Refined structure of the histidine-containing ­phosphotransfer (HPt) domain of the anaerobic sensor kinase ArcB from Escherichia coli at 1.57 Å resolution

The crystal structure of the histidine-containing phosphotransfer (HPt) domain of the anaerobic sensor kinase ArcB from Escherichia coli has been refined to 1.57 Å resolution, using the coordinates of the earlier 2.06 Å structure as a starting model. The final model contained 956 protein atoms, one zinc ion and 156 water molecules, with an R factor of 19.0%. The high-resolution electron-density maps clearly revealed additional solvent molecules and seven discrete rotamers in the protein side chains. One residue, Met755, was fully buried but was able to occupy the space in the hydrophobic core by means of the two-state conformation of its side chain. One water molecule was buried in the protein core and contributed to the rigidity of the HPt domain, cooperating in the coordination of the zinc ion.