g. EnvZ, KdpD and PhoR) identifies a predicted dimerization motif in the N-terminal part of Pph. The Pph sequence shows an identity of 27% and a similarity of about 57% compared to the dimerization domain of EnvZ (Figure 7A). To investigate whether the Pph protein can form a dimer in vitro, we performed gel filtration under non-reducing conditions. Crude soluble extracts of Pph expressing E. coli cells were

separated on a Sephadex G-200 column and analyzed by SDS-PAGE and Westernblotting. The Tigecycline cell line Pph protein eluted in fractions 43-46 (Figure 7B). The molecular weight of the Pph protein complexes was estimated by comparison with standard proteins on the same column. A majority of the Pph protein eluted at about 35 kDa (fraction 45) but a substantial amount was found as dimers at 70 kDa (fraction 43). A higher molecular weight form of Pph was found in fraction 22/23 above the exclusion limit of the column (200 kDa) and contains most likely higher aggregates which were also previously observed with Ppr [36, 37]. To verify the oligomeric states, fractions 43-46 were run on a non-reducing SDS-PAGE. Two protein bands with a molecular weight of about 35

JAK inhibitors in development and 70 kDa, respectively, were detected and analyzed by MALDI-TOF mass spectroscopy. The analysis clearly identified the Ppr photoreceptor (data not shown). Figure 7 Oligomeric state of the histidine kinase Pph. (A) Alignment of the dimerization domains of the Pph protein from R. centenaria and EnvZ from E. coli. The identity was 27% whereas the similarity was calculated with about 57%. The alignment was performed with the Clustal X software. (B) Purified Pph was analysed by gel filtration on a Sephadex G-200 column. Aliquots of the elution fractions (39-48) were separated by SDS-PAGE and blotted on a nitrocellulose membrane. The Pph protein was detected with a conjugate raised against the C-terminal StrepTag Interleukin-2 receptor II. The position of the Pph protein is indicated. The following proteins werde used as molecular weight markers: β-amylase (200 kDa), alcohol dehydrogenase (150 kDa), albumin (66 kDa), carboanhydrase (29 kDa) and cytochrome c (12 kDa) were used.

The Pph protein expressed in R. centenaria is found in a complex with Rc-CheW To test whether the Pph protein also assembles into a complex in R. centenaria cells, a plasmid containing an oxygen regulated puc promoter and an N-terminally his-tagged and C-terminally strep-tagged histidine kinase domain gene was constructed. This plasmid was transferred from E. coli RR28 [38] to R. centenaria by conjugation and the protein expression was induced by anaerobic growth conditions (see Experimental Procedures). The culture was continued at 42°C for 96 h and the Pph protein was purified using streptactin sepharose. The elution fractions were analyzed by SDS-PAGE, silver staining (Figure 8A) and Western blotting (Figure 8B). At the expected molecular weight of about 35 kDa no monomeric Pph protein was detectable (Figure 8A).