“
“Background In Escherichia coli, complex cellular responses are controlled by networks of transcriptional factors that regulate the expression of a diverse set of target genes, at various hierarchical levels. H-NS, a nucleoid-associated protein, is a top level regulator affecting the expression of at least 250 genes, mainly related to the bacterial #Temsirolimus randurls[1|1|,|CHEM1|]# response to environmental changes [1]. Among its various targets, it regulates in opposite directions the flagella-dependent motility and the acid stress resistance [1]; the first via the control of flhDC master flagellar operon by acting both directly and indirectly via regulators HdfR and RcsB [2–6]; the second

by repressing the genes involved in three amino acid decarboxylase systems, dependent on glutamate, lysine and arginine, via the RcsB-P/GadE regulatory complex [6]. In this regulatory process H-NS represses learn more the expression of gadE (encoding the central activator of the glutamate-dependent acid resistance pathway) both in a direct and an indirect way, via EvgA, YdeO, GadX and GadW [1, 7, 8], while it decreases rcsD expression, essential to the phosphorylation of RcsB (the capsular synthesis regulator component) required for the formation of the regulatory complex with GadE [6]. In the glutamate pathway, the RcsB-P/GadE regulatory complex controls the expression of two glutamate decarboxylase paralogues GadA and GadB, the glutamate/gamma-aminobutyrate antiporter GadC,

two glutamate synthase subunits GltB and GltD, the acid stress chaperones HdeA and HdeB,

the membrane protein HdeD, the transcriptional regulator YhiF (DctR) and the outer membrane STK38 protein Slp [6]. The complex also induces an arginine decarboxylase, AdiA, and an arginine:agmatine antiporter, AdiC (YjdE), essential for arginine-dependent acid resistance. Finally, the complex regulates a lysine decarboxylase, CadA, and a cadaverine/lysine antiporter, CadB, essential for lysine-dependent acid resistance [1, 6, 9]. Apart from the gadBC operon, the most important genes involved in acid resistance are present within the acid fitness island (AFI), a 15 kb region both repressed by H-NS and under the control of RpoS [10, 11]. Recent global chromatin immunoprecipitation studies revealed that H-NS binds to several loci within this region, including hdeABD [12, 13]. However, neither AdiY, the main regulator of the arginine-dependent response that controls adiA and adiC expression [14, 15] nor CadC, the main regulator of lysine-dependent response controlling cadBA [16], were yet found among the identified H-NS targets. In the present study, we aimed at further characterizing the H-NS-dependent cascade governing acid stress resistance pathways to identify the missing intermediary regulator(s) or functional protein(s) controlled by H-NS and to define the interplay between the different regulators and their targets. Methods Bacterial strains and plasmids Bacterial strains and plasmids used in this study are listed in Table 1.