Schematic presentation of the domain structure of the sensor kinase KdpD and the KdpD-Usp chimeras investigated in this study. The model is based on hydropathy plot analysis, studies with lacZ/phoA fusions , and use of the conserved domain architecture retrieval tool (CDART) . KdpD contains the conserved domains of histidine kinases: HATPase_c (Histidine kinase-like ATPases; Histidine kinase-, DNA gyrase B-, phytochrome-like ATPases, SMART00387) and HisKA (His Kinase A phosphoacceptor domain; dimerization and phosphoacceptor domain of histidine kinases, SMART00388). Within the input domain, the location of the highly conserved KdpD domain (pfam02702, presented in grey) and the Usp domain USP-OKCHK (cd01987, pfam00582, highlighted by dots) are shown. Amino acids comprising the KdpD-Usp domain (red box) were replaced with the corresponding amino acid sequences of four homologous KdpD-Usp domains (yellow boxes) or with the soluble Usp proteins (green boxes) of E. coli. UspC is the native binding partner of KdpD; the replacement of KdpD-Usp with UspC is marked by a blue box. The first and last amino acid of the homologous KdpD-Usp domains as well as the number of replaced amino acids comprising the respective soluble Usp protein are indicated above the Usp-domains of the chimeras.