Some
topoisomerase inhibitors trap covalent topoisomerase-
DNA complexes as topoisomerase-
drug-
DNA ternary complexes. Ternary complex formation results in inhibition of DNA replication and generation of permanent double-strand breaks. Recent demonstrations of the stimulation of covalent topoisomerase-
DNA complex formation by DNA lesions suggest that DNA damage may act as an endogenous topoisomerase
poison. We have investigated the effects of abasic (AP) sites on
topoisomerase IV (
Topo IV). AP sites can stimulate the formation of covalent
Topo IV-
DNA complexes when they are located either within the 4 base overhang generated by
DNA scission or immediately 5' to the point of scission (the -1 position). Thus, the AP site acts as a position-specific, endogenous topoisomerase
poison. Both
EDTA and
salt can reverse covalent
Topo IV-
DNA complexes induced by AP sites located within the 4 base overhang. Interestingly, an AP site at the -1 position inhibits
EDTA-mediated reversal of formation of the covalent
Topo IV-
DNA complex. Furthermore, we find that, unlike
quinolone-induced covalent
Topo IV-
DNA complexes, AP site-induced covalent
Topo IV-
DNA complexes do not inhibit the helicase activities of the DnaB and T7 Gene 4
proteins. These results suggest that the AP site-induced
poisoning of
Topo IV does not arrest replication fork progression.