Escherichia coli nucleotide excision repair (NER) is responsible for removing 
bulky DNA adducts by dual incisions of the UvrABC endonuclease. Although the 
activity of the UvrAB complex which can induce DNA conformational change is 
employed in NER, the involvement of DNA topology and DNA topoisomerases remains 
unclear. We examined the effect of topoisomerase inhibitions on a NER in vivo 
system. The repair analysis of intracellular plasmid revealed that the DNA 
damage on positive supercoils generated by gyrase inhibition remained 
unrepaired, whereas the DNA damage was repaired in topoisomerase I mutants. 
These results suggest that DNA topology affects the NER process and the removal 
of positive supercoils by gyrase is vital for the efficiency of the E. coli NER 
system.