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

The crystal structure of the histidine-containing phosphotransfer (HPt) dom ain of the anaerobic sensor kinase ArcB from Escherichia coli has been refi ned to 1.57 Angstrom resolution, using the coordinates of the earlier 2.06 Angstrom structure as a starting model. The final model contained 956 prote in atoms, one zinc ion and 156 water molecules, with an R factor of 19.0%. The high-resolution electron-density maps clearly revealed additional solve nt molecules and seven discrete rotamers in the protein side chains. One re sidue, Met755, was fully buried but was able to occupy the space in the hyd rophobic 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 rigid ity of the HPt domain, cooperating in the coordination of the zinc ion.