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Regions of E. coli topoisomerase I structure important for the control of the DNA cleavage-religation equilibrium. The active site nucleophile Tyr-319 and adjacent Met-320 residues in subdomain III are shown along with the TOPRIM residues Asp-111, Asp-113 and Gly-116 in subdomain I in the structure (Protein Data Bank number 1ECL) of the 67 kDa N-terminal fragment of E. coli topoisomerase I (41).
