1 and 2). The Usp domain within KdpD (I253-P365) shares similarities to the Usp proteins of the UspA subfamily [18]. The KdpD-Usp domain binds the universal stress protein UspC [19]. It has been puzzling how KdpD is activated under salt stress when K+ accumulates [20], although the kinase activity is inhibited by K+ [21]. Recent

data indicate that UspC scaffolds the KdpD/KdpE signaling cascade under salt stress by stabilizing the KdpD/KdpE~P/DNA complex [19]. This is in accord with the earlier finding according to which cells producing a truncated KdpD lacking the Usp domain exhibit reduced kdpFABC expression under salt stress [15]. Figure 1 Sequence alignment of the N-terminal domain of KdpD (KdpD/1-395) comprising the Usp-domain, marked by the blue line. The alignment was created and identities/similarities were determined using VectorNTI AlignX. E.c. (Escherichia coli), S.e. (Salmonella enterica SB525334 purchase serotype Typhimurium), A.t. (Agrobacterium tumefaciens), P.a. (Pseudomonas aeruginosa), S.c. (Streptomyces Cyclosporin A clinical trial coelicolor). Figure 2 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 [7], and use of the conserved domain architecture retrieval tool (CDART) [26]. 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, Rolziracetam 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. The Usp superfamily encompasses an ancient and conserved group of proteins that are found in bacteria, archaea, fungi, flies, and plants (see [22] for review). Usp-containing organisms are usually equipped with several copies of usp genes. The usp genes encode either small Usp proteins (one Usp domain), larger versions with two Usp domains in tandem, or Usp domains integrated in multi-domain proteins [18]. E. coli contains six Usp proteins that can be divided into two subfamilies on the basis of sequence similarities [23].