4). AroS was readily phosphorylated, with the maximum incorporation of [γ-32P]ATP reached within 5 min as shown by the intensities of the bands in the auotoradiograph (Fig. 4a). Identification of the putative phosphoacceptor residue was carried out by site-directed selleck chemicals llc mutagenesis of the only two histidine residues present in the phosphotransfer domain (DHp): His273 and His292. While the autophosphorylation activity of the AroS226–490H292N mutant was unaffected compared with the wild-type protein (Fig. 4c, lanes 2 and 3, respectively), the AroS226–490H273N mutant protein was defective in autophosphorylation (Fig. 4c, lane 1). Similar protein concentrations were used in these experiments
as can be seen in Fig. 4b and d. Thus, we demonstrated that AroS exhibits sensor histidine selleck products kinase activity and that His273 is required for autophosphorylation most likely as the phosphoaccepting residue. 1D 1H NMR spectra of AroS226–490, AroS226–490H273N and AroS226–490H292N mutant proteins, recorded on a 1H frequency of 700 MHz on a Bruker Advance III spectrometer at 25 °C, were similar (see Supporting Information, Fig. S1), exhibiting characteristic features of a folded polypeptide, thus excluding
the possibility that the loss of autophosphorylation of AroS226–490H273N is due to protein missfolding. To address whether AroR is the cognate response regulator for AroS, an expression construct coding for the receiver domain of AroR (residues 1–125) was cloned and expressed in E. coli and recombinant protein AroR1–125 was purified. The transphosphorylation reaction was carried out such that AroS226–490 was first incubated with [γ-32P]ATP for 10 min to generate a population of phosphorylated AroS226–490 and then purified AroR1–125 was added to the reaction mixture. The transphosphorylation reaction of AroS226–490 with AroR1–125 was incubated at room temperature for 1 and 10 min. Figure 5 clearly shows the autophosphorylation of AroS226–490 and the subsequent transfer of the phosphate group to AroR1–125 (Fig.
5a, lanes 3 and 4). Phosphorylation of AroR1–125 is AroS-dependent as omission of AroS226–490 from the reaction mixture (Fig. 5a, lane 2 and c, lane 2) leads to no Ketotifen AroR phosphorylation – an expected observation, given that the receiver domains are unable to undergo ATP-dependent autophosphorylation. Direct phosphotransfer from AroS to AroR confirms that these two proteins are a cognate sensor response regulator pair. To determine which aspartate residue is involved in the phosphorelay mechanism, purified protein variants of AroR1–125 containing single mutations (D13N, D53N and D58N) were tested for their ability to undergo transphosphorylation. Figure 5b shows that both AroR1–125D13N and AroR1–125D53N mutants show a reduced phosphorylation level (Fig. 5b, lanes 3–6) compared with wild-type AroR1–125 (Fig.