5 mM desthiobiotin. The success of the purification

was verified by SDS-PAGE, silver staining and Western blot analysis with the antibodies raised against the his-tag or the strep-tagII, respectively. Determination of the dissociation constant of Pph and Rc-CheW by resonant mirror spectroscopy The Pph protein was purified from inclusion selleck chemicals bodies as described above and the aminosilane cuvette was activated as described by the manufacturer (Iasys, Biosensors). 200 μl of the purified Pph protein (50 μg/ml) was added to the activated cuvette and the immobilization was recorded for 30 minutes. The unbound protein was removed by extensive washing and increasing amounts of purified Rc-CheW (see above) were added. see more After 30 minutes

of incubation the free Rc-CheW was washed out and the amount of bound Rc-CheW was determined for each experiment. The fractional saturation was calculated and depicted against the amount of the added Rc-CheW concentration. The resulting Scatchard Plot is illustrated as the inlet of Figure 5. In vitro transcription and translation The histidine kinase domain Pph as well as Rc-CheAY were transcribed in vitro from the selleckchem plasmids pSK4 and pET28-CheAY, respectively, using a T7 transcription kit (Fermentas) according to the manufacturers manual. The translation reaction was performed as described previously [60] by using an E. coli based cell free expression system The proteins were labeled with 10 μCi of [35S]methionine (ICN) in each experiment. The high speed supernatant (S-135) was prepared as described from E. coli MRE600 [61]. Pull-down assays 50 μg of the purified his6-Rc-CheW protein was mixed with 25 μl of the in vitro translated Pph protein and 25 μl Rc-CheAY when indicated. The protein mixture was incubated overnight at 37°C.

Then, the his6-Rc-CheW protein was bound to a column containing 50 μl Sepharose 6b (GE Healthcare) aminophylline charged with Cu(II) ions and pre-equilibrated with buffer I (20 mM sodium phosphate pH 7.7, 200 mM NaCl, 50 mM imidazole pH 8.0). After 30 minutes at room temperature, the unbound proteins were removed by washing the column five times with 500 μl buffer I followed by an elution with 1.5 ml buffer II (20 mM sodium phosphate pH 7.7, 200 mM NaCl, 500 mM imidazole pH 8.0). All fractions were TCA precipitated and analyzed by SDS-PAGE. The gels were stained with coomassie brilliant blue and the radiolabeled bands were quantified using a Fuji BAS 1500 phosphorimager. Gelfiltration assay 1L terrific broth [62] in a Fernbach flask was inoculated with an overnight culture of E. coli C41 (DE3) harbouring pET16b-Pph. The cells were incubated at 18°C with gentle shaking for 48 hours. This procedure prevents the formation of inclusion bodies [36]. Then the cells were harvested by centrifugation and resuspended in 20 mM Tris pH 7.4, 40 mM NaCl, 20% glycerol.