The 85 kDa band was recognized by an antibody to the strep-tag epitope (Figure 8B), that is present at the C-terminus of Pph. The 85 kDa band was also recognized by the antibody to Rc-CheW (Figure 8C), suggesting that this band contains a Pph
dimer and Rc-CheW protein. The 60 kDa band represents a non-identified protein that bound to the immobilized Pph. In conclusion, a stable complex of Pph and CheW can ERK inhibitor be isolated from R. centenaria cells confirming our in vitro findings. Figure 8 Protein complexes containing Pph isolated from R. centenaria . The Pph protein C-terminally fused to a strep-tag was expressed in R. centenaria and bound to a streptactin-Sepharose MI-503 nmr column. The elution fractions were analyzed by SDS-PAGE, silver staining (A) and Western blot with antibodies to strep-tag II (B) or to Rc-CheW (C), respectively. The crude protein extract (lanes 1 and 4), the last washing step (lanes 2 and 5) as well as the elution step (lanes 3 and 6) are shown. The positions of molecular weight markers are indicated. Discussion Since photosynthetic bacteria have to locate their habitat with optimal light conditions, specialized sensor systems and signal transduction cascades
involving different chromophores arose during evolution (for review see [39]). The blue light sensitive Ppr protein of R. centenaria consists of three distinct domains, the Pyp domain containing a cinnamic
acid chromophore, the phytochrome-like bilin binding domain and the histidine kinase domain Pph (Figure Resveratrol 1; [22]). The structural organization suggests that the protein is involved in a light-dependent signaling pathway similar to chemotaxis. Since R. centenaria exhibits a strikingly obvious phototactic behavior it is compelling to assume that the Ppr protein is involved in this reaction. Light with a wavelength of above 650 nm is attractive, whereas light with less than 650 nm acts as a repellent [10]. The absorption maximum of a prototypical cinnamic acid chromophore in a Pyp light sensor is at about 450 nm [40], whereas the phytochrome-linked biliverdin absorbs red light, suggesting that the latter could function as an attractant sensor. Recently, Cusanovich and co-workers showed that the holo-Ppr of R. centenaria has absorption maxima at 425 nm (Pyp), 400, 642 and 701 nm (phytochrome) [36] corresponding to the typical absorption spectrum of Pyp [40] and phytochromes [41]. The phytochromes TaxD1, Cph2 and PlpA were found to be involved in the phototactic reaction of Synechocystis sp. PCC 6803, a finding that supports the idea of a participation of the Ppr sensor in the phototactic response of R. centenaria [42, 43]. The data presented here show that the histidine kinase Pph domain of the Ppr receptor is found in a complex with Rc-CheW when isolated from R. centenaria (Figure 8).
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