA) or relaxed pUC19. The unwinding assay was used to calculate the apparent unwinding angle per bound antibiotic molecule, phi = 19 +/- 2.7 degrees. Moreover, an apparent binding constant for elsamicin was derived, under the experimental conditions of the topoisomerase I assays, using the Scatchard equation. The effects of elsamicin A on the mammalian topoisomerase I catalytic cycle do not seem to involve inhibition of the enzyme. Neither symptoms of trapping of covalent DNA-topoisomerase I cleavable complexes nor "nonspecific" inhibition, based solely on DNA binding, was apparent. Utilizing an experimental approach based on the use of relaxed plasmid DNA, we suggest that elsamicin might not be a topoisomerase I inhibitor.