To study if the reduction in growth rate seen using the ysxC conditional lethal strain LC109 (SH1000 Pspac~ysxC/pGL485) correlated with a concomitant depletion of YsxC, protein BV-6 cost levels after growth without IPTG were analysed. As indicated above, cells showed a severe growth defect when IPTG was lacking, thus

limiting the yield for biochemical analysis. To overcome this, a higher initial inoculum (OD600 = 0.01) was used and cultures were grown with choramphenicol and IPTG (with 500 μM or without). At this inoculum density, without IPTG the growth rate of LC109 (SH1000 Pspac~ysxC/pGL485) was still approximately 1 log below that of SH1000 after 5 hours of growth (data not shown). Equal amounts of material purified by ultracentrifugation were analysed by SDS-PAGE (data not shown) and Western blotting, probing with anti-YsxC polyclonal antibody

(See Methods; Figure 2C). In SH1000 there is a major YsxC cross-reactive band of ~26 kD and a minor band of ~25 kD, corresponding to a size similar to the predicted molecular weight, i.e., 23 kD. Both bands show lower intensity in LC109 (SH1000 Pspac~ysxC/pGL485) grown without IPTG. Hence, ysxC downregulation is accompanied by a decrease in YsxC concentration in the cell. Purification of YsxC interacting partners One method used to elucidate the function of a protein of interest buy SRT2104 is to search for protein

partners with which it interacts in the cell. In order to identify proteins interacting with YsxC, the protein was TAP-tagged [strain LC103 (SH1000 spa::tet ysxC::TAP)] and an interactive complex purified as described in Materials and Methods. The resulting proteins were separated by SDS PAGE and silver stained (Figure 3). 16 distinctive protein bands found in the eluted YsxC complex were trypsin digested and the amino acid sequence of the resulting fragments determined by Niclosamide mass spectrometry. Subsequently, a MASCOT search for proteins in the database containing these sequences was carried out. Table 1 shows the most probable identity of each of the bands as per its Mowse score. 10 of the 16 bands were identified as proteins from S. aureus, one band was not identified, and four of them (check details casein and keratin) corresponded to preparation contaminants. Figure 3 Identification of YsxC interacting proteins. Proteins were separated on a 4-12% (w/v) SDS-PAGE gradient gel and silver stained. Lane: 1, molecular mass markers of sizes shown; 2, YsxC complex proteins from 15 l of original culture. The band numbers correspond to those that were analysed by mass spectrometry. Table 1 MASCOT search results for YsxC partners Band no. Gene name Protein Mowse score (threshold level) * No.