Crystals of cytochrome c-553 from Bacillus pasteurii show diffraction to 0.97 A resolution

We report here the purification and characterization of a c-type cytochrome present in the soluble fraction of the gram-positive, alkaliphilic, and highly ureolytic soil bacterium Bacillus pasteurii. The cytochrome is acidic (pI = 3.3), has a molecular mass of 9.5 kDa, and appears to dimerize in 150 mM ionic strength solution. The electronic spectrum is typical of a low-spin hexa-coordinated heme iron. Crystals of the protein in the oxidized state were grown by vapor diffusion at pH 5, by using 3.2 M ammonium sulfate as precipitant. Diffraction data at ultrahigh resolution (0.97 A) and completeness (99.9%) have been collected under cryogenic conditions, by using synchrotron radiation. The crystals belong to the orthorhombic space group P2(1)2(1)2(1), with cell constants a = 37.14, b = 39.42, c = 44.02 A, and one protein monomer per asymmetric unit. Attempts to solve the crystal structure by ab initio methods are in progress.