Novel synthetic molecules targeting the bacterial RNA polymerase assembly

Inhibition of bacterial RNA polymerase function and protein-protein interactions: a promising approach for next-generation antibacterial therapeutics

The increasing prevalence of multidrug-resistant pathogens necessitates the urgent development of new antimicrobial agents with innovative modes of action for the next generation of antimicrobial therapy. Bacterial transcription has been identified and widely studied as a viable target for antimicrobial development. The main focus of these studies has been the discovery of inhibitors that bind directly to the core enzyme of RNA polymerase (RNAP). Over the past two decades, substantial advancements have been made in understanding the properties of protein-protein interactions (PPIs) and gaining structural insights into bacterial RNAP and its associated factors. This has led to the crucial role of computational methods in aiding the identification of new PPI inhibitors to affect the RNAP function. In this context, bacterial transcriptional PPIs present promising, albeit challenging, targets for the creation of new antimicrobials. This review will succinctly outline the structural foundation of bacterial transcription networks and provide a summary of the known small molecules that target transcription PPIs.

Thiazolidin-4-Ones as a Promising Scaffold in the Development of Antibiofilm Agents-A Review

Thiazolidin-4-ones have a broad range of medical and clinical implementation, which is important for pharmaceutical and medicinal chemistry. This heterocyclic core has been reported to possess a diversity of bioactivities, including antimicrobial and antibiofilm-forming potential. The resistance of biofilms to antibiotics or disinfectants is a serious medical problem. Therefore, there is a natural need to discover new effective structures with properties that inhibit biofilm formation. This review aims to analyze the antibiofilm features of thiazolidin-4-ones described in the literature over the last two decades. The information gathered in this review could benefit the rational design of new effective antibiofilm small molecules with thiazolidin-4-one cores.

Inhibitors of bacterial RNA polymerase transcription complex

A series of hybrid compounds that incorporated anthranilic acid with activated 1H-indoles through a glyoxylamide linker were designed to target bacterial RNA polymerase holoenzyme formation using computational docking. Synthesis, in vitro transcription inhibition assays, and biological testing of the hybrids identified a range of potent anti-transcription inhibitors with activity against a range of pathogenic bacteria with MICs as low as 3.1 μM. A structure activity relationship study identified the key structural components necessary for inhibition of both bacterial growth and transcription. Correlation of in vitro transcription inhibition activity with in vivo mechanism of action was established using fluorescence microscopy and resistance passaging using Gram-positive bacteria showed no resistance development over 30 days. Furthermore, no toxicity was observed from the compounds in a wax moth larvae model, establishing a platform for the development of a series of new antibacterial drugs with an established mode of action.

RNA polymerases from low G+C gram-positive bacteria

The low G + C Gram-positive bacteria represent some of the most medically and industrially important microorganisms. They are relied on for the production of food and dietary supplements, enzymes and antibiotics, as well as being responsible for the majority of nosocomial infections and serving as a reservoir for antibiotic resistance. Control of gene expression in this group is more highly studied than in any bacteria other than the Gram-negative model  Escherichia coli, yet until recently no structural information on RNA polymerase (RNAP) from this group was available. This review will summarize recent reports on the high-resolution structure of RNAP from the model low G + C representative  Bacillus subtilis, including the role of auxiliary subunits δ and ε, and outline approaches for the development of antimicrobials to target RNAP from this group.

Mechanisms of antibiotics inhibiting bacterial RNA polymerase

Transcription, the first phase of gene expression, is performed by the multi-subunit RNA polymerase (RNAP). Bacterial RNAP is a validated target for clinical antibiotics. Many natural and synthetic compounds are now known to target RNAP, inhibiting various stages of the transcription cycle. However, very few RNAP inhibitors are used clinically. A detailed knowledge of inhibitors and their mechanisms of action (MOA) is vital for the future development of efficacious antibiotics. Moreover, inhibitors of RNAP are often useful tools with which to dissect RNAP function. Here, we review the MOA of antimicrobial transcription inhibitors.

