Figure 2. (A) The amino-acid sequences and net charges of the three zinc-finger domains of Egr-1 and Sp1. (B) Electrostatic binding free energies for Egr-1’s three zinc finger domains (C) Inter-domain interfaces in the crystal structure of the Egr-1 / target DNA complex. Two hydrogen bonds involving a salt bridge stabilize inter-domain interactions between ZF2 and ZF3, whereas there is no corresponding stabilization between ZF1 and ZF2.