Solution structure and biophysical properties of MqsA, a Zn-containing antitoxin from Escherichia coli

The gene ygiT (mqsA) of Escherichia coli encodes MqsA, the antitoxin of the motility quorum sensing regulator (MqsR). Both proteins are considered to form a DNA binding complex and to be involved in the formation of biofilms and persisters. We have determined the three-dimensional solution structure of MqsA by high-resolution NMR. The protein comprises a well-defined N-terminal domain with a Zn finger motif usually found in eukaryotes, and a defined C-terminal domain with a typical prokaryotic DNA binding helix-turn-helix motif. The two well-defined domains of MqsA have almost identical structure in solution and in the two published crystal structures of dimeric MqsA bound to either MqsR or DNA. However, the connection of the two domains with a flexible linker yields a large variety of possible conformations in solution, which is not reflected in the crystal structures. MqsA binds Zn with all four cysteines, a stoichiometry of 1:1 and a femtomolar affinity (K(a)≥ 10¹⁷M⁻¹ at 23°C, pH 7.0).