Figure 4.

Footprinting of three additional cysteine-substitution mutants of yeast DNA topoisomerase II. Peptide cleavage at S-cyanocysteines was carried out in the presence of excess NEM. The absence or presence of AMPPNP during cyanylation by NTCB is indicated by a minus or plus sign in the top margin. In the autoradiogram shown in a, lane 1 of the gel contained size markers obtained by cleavage of the HMK-tagged yeast DNA topoisomerase II and treatment of the cleavage products with NEM. The proteins analyzed in lanes 2–5 are specified in the upper margin. (b) The autoradiogram of a similar gel. The lane containing the size markers for the gel on the right was not shown, and the arrows in the left and right margins mark the positions of the radiolabeled bands corresponding to cleavage at Cys-141 in the R141C mutant and Cys-601 of the Y601C mutant, respectively.