The lexA and recA genes were amplified by PCR from the chromosomal DNA using specific primers (DinR_U 5′-GCGCGGATCCAGTGATGTTATGTATTTAGATC-3′ – DinR_D 5′-CGCACGCGTCTATTTAATAACTCTAAATAC-3′) and (RecA_U 5′-GCGCGGATCCAGTGTAGATCAAGAAAAATTAAAAG-3′ – RecA_D 5′-CGCACGCGTTTATTCTTCTACAATTTCTTTTG-3′), respectively. The PCR products were then purified and cut with BamHI and MluI and cloned into pET8c vector digested by the same enzyme to create plasmids pDinRCD and pRecACD for expression of Tanespimycin cell line proteins fusion with N-terminal His6 STI571 mouse tag. Large-scale expression of proteins was performed in the E. coli BL21 (DE3) strain and purified from the bacterial cytoplasm by Ni-NTA affinity chromatography

as described for the E. coli key SOS proteins [25]. PD10 desalting columns (GE Healthcare) were used for exchange of the buffer. The proteins were stored at −80°C in 20 mM NaH2PO4 (pH 7.4),

0.2 mM NaCl. CH5183284 Protein concentrations were determined using NanoDrop1000 (Thermo Scientific) and extinction coefficients at 280 nm of 7450 M−1 cm−1 for recombinant LexA and 16055 M−1 cm−1 for recombinant RecA. Surface plasmon resonance assays C. difficile LexA-operator measurements were performed on a Biacore T100 (GE Healthcare) at 25°C as described [6]. The 3′-biotynilated 5-CGCTCGAGTAGTAAC-TEG-Bio-3′primer was immobilized on the flow cell 2 (Fc2) of the streptavidin sensor chip (GE Healthcare) in SPR buffer containing 20 mM Tris–HCl (pH 7.4), 140 mM NaCl, 0.005% surfactant P20 (GE Healthcare). To prepare double stranded

DNA (dsDNA) fragments with the predicted C. difficile LexA operators, complementary pairs of primers presented in Additional file 3: Table S2 were dissolved in 20 mM NaH2PO4 (pH 7.4), 0.14 M NaCl and mixed in 1:1.5 (mol : mol) ratio for the longer to shorter primer, respectively. Primers were annealed in temperature gradient from 95°C to 4°C (~ 1.5 h) in PCR machine (Eppendorf). So prepared DNA fragments were approximately 22 bp duplex DNAs with 15-nucleotide overhangs complementary to the chip-immobilized primer. Approximately 44 response units of either DNA fragment were hybridised Morin Hydrate at 2 μl min−1 to the Fc2. The interaction of C. difficile LexA with the chip-immobilized DNAs was analysed by injecting repressor in SPR buffer in 20 nM concentration across the chip surface at 100 μl min−1 for a minute and dissociation was followed for 9 minutes. The regeneration of the surface was achieved injecting 12 s pulse of 50 mM NaOH at 100 μl min−1. The experiments were performed in triplicates and the representative sensorgrams are shown. Data were fitted to a 1:1 binding model to obtain the dissociation rates constants. Program MEME was used to determine LexA binding motifs [33]. SPR C. difficile RecA*-LexA interaction measurements were performed on a Biacore X (GE Healthcare) at 25°C as described to study the interaction among the key E. coli SOS proteins [25]. Experiments were performed in SPR_2 buffer (20 mM NaH2PO4 (pH 7.