GadX has been shown to suppress the expression of perA encoded by a plasmid of enteropathogenic E. coli [14], but activate gadX, gadA, gadB, and gadC in response to acid stress [15–19]. GadA and GadB are isozymes of glutamate decarboxylases that convert glutamate to γ-aminobutyric acid (GABA) which is then exported by the antiporter protein GadC [20, 21]. An intracellular proton is consumed during GABA production [22], but the released GABA, which is less acidic than glutamate, provides local buffering of the extracellular environment. The expression of gadX is activated by the alternative sigma factor RpoS during the stationary phase

of growth [15, 19, 21], but is repressed during the exponential Paclitaxel chemical structure phase by the

nucleoid protein H-NS due to its binding to the gadX promoter or its destabilizing effect on RpoS [23–25]. However, the acid stress increases the RpoS level and thus induces gadX expression even during the exponential phase of growth [26]. GadW, like GadX, belongs to the family of AraC-like regulators. It represses the expression of gadX and inhibits the activation of gadA and gadBC by GadX [15, 18, 27]. In addition to these trans-acting proteins, the production of GadX is also controlled by gadY that is located between gadX and gadW in an opposite orientation Selleck PLX4032 to gadX [28, 29]. The gadY gene has no known protein products. It produces three RNA species of 105, 90, and 59 nucleotides with a common 3′ end [28]. The 3′ ends of gadX and gadY RNAs overlap by at least 30 nucleotides and are complementary to each other. Annealing of gadY RNA to the 3′ end of gadX mRNA stabilizes gadX mRNA, resulting in an increased production of the GadX protein [28]. BtuB is also involved in the import of colicins such as colicin E7 (ColE7) [30–34]. ColE7 is composed of three domains responsible for the translocation

of ColE7 through the OmpF porin, binding of ColE7 to BtuB, and cleavage of DNA [35, 36], respectively. The import Idoxuridine of ColE7 is dependent on the Tol (tolerance to colicin) system that is composed of TolQ, TolR, TolA, and TolB proteins [35, 36]. Deletion or mutation of BtuB, OmpF, or any of the Tol proteins renders E. coli resistant to ColE7 [33, 37, 38]. Based on this information, we used a ColE7 resistance assay in this study to search for transcriptional regulators of btuB from a genomic library of E. coli strain DH5α and found that gadX and gadY genes down regulate the expression of btuB. Results Screening of genes conferring E. coli resistance to ColE7 To search for genes that can confer E. coli resistance to ColE7, plasmids in the genomic library were transformed into the ColE7-sensitive E. coli strain DH5α, and the transformants were plated on LB agar plates containing 50 μg/ml of ampicillin and 5.0 ng/ml of His6-tagged ColE7/ImE7. Two colonies were seen after incubation at 37°C overnight.