Transcriptional regulation of the Pseudomonas aeruginosa iron-sulfur cluster assembly pathway by binding of IscR to multiple sites

Fe-S cluster homeostasis and beyond: The multifaceted roles of IscR

The role of IscR in regulating the transcription of genes involved in Fe-S cluster homeostasis has been well established for the model organism Escherichia coli K12. In this bacterium, IscR coordinates expression of the Isc and Suf Fe-S cluster assembly pathways to meet cellular Fe-S cluster demands shaped by a variety of environmental cues. However, since its initial discovery nearly 25 years ago, there has been growing evidence that IscR function extends well beyond Fe-S cluster homeostasis, not only in E. coli, but in bacteria of diverse lifestyles. Notably, pathogenic bacteria have exploited the ability of IscR to respond to changes in oxygen tension, oxidative and nitrosative stress, and iron availability to navigate their trajectory in their respective hosts as changes in these cues are frequently encountered during host infection. In this review, we highlight these broader roles of IscR in different cellular processes and, in particular, discuss the importance of IscR as a virulence factor for many bacterial pathogens.

When iron and sulfur met on an anoxic planet and eventually made clusters essential for life

[FeS] clusters are co-factors that are essential for life and are synthesized by dedicated multiprotein cellular machineries. In this review, we present the current scenario for the emergence and the diversification of the [FeS] cluster biosynthesis machineries. In addition to well-known NIF, ISC and SUF machineries, two alternative minimal systems, SMS, and MIS, were recently identified. Taxonomic distribution and phylogeny analyses indicate that SMS and MIS were present in the Last Universal Common Ancestor (LUCA), well before the increase of oxygen on Earth. ISC, SUF and NIF systems emerged later in the history of life. The possible reasons for the emergence and diversification of these machineries are discussed.

Genetic dissection of the bacterial Fe-S protein biogenesis machineries

Iron‑sulfur (Fe-S) clusters are one of the most ancient and versatile inorganic cofactors present in the three domains of life. Fe-S clusters are essential cofactors for the activity of a large variety of metalloproteins that play crucial physiological roles. Fe-S protein biogenesis is a complex process that starts with the acquisition of the elements (iron and sulfur atoms) and their assembly into an Fe-S cluster that is subsequently inserted into the target proteins. The Fe-S protein biogenesis is ensured by multiproteic systems conserved across all domains of life. Here, we provide an overview on how bacterial genetics approaches have permitted to reveal and dissect the Fe-S protein biogenesis process in vivo.

Monothiol Glutaredoxin Is Essential for Oxidative Stress Protection and Virulence in Pseudomonas aeruginosa

Glutaredoxins (Grxs), ubiquitous redox enzymes belonging to the thioredoxin family, catalyze the reduction of thiol-disulfide exchange reactions in a glutathione-dependent manner. A Pseudomonas aeruginosa ΔgrxD mutant exhibited hypersensitivity to oxidative stress-generating agents, such as paraquat (PQ) and cumene hydroperoxide (CHP). In vitro studies showed that P. aeruginosa GrxD acts as an electron donor for organic hydroperoxide resistance enzyme (Ohr) during CHP degradation. The ectopic expression of iron-sulfur cluster ([Fe-S]) carrier proteins, including ErpA, IscA, and NfuA, complements the function of GrxD in the ΔgrxD mutant under PQ toxicity. Constitutively high expression of iscR, nfuA, tpx, and fprB was observed in the ΔgrxD mutant. These results suggest that GrxD functions as a [Fe-S] cluster carrier protein involved in [Fe-S] cluster maturation. Moreover, the ΔgrxD mutant demonstrates attenuated virulence in a Drosophila melanogaster host model. Altogether, the data shed light on the physiological role of GrxD in oxidative stress protection and virulence of the human pathogen, P. aeruginosa. IMPORTANCE Glutaredoxins (Grxs) are ubiquitous disulfide reductase enzymes. Monothiol Grxs, containing a CXXS motif, play an essential role in iron homeostasis and maturation of [Fe-S] cluster proteins in various organisms. We now establish that the human pathogen Pseudomonas aeruginosa GrxD is crucial for bacterial virulence, maturation of [Fe-S] clusters and facilitation of Ohr enzyme activity. GrxD contains a conserved signature monothiol motif (C₂₉GFS), in which C29 is essential for its function in an oxidative stress protection. Our findings reveal the physiological roles of GrxD in oxidative stress protection and virulence of P. aeruginosa.

