Crystal structure of activated ModE reveals conformational changes involving both oxyanion and DNA-binding domains.

ModE is a bacterial transcriptional regulator that orchestrates many aspects of molybdenum metabolism by binding to specific DNA sequences in a molybdate-dependent fashion. We present the crystal structure of Escherichia coli ModE in complex with molybdate, which was determined at 2.75A from a merohedrally twinned crystal (twin fraction approximately 0.30) with space group P4(3). We now have structures of ModE in both its "switched on" (ligand-bound) and "switched off" (apo) states. Comparison with the apo structure shows that ligand binding leads to extensive conformational changes not only in the molybdate-binding domain, but also in the DNA-binding domain. The most obvious difference is the loss of the pronounced asymmetry between the two chains of the ModE dimer, which had been a characteristic property of the apo structure. Another major change concerns the relative orientation of the two DNA-interacting winged helix-turn-helix motifs. Manual docking of an idealized DNA structure suggests that this conformational change should improve DNA binding of the activated molybdate-bound ModE.