Crystallization and preliminary X-ray diffraction analysis of biotin acetyl-CoA carboxylase ligase (BirA) from Mycobacterium tuberculosis

The gene encoding biotin acetyl-CoA carboxylase ligase (BirA) from Myco­bacterium tuberculosis was cloned and expressed in Escherichia coli with a C-­terminal Strep-tag. PEG 4000 as well as PEG 8000 were used as precipitants at pH 7.5 to crystallize the protein using the vapour-diffusion technique. X-ray characterization of crystals at room temperature indicated that the crystals belonged to the orthorhombic space group P2₁2₁2₁, with unit-cell parameters a = 79.7, b = 62.8, c = 105.8 Å. Assuming the presence of two BirA molecules in the asymmetric unit, the solvent content of the crystals was 44% (V_M = 2.2 Å³ Da^-1). When transferred to a cryoprotectant, crystals grown in the same drop exhibited a difference in one unit-cell parameter, with a = 60.1, b = 64.0, c = 103.6 Å, but belonged to the same P2₁2₁2₁ space group. These crystals, with two molecules of BirA present per asymmetric unit, appeared to have a very low solvent content of 28% (V_M = 1.7 Å³ Da^-1).