FIG. 4.

¹²⁵I-transferrin binding isotherms. Whole, iron-stressed gonococci were mixed with various amounts of ¹²⁵I-labeled human transferrin, and specific binding of the ligand was calculated by subtracting nonspecific binding (with excess competitor) from total binding. The amount of specific transferrin bound is reported on the y axis as nanograms of transferrin (Tf) bound per microgram of total cell protein (TCP). Isotherms are grouped for display according to the putative location of the epitope tag within TbpA: plug (A), extracellular loops (B), and periplasmic turns and β strands (C). Representative samples from each group are shown. The data shown represent the means and standard deviations generated from three independent binding experiments.