The six oligonucleotide probes used in EMSA experiments were purchased from MDBio Inc. (Taiwan) ( Figure 1A ). The viron wX174 and M13 ssDNA were purchased from New England Biolabd (USA). For the preparation of double-stranded IcaR DNA1, equimolar amounts (100 mM each) of complementary oligonucleotides were mixed, fully denatured by heating at 95uC for 5 min in 10 mM Tris-HCl pH 8.0, 20 mM NaCl and allowed to cool gradually to room temperature. Gel shift assays were performed by incubating 1 mM of ssDNA or dsDNA with 1-4 mM purified recombinant proteins under binding conditions (20 mM Tris-HCl, pH 8.0, 150 mM KCl, 0.1 mM MgCl 2 , 0.05 mM EDTA, 12.5% Glycerol, 0.1 mM DTT and 1 mg/ml BSA) for 15 min at room temperature with gentle vortex. After incubation, 15 ml of the reaction solution was mixed with 3 ml of the sample loading dye and subsequently electrophoresed on 6% preequilibrated polyacrylamide gels in 1/2 Tris/acetate/EDTA (TAE) at 100 V for 30 min and visualized using SYBR Green I nucleic acid gel stain (Invitrogen). For competition assay, IcaR DNA1 probe duplex of 1 mM was pre-incubated with 2 mM TcaR (dimer) at room temperature for 15 min, then mixed with increasing concentration of GC33 ssDNA. In the assay for analyzing the effect of different antibiotics on the interaction between TcaR and ssDNA, 1 mM GC33 probe was pre-incubated with 2 mM TcaR (dimer) at room temperature for 15 min before mixing with 2 mM or 20 mM antibiotics.