Supercompetent DH5α cells used for cloning were from Bioline. Antibiotics were purchased from Sigma, fluorescent substrates from Molecular Probes, and dodecyl-β-d-maltoside (DDM) from Glycon. A mutation of phenylalanine residues 4 and 5 to alanine residues (FAFA click here mutation) was introduced in the mexB gene in the E. coli vectors pMexB and pMABO by PCR using Pfu DNA polymerase (Stratagene) and the forward primer 5′-ATGTCGAAGGCTGCCATTGATAGGCCCATTTTCGC-3′ and reverse primer 5′-CCTATCAATGGCAGCCTTCGACATATGTATATCTCC-3′. Single F4A and F5A mutations were made using forward primer 5′-ATGTCGAAGTGTTTCATTGATAGGCCCATTTTC-3′, reverse primer 5′-ATCAATGAAACACTTCGACATATGTATATCTCC-3′ and forward primer 5′-ATGTCGAAGTTTTGCATTGATAGGCCCATTTTC-3′,

reverse primer 5′-CCTATCAATGCAAAACTTCGACATATGTATATC-3′ respectively. The mutated mexB genes were sequenced to ensure that only the intended changes were introduced. Escherichia coli BW25113 cells with deletions in AcrB or AcrA and AcrB

were used to propagate the control (pUC18, pET41a+), the MexAB-OprM (pMABO) or the MexB (pMexBH) expressing plasmids, respectively. All experiments employed basal levels of expression without induction. Cytotoxicity assays were carried out according to the 96-well microtitre broth dilution method (Jorgensen et al., 1999). Briefly, cells were grown to an OD660 nm of 0.2 in LB medium containing Selleck LGK-974 carbenicillin for pUC18 and pMABO (50 μg mL−1) or kanamycin for pET41a+ and pMexBH (25 μg mL−1) containing cells. Cytotoxic drugs were added to the cell suspensions at increasing concentrations, and ADP ribosylation factor the cultures were incubated at 37 °C with shaking. The A630 nm of the cultures were measured in a BioTek plate reader (Geneflow) after 18 h, and the lowest concentration of drug needed to prevent growth (no increase in turbidity compared

to the turbidity at time zero) was determined (MIC). LB-Broth Miller (Formedium) containing 50 μg mL−1 carbenicillin was inoculated with an overnight culture of E. coli cells (1 : 500 dilution) and incubated with shaking at 37 °C until an OD660 nm of 0.5 was reached. Substrate transport was then performed as described previously (Welch et al., 2010). Initial substrate transport rates were determined over the first 120 s, during which uptake was linear (Venter et al., 2003). Phenylalanine residues are important for drug transport by multidrug transporters (Yu et al., 2005; Bohnert et al., 2008; Vargiu et al., 2011). Alignment of MexB with several other RND-type multidrug transporters from Gram-negative bacteria identified two conserved phenylalanline residues at the N-terminus (Fig. 1a). From the crystal structure of AcrB, these Phe residues have been predicted to line the opening of a pore facing the cytoplasm (Das et al., 2007). The Phe residues at positions 4 and 5 in MexB are also aligned around a pore formed between the protomers (Fig. 1b and c).