SS carried out the overexpression of Obg and its biochemical analysis. VLS
read the manuscript critically, participated in interpretation of the data, and worked with the other authors to prepare the final version of the paper. SD conceived the study, participated in its design and interpretation of results and wrote the manuscript. All authors read and approved the manuscript.”
“Background The two major porins of Escherichia coli, namely OmpF and OmpC, form non-specific transport channels selleck products and allow for the passive diffusion of small, polar molecules (such as water, ions, amino acids, and other nutrients, as well as waste products) across the cell membrane. High and low levels of OmpF and OmpC are respectively expressed at low osmolarities in E. coli; as the medium osmolarity increases, OmpF expression is repressed, while OmpC is activated [1, 2]. OmpF forms a larger pore (hence a faster flux) than OmpC
. OmpC expression is favored when the enteric bacteria, such as E. coli, live in the mammalian gut where a high osmolarity (300 mM of NaCl or higher) is observed; in addition, the smaller pore size of OmpC can aid in the exclusion of harmful molecules in the gut. OmpF can predominate in the aqueous habitats, and its larger pore size can assist in scavenging for scarce nutrients from the external aqueous environments. OmpX represents the smallest known channel protein. OmpX expression in Enterobacter is inducible under high osmolarity, selleckchem which is accompanied by the repressed expressions of OmpF and OmpC [4–6]. The over-expression of OmpX can balance the decreased expression of non-specific porins, OmpF and OmpC, for the exclusion of small harmful molecules. However, whether or not OmpX functions as a porin to modulate the membrane permeability is still unclear. The osmosensor Lck histidine protein kinase EnvZ can phosphorylate the response regulator OmpR, which constitutes a two-component signal transduction
and regulatory system. The reciprocal regulation of OmpF and OmpC in E. coli is mediated by phosphorylated OmpR (OmpR-P) [2, 7, 8] (Figure 1). OmpR-P binds to four (F4, F1, F2, and F3 from the 5′ to 3′ direction) and three (C1, C2, and C3) sites within the upstream regions of ompF and ompC, respectively, with each containing two tandem 10 bp subsites (‘a’ and ‘b’) bound by two OmpR-P molecules. At low osmolarity, OmpR-P tandemly binds to F1 and F2 (and somewhat loosely to F3) in order to activate the transcription of ompF; meanwhile OmpR-P occupies C1 but not C2 and C3, which is not sufficient to stimulate the transcription of ompC. With increasing osmolarity, the cellular levels of OmpR-P elevate, and OmpR-P binds to C2 and C3 cooperatively, allowing for the transcription of ompC. At high osmolarity, OmpR-P is also capable of binding to F4, which is a weak site upstream F1-F2-F3.