Small molecule inhibitors of bacterial transcription complex formation

Knoevenagel condensation was employed to generate a set of molecules potentially capable of inhibiting the RNA polymerase-σ⁷⁰/σ^A interaction in bacteria. Synthesis was achieved via reactions between a variety of indole-7-carbaldehydes and rhodanine, N-allylrhodanine, barbituric acid or thiobarbituric acid. A library of structurally diverse compounds was examined by enzyme-linked immunosorbent assay (ELISA) to assess the inhibition of the targeted protein-protein interaction. Inhibition of bacterial growth was also evaluated using Bacillus subtilis and Escherichia coli cultures. The structure-activity relationship studies demonstrated the significance of particular structural features of the synthesized molecules for RNA polymerase-σ⁷⁰/σ^A interaction inhibition and antibacterial activity. Docking was investigated as an in silico method for the further development of the compounds.

Novel Chemical Scaffolds for Inhibition of Rifamycin-Resistant RNA Polymerase Discovered from High-Throughput Screening

Rifampin has been a cornerstone of tuberculosis (TB) treatment since its introduction. The rise of multidrug-resistant and extensively drug-resistant TB makes the development of novel therapeutics effective against these strains an urgent need. Site-specific mutations in the target enzyme of rifampin, RNA polymerase (RNAP) comprises the majority (~97%) of rifamycin-resistant (Rif^R) strains of Mycobacterium tuberculosis (MTB). To identify novel inhibitors of bacterial RNAP, an in vitro plasmid-based transcription assay that uses malachite green (MG) to detect transcribed RNA containing MG aptamers was developed. This assay was optimized in a 384-well plate format and used to screen 150,000 compounds against an Escherichia coli homolog of the most clinically relevant Rif^R RNAP (βS531L) containing a mutation (β'V408G) that compensates for the fitness defect of this Rif^R mutant. Following confirmation and concentration-response studies, 10 compounds were identified with similar in vitro inhibition values across a panel of wild-type and Rif^R E. coli and MTB RNAPs. Four compounds identified from the screen are active against MTB in culture at concentrations below 200 µM. Initial follow-up has resulted in the elimination of one scaffold due to potential pan-assay interference.

Bacterial RNA Polymerase-DNA Interaction-The Driving Force of Gene Expression and the Target for Drug Action

DNA-dependent multisubunit RNA polymerase (RNAP) is the key enzyme of gene expression and a target of regulation in all kingdoms of life. It is a complex multifunctional molecular machine which, unlike other DNA-binding proteins, engages in extensive and dynamic interactions (both specific and nonspecific) with DNA, and maintains them over a distance. These interactions are controlled by DNA sequences, DNA topology, and a host of regulatory factors. Here, we summarize key recent structural and biochemical studies that elucidate the fine details of RNAP-DNA interactions during initiation. The findings of these studies help unravel the molecular mechanisms of promoter recognition and open complex formation, initiation of transcript synthesis and promoter escape. We also discuss most current advances in the studies of drugs that specifically target RNAP-DNA interactions during transcription initiation and elongation.

Bacterial Transcription as a Target for Antibacterial Drug Development

Transcription, the first step of gene expression, is carried out by the enzyme RNA polymerase (RNAP) and is regulated through interaction with a series of protein transcription factors. RNAP and its associated transcription factors are highly conserved across the bacterial domain and represent excellent targets for broad-spectrum antibacterial agent discovery. Despite the numerous antibiotics on the market, there are only two series currently approved that target transcription. The determination of the three-dimensional structures of RNAP and transcription complexes at high resolution over the last 15 years has led to renewed interest in targeting this essential process for antibiotic development by utilizing rational structure-based approaches. In this review, we describe the inhibition of the bacterial transcription process with respect to structural studies of RNAP, highlight recent progress toward the discovery of novel transcription inhibitors, and suggest additional potential antibacterial targets for rational drug design.

