Expression and purification of F-plasmid RepE and preliminary X-ray crystallographic study of its complex with operator DNA.

The replication initiator factor RepE of the F plasmid in Escherichia coli is an essential protein that stringently regulates the F-plasmid copy number. The RepE protein has a dual function: its monomer functions as a replication initiator, while its dimer acts as a transcriptional repressor of the repE gene. The wild-type dimeric RepE protein was expressed as an N-terminal histidine-tagged protein, purified under native conditions with a high salt concentration and crystallized in complex with the repE operator DNA using the sitting-drop vapour-diffusion technique. The crystals diffracted to a resolution of 3.14 A after the application of dehydration and crystal annealing and belong to space group P2(1), with unit-cell parameters a = 60.73, b = 99.32, c = 95.00 A, beta = 108.55 degrees.