Abstract
Prominent DNA gyrase-mediated cleavage sites, induced by oxolinic acid, occur at specific, but infrequent, locations on the Escherichia coli chromosome. These sites, which we call toposites, may represent high affinity DNA gyrase binding sites or may mark chromosomal regions that accumulate superhelical stress. Toposites are usually grouped in 5 to 10 kb clusters that are mostly 50 to 100 kb apart. The total number of clusters on the chromosome is between 50 and 100. The location of sites depends on the local sequence. The extent of DNA gyrase cleavage at toposites can be strongly modulated by transcription occurring at as far as 35 kb away.