SirR, a metal-dependent transcriptional repressor from Mycobacterium tuberculosis (Rv2788), was cloned in pQE30 expression vector with an N-terminal His(6) tag for heterologous overexpression in Escherichia coli M15 (pREP4) cells and purified to homogeneity using chromatographic procedures. The purified protein was crystallized using the sitting-drop vapour-diffusion technique. The crystals belonged to the tetragonal space group P4(1)2(1)2/P4(3)2(1)2, with unit-cell parameters a = 105.21, b = 105.21, c = 144.85 A. The X-ray diffraction data were processed to a maximum resolution of 2.5 A. The Matthews coefficient suggests the presence of two (V(M) = 4.01 A(3) Da(-1)) to four (V(M) = 2.0 A(3) Da(-1)) molecules in the asymmetric unit. Calculation of the self-rotation function shows a crystallographic fourfold symmetry axis along the z axis (chi = 90 degrees) and also a twofold symmetry axis around the z axis (chi = 180 degrees).