Crystallization and preliminary X-ray crystallographic studies on the parD-encoded protein Kid from Escherichia coli plasmid R1

DNA replication in Escherichia coli and therefore bacterial proliferation relies upon the efficient functioning of the DnaB helicase. The toxin protein Kid from the plasmid-stability system parD encoded on plasmid R1 of E. coli is thought to target and block DnaB-dependent DNA replication. The toxicity of Kid is antagonized through interaction with the Kis antidote protein and the resultant complex can then act as a transcriptional regulator for the parD system. Crystals of selenomethionine-incorporated Kid have been obtained by the hanging-drop vapour-diffusion method using potassium phosphate as the precipitant. The crystals belong to the monoclinic system, space group P21, have unit-cell parameters a = 32.9, b = 45.0, c = 64.4 Å, β = 96.2° and diffract to a dmin of better than 1.8 Å on a synchrotron-radiation source. The determination of the structure of Kid will permit a better understanding of its interactions with DnaB and Kis and allow the evolutionary relationships of Kid to other toxins of plasmid and chromosomal origin to be explored.