Figure 1.

Structures of E.coli topoisomerase I and topoisomerase III. (A) Active site of topoisomerase I–ssDNA complex. (B) Active site of topoisomerase III–ssDNA complex. Residues within 10 Å of the DNA substrate are shown; only residues discussed in the text are labeled. The structure of topoisomerase I [1MW8 (18)] and topoisomerase III [1I7D (17)] were superimposed using the backbone of comparable secondary structure elements and are shown from the same orientation. The ssDNA substrate is shown in orange. The insets show the structure of the entire enzyme; the active site and binding region is highlighted. The 2 nt at the 3′ end of the topoisomerase III DNA strand have been omitted for clarity.