Figure 7.

Strand passage mechanism of topoI. TopoI-mediated relaxation is proposed to occur by recognizing and binding to a single-stranded region within the supercoiled substrate (A and B), followed by nicking of the scissile strand. The enzyme undergoes conformational changes resulting in opening of the DNA gate (C). The complementary non-scissile strand held by the basic amino acid stretches in the CTD is passed through the opened DNA gate (D). Subsequently, two ends of the DNA are brought closer for religation (E) leading to the relaxation of supercoiled DNA. The enzyme releases the DNA or begins another catalytic cycle. Basic amino acid stretches are shown in cyan.