The strong effect of elsamicin A on the mobility of DNA in agarose gel
s has been characterized, This antibiotic forms tight complexes that a
re resistant to an electrophoretic field, though they are not covalent
and can be removed by phenol or I-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 (d
ouble-stranded, covalently closed DNA) or relaxed pUC19. The unwinding
assay was used to calculate the apparent unwinding angle per bound an
tibiotic molecule, phi = 19 +/- 2.7 degrees. Moreover, an apparent bin
ding constant for elsamicin was derived, under the experimental condit
ions of the topoisomerase I assays, using the Scatchard equation, The
effects of elsamicin A on the mammalian topoisomerase I catalytic cycl
e do not seem to involve inhibition of the enzyme. Neither symptoms of
trapping of covalent DNA-topoisomerase I cleavable complexes nor ''no
nspecific'' inhibition, based solely on DNA binding, was apparent. Uti
lizing an experimental approach based on the use of relaxed plasmid DN
A, we suggest that elsamicin might not be a topoisomerase I inhibitor.