Crystallization and preliminary X-ray crystallographic analysis of FlhD/FlhC complex from Escherichia coli

The heterotetrameric (C₂D₂) FlhD/FlhC complex was first discovered as a transcriptional activator of the flagellar genes in Escherichia coli. Recent studies now show that FlhD/FlhC also regulates several non-flagellar target genes in E. coli. The FlhD/FlhC complex also plays several important roles in other microorganisms. The molecular interactions between FlhD and FlhC, as well as the mechanisms by which the complex may vary its DNA-binding specificity, are not clear. Determination of the FlhD/FlhC crystal structure will provide insight into these protein-protein and protein-DNA interactions. The initial steps in this investigation are reported here: the overexpression, purification and crystallization of the FlhD/FlhC complex, the characterization of this crystal form and the recording and processing of an initial diffraction data set. The obtained crystal form of the FlhD/FlhC complex is hexagonal (space group P6₁, unit-cell parameters a = b = 150.5, c = 115.9 Å). The crystal density is very low (V_M = 5.5), with 81.7% of its volume occupied by solvent. A single C₂D₂ tetramer is present in the crystallographic asymmetric unit. A complete native data set has been collected to 4.5 Å resolution.