Crystallization and preliminary X-ray crystallographic analysis of SMU.412c protein from the caries pathogen Streptococcus mutans

The smu.412c gene encodes a putative histidine triad-like protein (SMU.412c) with 139 residues that is involved in cell-cycle regulation in Streptococcus mutans. The gene was cloned into the expression vector pET28a and subsequently expressed in Escherichia coli strain BL21 (DE3) to give a substantially soluble form of SMU.412c with a His₆ tag at its N-terminus. The recombinant protein was purified to homogeneity in a two-step procedure involving Ni²⁺-chelating and size-exclusion chromatography. Crystals suitable for X-ray diffraction were obtained using the sitting-drop vapour-diffusion method and diffracted to 1.8 Å resolution on beamline BL6A at Photon Factory, Tsukuba, Japan. The crystal belonged to space group P4₁2₁2, with unit-cell parameters a = b = 53.5, c = 141.1 Å.