The mature SsTroA domain (residues 36-317) was PCRamplified from genomic DNA extracted from S. suis 05ZYH33. Primers, plasmids and bacterial strains used for gene cloning are all summarized in Table S4 . The PCR product was cloned into the BamHI/XhoI restriction sites of the pGEX-6P-1 expression vector (Amersham) to generate the pGEX-SsTroA plasmid. This construct was then transformed into E. coli BL21 (DE3). E. coli cells were grown at 37uC to mid-log-phase in LB broth supplemented with 100 mg/ml ampicillin, and then SsTroA expression was induced with 1 mM isopropyl-b-D-thiogalactopyranoside (IPTG) for 4 hours at 37uC. Cultures were harvested by centrifugation, and the cells were homogenized in PBS (140 mM NaCl, 2.7 mM KCl, 10 mM Na 2 HPO 4 , and 1.8 mM KH 2 PO 4 , pH 7.4) prior to sonication. The cell lysate was then centrifuged for 20 minutes at 35 000 g, the clarified supernatant was loaded onto a glutathione Sepharose 4B column (GE Healthcare) for 2 hours, and bound proteins were cleaved from the resin by incubating with PreScission Protease in accordance with the manufacturer's instruction for batch purification. The resulting supernatant was directly loaded onto a Superdex 200 column (GE Healthcare) equilibrated with 20 mM Tris-HCl (pH 8.0) and 50 mM NaCl. The purified protein was verified by SDS-PAGE and high purity fractions containing SsTroA were concentrated using Amicon Ultra 15 centrifugal filters (Millipore) with a 10-kDa molecular mass cut-off. Protein concentrations were determined by the BCA assay (Pierce) according to the manufacturer's protocol using bovine serum albumin as the standard. To produce uniformly 15 N-labeled SsTroA samples for NMR studies, E. coli BL21 (DE3) were grown in M9 minimal medium containing 0.1% 15 NH 4 Cl (Cambridge Isotope Laboratories) as the sole source of nitrogen. The other steps of protein expression and purification were the same as described above.