Regulation of in vitro expression of the Escherichia coli frd operon: alanine and Fnr represent positive and negative control elements

A Dietary Source of High Level of Fluoroquinolone Tolerance in mcr-Carrying Gram-Negative Bacteria

The emergence of antibiotic tolerance, characterized by the prolonged survival of bacteria following antibiotic exposure, in natural bacterial populations, especially in pathogens carrying antibiotic resistance genes, has been an increasing threat to public health. However, the major causes contributing to the formation of antibiotic tolerance and underlying molecular mechanisms are yet poorly understood. Herein, we show that potassium sorbate (PS), a widely used food additive, triggers a high level of fluoroquinolone tolerance in bacteria carrying mobile colistin resistance gene mcr. Mechanistic studies demonstrate that PS treatment results in the accumulation of intracellular fumarate, which activates bacterial two-component system and decreases the expression level of outer membrane protein OmpF, thereby reducing the uptake of ciprofloxacin. In addition, the supplementation of PS inhibits aerobic respiration, reduces reactive oxygen species production and alleviates DNA damage caused by bactericidal antibiotics. Furthermore, we demonstrate that succinate, an intermediate product of the tricarboxylic acid cycle, overcomes PS-mediated ciprofloxacin tolerance. In multiple animal models, ciprofloxacin treatment displays failure outcomes in PS preadministrated animals, including comparable survival and bacterial loads with the vehicle group. Taken together, our works offer novel mechanistic insights into the development of antibiotic tolerance and uncover potential risks associated with PS use.

Bacillus subtilis Fnr senses oxygen via a [4Fe-4S] cluster coordinated by three cysteine residues without change in the oligomeric state

The oxygen regulator Fnr is part of the regulatory cascade in Bacillus subtilis for the adaptation to anaerobic growth conditions. In vivo complementation experiments revealed the essential role of only three cysteine residues (C227, C230, C235) at the C-terminus of B. subtilis Fnr for the transcriptional activation of the nitrate reductase operon (narGHJI) and nitrite extrusion protein gene (narK) promoters. UV/VIS, electron paramagnetic spin resonance (EPR) and Mössbauer spectroscopy experiments in combination with iron and sulphide content determinations using anaerobically purified recombinant B. subtilis Fnr identified the role of these three cysteine residues in the formation of one [4Fe-4S]2+ cluster per Fnr molecule. The obtained Mössbauer parameters are supportive for a [4Fe-4S]2+ cluster with three cysteine ligated iron sites and one non-cysteine ligated iron site. Gel filtration experiments revealed a stable dimeric structure for B. subtilis Fnr which is independent of the presence of the [4Fe-4S]2+ cluster. Gel mobility shift and in vitro transcription assays demonstrated the essential role of an intact [4Fe-4S]2+ cluster for promoter binding and transcriptional activation. An amino acid exchange introduced in the proposed alphaD-helix of B. subtilis Fnr (G149S) abolished its in vivo and in vitro activities indicating its importance for intramolecular signal transduction. The clear differences in the localization and coordination of the [4Fe-4S] cluster and in the organization of the oligomeric state between Escherichia coli and B. subtilis Fnr indicate differences in their mode of action.

FNR and its role in oxygen-regulated gene expression in Escherichia coli

Bacteria which can grow in different environments have developed regulatory systems which allow them to exploit specific habitats to their best advantage. In the facultative anaerobe Escherichia coli two transcriptional regulators controlling independent networks of oxygen-regulated gene expression have been identified. One is a two-component sensor-regulator system (ArcB-A), which represses a wide variety of aerobic enzymes under anaerobic conditions. The other is FNR, the transcriptional regulator which is essential for expressing anaerobic respiratory processes. The purpose of this review is to summarize what is known about FNR. The fnr gene was initially defined by the isolation of some pleiotropic mutants which characteristically lacked the ability to use fumarate and nitrate as reducible substrates for supporting anaerobic growth and several other anaerobic respiratory functions. Its role as a transcriptional regulator emerged from genetic and molecular studies in which its homology with CRP (the cyclic AMP receptor protein which mediates catabolite repression) was established and has since been particularly important in identifying the structural basis of its regulatory specificities. FNR is a member of a growing family of CRP-related regulatory proteins which have a DNA-binding domain based on the helix-turn-helix structural motif, and a characteristic beta-roll that is involved in nucleotide-binding in CRP. The FNR protein has been isolated in a monomeric form (Mr 30,000) which exhibits a high but as yet non-specific affinity for DNA. Nevertheless, the DNA-recognition site and important residues conferring the functional specificity of FNR have been defined by site-directed mutagenesis. A consensus for the sequences that are recognized by FNR in the promoter regions of FNR-regulated genes, has likewise been identified. The basic features of the genes and operons regulated by FNR are reviewed, and examples in which FNR functions negatively as an anaerobic repressor as well as positively as an anaerobic activator, are included. Less is known about the way in which FNR senses anoxia and is thereby transformed into its 'active' form, but it seems likely that cysteine residues and possibly a metal ion are involved. Four of the five cysteine residues of FNR are clustered in an essential N-terminal 'domain' which is conserved in FNR and the HlyX protein of Actinobacillus pleuropneumoniae, but not in CRP or the FixK protein of Rhizobium meliloti. The relationships between FNR and other oxygen-related systems in E. coli are discussed, as well as parallel systems in other organisms.(ABSTRACT TRUNCATED AT 400 WORDS)

Covalent cofactor binding to flavoenzymes requires specific effectors

Modification by covalent FAD attachment to a histidine residue via an 8 alpha-(N3-histidyl)-riboflavin linkage occurs in several flavoenzymes. Among them is 6-hydroxy-D-nicotine oxidase (6-HDNO) of Arthrobacter oxidans and the flavoprotein subunits of the fumarate reductase and succinate dehydrogenase complex of Escherichia coli and other bacterial and eukaryotic cells. We found that 6-HDNO holoenzyme formation from apo-6-HDNO, monitored by [14C]FAD incorporation and increase in enzyme activity, can be mediated not only by phosphoenolpyruvate [Nagursky, H., Bichler, V. and Brandsch, R. (1988) Eur. J. Biochem. 177, 319-325], but also by one of the glycolytic intermediates glyceraldehyde-3-P, glycerate-3-P, or the intermediate in glycerol utilization by bacteria, glycerol-3-P. Apoflavoprotein of fumarate reductase and succinate dehydrogenase was obtained in an E. coli riboflavin-requiring strain (E. coli RR28rf) overexpressing the frdABCD or the sdhCDAB operon from the recombinant plasmids pGS39 and pGS141, respectively. In extracts obtained from these cells, flavoprotein flavinylation, analyzed as covalent [14C]FAD incorporation into the apoflavoprotein polypeptide by polyacrylamide gel electrophoresis and fluorography, was stimulated severalfold by the citric acid cycle intermediates citrate, isocitrate, succinate and fumarate. Our results suggest that covalent modification and thus activation of these enzymes is dependent on specific metabolic intermediates which may act as allosteric effectors in the reaction.