In order to determine whether the phosphorylated SarA protein could bind to these promoters with the same affinity, DNA fragments containing the Prot, PfnbA, agr P2 and sarA P1 promoter region were amplified by PCR using S. aureus chromosomal DNA as a template. The primers used in these assays are DNA Damage inhibitor listed in Table 2. Before PCR, the forward primers were end-labeled with [γ-32P]ATP and T4 polynucleotide kinase (Promega), and were purified by ProbeQuant G-50 columns (GE Healthcare). The labeled fragments (0.3 ng/5000 c.p.m.) were incubated at room temperature for 20 min with varying amounts of purified SarA protein, in 20 μL binding buffer containing 10 mM Tris-HCl,

pH 7.5, 0.1 mM EDTA, 50 mM NaCl, 1 mM DTT, 5% w/v glycerol and 1 μg calf thymus DNA (Sigma Aldrich). When needed, SarA was incubated with

either Stk1 or SA0077, as described above. SarA was then diluted twice in the assay. Controls were performed using Stk1-K39A and SA0077-K152A mutants, both unable to phosphorylate any substrate. Samples were analyzed on 6% polyacrylamide gels in 0.5 × Tris–borate–EDTA buffer. After electrophoresis, gels were dried and autoradiographed. Special attention was paid to Idelalisib in vivo the staphylococcal accessory regulator SarA because, first, it is known to regulate the expression of >100 virulence factors in S. aureus (Chien et al., 1999) and, second, its activity had been previously proposed to be controlled by post-translational modification, although no experimental support was provided for this hypothesis (Blevins et al., 1999; Wolz et al., 2000; Schumacher et al., 2001; Bronner et al., 2004). To detect post-translational modification

of SarA, this protein was first overproduced in an RN4220 strain carrying the plasmid pMK4-sarA. The total protein extracts prepared from bacteria were subjected to one-dimensional separation (Laemmli, 1970). After migration and Coomassie blue staining, the presence of a band at 16 kDa was detected (not shown). The analysis by MS showed that this Urocanase band corresponded to protein SarA. Then, proteins from the parental strain and from the parental strain carrying either pMK4 or pMK4-sarA were labeled in vivo with [32P]-orthophosphate for 2 h at 37 °C in the exponential phase on a minimum medium described previously (Toledo-Arana et al., 2005). Interestingly, we could detect on the autoradiography of Fig. 1 the presence of a 16-kDa band in the strain carrying pMK4-sarA, which was absent in the parental strain and in the parental strain containing the blank vector pMK4, thus showing that the virulence regulator SarA was phosphorylated in vivo. In order to assay SarA for phosphorylation, it was first necessary to overproduce and purify this protein. For this purpose, the sarA gene was prepared by PCR and cloned in plasmid pET15b. The resulting construct, pET15b-sarA, was used to transform competent E. coli cells.