Abstract
BackgroundIn the presence of sufficient iron, the Escherichia coli protein Fur (Ferric Uptake Regulator) represses genes controlled by the Fur box, a consensus sequence near or within promoters of target genes. De-repression of Fur-controlled genes occurs upon iron deprivation. In the E. coli chromosome, there is a bidirectional intercistronic promoter region with two non-overlapping Fur boxes. This region controls Fur-regulated expression of entCEBAH in the clockwise direction and fepB in the anticlockwise direction.ResultsWe cloned the E. coli bidirectional fepB/entC promoter region into low-copy-number plasmid backbones (pACYC184 and pBR322) along with downstream sequences encoding epitope tags and a multiple cloning site (MCS) compatible with the bacterial adenylate cyclase two-hybrid (BACTH) system. The vector pFCF1 allows for iron-controlled expression of FLAG-tagged proteins, whereas the pFBH1 vector allows for iron-controlled expression of HA-tagged proteins. We showed that E. coli knockout strains transformed with pFCF1-entA, pFCF1-entE and pFBH1-entB express corresponding proteins with appropriate epitope tags when grown under iron restriction. Furthermore, transformants exhibited positive chrome azurol S (CAS) assay signals under iron deprivation, indicating that the transformants were functional for siderophore biosynthesis. Western blotting and growth studies in rich and iron-depleted media demonstrated that protein expression from these plasmids was under iron control. Finally, we produced the vector pFCF2, a pFCF1 derivative in which a kanamycin resistance (KanR) gene was engineered in the direction opposite of the MCS. The entA ORF was then subcloned into the pFCF2 MCS. Bidirectional protein expression in an iron-deprived pFCF2-entA transformant was confirmed using antibiotic selection, CAS assays and growth studies.ConclusionsThe vectors pFCF1, pFCF2, and pFBH1 have been shown to use the fepB/entC promoter region to control bidirectional in trans expression of epitope-tagged proteins in iron-depleted transformants. In the presence of intracellular iron, protein expression from these constructs was abrogated due to Fur repression. The compatibility of the pFCF1 and pFBH1 backbones allows for iron-controlled expression of multiple epitope-tagged proteins from a single co-transformant.Electronic supplementary materialThe online version of this article (doi:10.1186/s12896-016-0298-1) contains supplementary material, which is available to authorized users.
Highlights
In the presence of sufficient iron, the Escherichia coli protein Fur (Ferric Uptake Regulator) represses genes controlled by the Fur box, a consensus sequence near or within promoters of target genes
Construction of pFCF1, pFCF2 and pFBH1 The vector pFCF1 was constructed by inserting gBlock1 (Additional file 2: Figure S1) into a pACYC184 backbone. gBlock1 encodes the bidirectional Fur promoter region (Fig. 1a) followed by a downstream FLAG epitope tag sequence, TEV protease cleavage site, and a multiple cloning site (MCS) (Fig. 2a)
The vector pFBH1 was constructed by insertion of gBlock3 (Additional file 4: Figure S3) between the EcoRI and SalI sites of linearized pBR322. gBlock3 contained DNA encoding the bidirectional Fur promoter region (Fig. 1a), the HA tag sequence, a TEV protease cleavage site, and the MCS from pUT18C (Fig. 3a)
Summary
In the presence of sufficient iron, the Escherichia coli protein Fur (Ferric Uptake Regulator) represses genes controlled by the Fur box, a consensus sequence near or within promoters of target genes. In the E. coli chromosome, there is a bidirectional intercistronic promoter region with two non-overlapping Fur boxes This region controls Fur-regulated expression of entCEBAH in the clockwise direction and fepB in the anticlockwise direction. In the classical holo-Fur repression mechanism, iron-bound Fur binds to a Fur box sequence that overlaps with, or is proximal to, promoters of ironresponsive genes, preventing their transcription [11]. When intracellular iron is depleted, Fe2+ is released from Fur, causing conformational changes in the protein resulting in dissociation from the Fur box [12] This derepression results in the up-regulation of Fur-controlled genes. Numerous genes are controlled by holo-Fur, including those that encode: (i) proteins involved in siderophore-mediated iron uptake [13], (ii) small RNAs such as ryhB that regulate bacterial iron uptake [14], (iii) some TCA cycle enzymes [15], (iv) superoxide dismutase [14, 16], and (v) Fur itself [17]
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