Practice question

AraC protein embodies remarkable dual regulatory function determined by arabinose-dependent conformational switch. Polypeptide of 292 residues contains N-terminal dimerization and arabinose-binding domain linked via flexible interdomain arm to C-terminal DNA-binding domain possessing two helix-turn-helix motifs. Apo form without arabinose favors interaction binding to distal O2 operator and I1 half-site forming DNA repression loop that blocks polymerase. Arabinose-bound form reorganizes N-terminal arm, neutralizes arm dimer interactions, shifts DNA-binding specificity to adjacent I1-I2 half-sites upstream of PBAD, and exposes activation surface capable of direct contacts with RNA polymerase alpha subunit, behaving simultaneously as both repressor and activator depending on ligand status.