Genomic Characterization of Carbapenem-Non-susceptible Pseudomonas aeruginosa Clinical Isolates From Saudi Arabia Revealed a Global Dissemination of GES-5-Producing ST235 and VIM-2-Producing ST233 Sub-Lineages

Carbapenem-resistant P. aeruginosa has become a major clinical problem due to limited treatment options. However, studies assessing the trends in the molecular epidemiology and mechanisms of antibiotic resistance in this pathogen are lacking in Saudi Arabia. Here, we reported the genome characterization in a global context of carbapenem non-susceptible clinical isolates from a nationally representative survey. The antibiotic resistance profiles of the isolates (n = 635) collected over 14 months between March 2018 and April 2019 from different geographical regions of Saudi Arabia showed resistance rates to relevant β-lactams, aminoglycosides and quinolones ranging between 6.93 and 27.56%. Overall, 22.52% (143/635) of the isolates exhibited resistance to both imipenem and meropenem that were mainly explained by porin loss and efflux overexpression. However, 18.18% of resistant isolates harbored genes encoding GES (69.23%), VIM (23.07%), NDM (3.85%) or OXA-48-like (3.85%) carbapenemases. Most common GES-positive isolates produced GESs −5, −15 or −1 and all belonged to ST235 whereas the VIM-positive isolates produced mainly VIM-2 and belonged to ST233 or ST257. GES and VIM producers were detected at different sampling periods and in different surveyed regions. Interestingly, a genome-wide comparison revealed that the GES-positive ST235 and VIM-2-positive ST233 genomes sequenced in this study and those available through public databases from various locations worldwide, constituted each a phylogenetically closely related sub-lineage. Profiles of virulence determinants, antimicrobial resistance genes and associated mobile elements confirmed relatedness within each of these two different sub-lineages. Sequence analysis located the bla_GES gene in nearly all studied genomes (95.4%) in the same integrative conjugative element that also harbored the acc(6′)-Ib, aph(3′)-XV, aadA6, sul1, tet(G), and catB resistance genes while bla_VIM–2 in most (98.89%) ST233-positive genomes was co-located with aac(6′)-I1, dfrB-5, and aac(3′)-Id in the same class I integron. The study findings revealed the global spread of GES-5 ST235 and VIM-2 ST233 sub-lineages and highlighted the importance of routine detection of rare β-lactamases.

Oxidative Stress Response in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a Gram-negative environmental and human opportunistic pathogen highly adapted to many different environmental conditions. It can cause a wide range of serious infections, including wounds, lungs, the urinary tract, and systemic infections. The high versatility and pathogenicity of this bacterium is attributed to its genomic complexity, the expression of several virulence factors, and its intrinsic resistance to various antimicrobials. However, to thrive and establish infection, P. aeruginosa must overcome several barriers. One of these barriers is the presence of oxidizing agents (e.g., hydrogen peroxide, superoxide, and hypochlorous acid) produced by the host immune system or that are commonly used as disinfectants in a variety of different environments including hospitals. These agents damage several cellular molecules and can cause cell death. Therefore, bacteria adapt to these harsh conditions by altering gene expression and eliciting several stress responses to survive under oxidative stress. Here, we used PubMed to evaluate the current knowledge on the oxidative stress responses adopted by P. aeruginosa. We will describe the genes that are often differently expressed under oxidative stress conditions, the pathways and proteins employed to sense and respond to oxidative stress, and how these changes in gene expression influence pathogenicity and the virulence of P. aeruginosa. Understanding these responses and changes in gene expression is critical to controlling bacterial pathogenicity and developing new therapeutic agents.

A transcriptional regulator, IscR, of Burkholderia multivorans acts as both repressor and activator for transcription of iron-sulfur cluster-biosynthetic isc operon

Bacterial iron-sulfur (Fe-S) clusters are essential cofactors for many metabolic pathways, and Fe-S cluster-containing proteins (Fe-S proteins) regulate the expression of various important genes. However, biosynthesis of such clusters has remained unknown in genus Burkholderia. Here, we clarified that Burkholderia multivorans ATCC 17616 relies on the ISC system for the biosynthesis of Fe-S clusters, and that the biosynthetic genes are organized as an isc operon, whose first gene encodes IscR, a transcriptional regulatory Fe-S protein. Transcription of the isc operon was repressed and activated under iron-rich and -limiting conditions, respectively, and Fur, an iron-responsive global transcriptional regulator, was indicated to indirectly regulate the expression of isc operon. Further analysis using a ΔiscR mutant in combination with a constitutive expression system of IscR and its derivatives indicated transcriptional repression and activation of isc operon by holo- and apo-forms of IscR, respectively, through their binding to the sequences within an isc promoter-containing (Pisc) fragment. Biochemical analysis using the Pisc fragment suggested that the apo-IscR binding sequence differs from the holo-IscR binding sequence. The results obtained in this study revealed a unique regulatory system for the expression of the ATCC 17616 isc operon that has not been observed in other genera.

Making iron-sulfur cluster: structure, regulation and evolution of the bacterial ISC system

Iron sulfur (Fe-S) clusters rank among the most ancient and conserved prosthetic groups. Fe-S clusters containing proteins are present in most, if not all, organisms. Fe-S clusters containing proteins are involved in a wide range of cellular processes, from gene regulation to central metabolism, via gene expression, RNA modification or bioenergetics. Fe-S clusters are built by biogenesis machineries conserved throughout both prokaryotes and eukaryotes. We focus mostly on bacterial ISC machinery, but not exclusively, as we refer to eukaryotic ISC system when it brings significant complementary information. Besides covering the structural and regulatory aspects of Fe-S biogenesis, this review aims to highlight Fe-S biogenesis facets remaining matters of discussion, such as the role of frataxin, or the link between fatty acid metabolism and Fe-S homeostasis. Last, we discuss recent advances on strategies used by different species to make and use Fe-S clusters in changing redox environmental conditions.