Crystallization and preliminary X-ray diffraction studies of the transcriptional regulator TraR bound to its cofactor and to a specific DNA sequence

TraR is an Agrobacterium tumefaciens transcriptional regulator which binds the pheromone N-3-oxooctanoyl-L-homoserine lactone (AAI) in response to the bacterial population density. The TraR-AAI complex dimerizes and interacts with a specific 18-base-pair DNA sequence (TraBox), activating promoters containing this site. TraR was overexpressed and purified from Escherichia coli. Crystals of the ternary complex, in which dimeric TraR-AAI is bound to the TraBox sequence, have been obtained by the vapour-diffusion method. The crystals belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 66.99, b = 94.67, c = 209.66 Angstrom, with two (TraR-AAI)(2)-TraBox complexes in the asymmetric unit. A three-wavelength MAD data set for the seleno-l-methionine-substituted form has been collected to a resolution of 3 Angstrom. 20 of the 24 crystallographically independent selenium sites were located as part of the MAD-phasing process.