Replicative helicases can translocate through abasic site-induced covalenttopoisomerase IV-DNA complexes

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 a n endogenous topoisomerase poison. We have investigated the effects of abas ic (AP) sites on topoisomerase IV (Topo IV). AP sites can stimulate the for mation of covalent Topo IV-DNA complexes when they are located either withi n the 4 base overhang generated by DNA scission or immediately 5' to the po int of scission (the -1 position). Thus, the AP site acts as a position-spe cific, endogenous topoisomerase poison. Both EDTA and salt can reverse cova lent Topo IV-DNA complexes induced by AP sites located within the 4 base ov erhang. 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 sit e-induced covalent Topo IV-DNA complexes do not inhibit the helicase activi ties 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 progress ion.