Identification of inhibitors of a bacterial sigma factor using a new high-throughput screening assay

Gram-negative bacteria are formidable pathogens because their cell envelope presents an adaptable barrier to environmental and host-mediated challenges. The stress response pathway controlled by the alternative sigma factor σ(E) is critical for maintenance of the cell envelope. Because σ(E) is required for the virulence or viability of several Gram-negative pathogens, it might be a useful target for antibiotic development. To determine if small molecules can inhibit the σ(E) pathway, and to permit high-throughput screening for antibiotic lead compounds, a σ(E) activity assay that is compatible with high-throughput screening was developed and validated. The screen employs a biological assay with positive readout. An Escherichia coli strain was engineered to express yellow fluorescent protein (YFP) under negative regulation by the σ(E) pathway, such that inhibitors of the pathway increase the production of YFP. To validate the screen, the reporter strain was used to identify σ(E) pathway inhibitors from a library of cyclic peptides. Biochemical characterization of one of the inhibitory cyclic peptides showed that it binds σ(E), inhibits RNA polymerase holoenzyme formation, and inhibits σ(E)-dependent transcription in vitro. These results demonstrate that alternative sigma factors can be inhibited by small molecules and enable high-throughput screening for inhibitors of the σ(E) pathway.

Surface plasmon resonance – more than a screening technology: insights in the binding mode of σ70:core RNAP inhibitors

Aim: Antibiotic resistance has become a major health problem. The σ70:core interface of bacterial RNA polymerase is a promising drug target. Recently, the coiled-coil and lid-rudder-system of the β’ subunit has been identified as an inhibition hot spot. Materials & methods & Results: By using surface plasmon resonance-based assays, inhibitors of the protein–protein interaction were identified and competition with σ70 was shown. Effective inhibition was verified in an in vitro transcription and a σ70:core assembly assay. For one hit series, we found a correlation between activity and affinity. Mutant interaction studies suggest the inhibitors’ binding site. Conclusion: Surface plasmon resonance is a valuable technology in drug design, that has been used in this study to identify and evaluate σ70:core RNA polymerase inhibitors.

New insights into the bacterial RNA polymerase inhibitor CBR703 as a starting point for optimization as an anti-infective agent

CBR703 was reported to inhibit bacterial RNA polymerase (RNAP) and biofilm formation, considering it to be a good candidate for further optimization. While synthesized derivatives of CBR703 did not result in more-active RNAP inhibitors, we observed promising antibacterial activities. These again correlated with a significant cytotoxicity toward mammalian cells. Furthermore, we suspect the promising effects on biofilm formation to be artifacts. Consequently, this class of compounds can be considered unattractive as antibacterial agents.

Synthesis and biological activity of novel bis-indole inhibitors of bacterial transcription initiation complex formation

The synthesis of novel bis-indole amides and glyoxylamides as bacterial transcription complex formation inhibitors and their structure–activity relationships are discussed.

Inhibitors of bacterial transcription initiation complex formation

Antibiotic resistance is a growing global problem, with very few new compounds in development. Bacterial transcription is an underutilized target for antibiotics, which has been attributed to the similarity of the active site of RNA polymerases (RNAPs) across all domains of life and the ease with which resistance can arise through point mutation at multiple sites within this conserved region. In this study we have taken a rational approach to design a novel set of compounds that specifically target the formation of transcription initiation complexes by preventing the unique bacterial σ initiation factor from binding to RNAP. We have identified the region of RNAP to which these compounds bind and demonstrate that one compound, GKL003, has an inhibition constant in the low nanomolar range. This compound has activity against both Gram-positive and -negative organisms, including a community acquired methicillin-resistant strain of the major pathogen Staphylococcus aureus.

Peptide-based investigation of the Escherichia coli RNA polymerase σ(70):core interface as target site

The number of bacterial strains that are resistant against antibiotics increased dramatically during the past decades. This fact stresses the urgent need for the development of new antibacterial agents with novel modes of action targeting essential enzymes such as RNA polymerase (RNAP). Bacterial RNAP is a large multi-subunit complex consisting of a core enzyme (subunits: α(2)ββ'ω) and a dissociable sigma factor (σ(70); holo enzyme: α(2)ββ'ωσ(70)) that is responsible for promoter recognition and transcription initiation. The interface between core RNAP and σ(70) represents a promising binding site. Nevertheless, detailed studies investigating its druggability are rare. Compounds binding to this region could inhibit this protein-protein interaction and thus holo enzyme formation, resulting in inhibition of transcription initiation. Sixteen peptides covering different regions of the Escherichia coli σ(70):core interface were designed; some of them-all derived from σ(70) 2.2 region-led to a strong RNAP inhibition. Indeed, an ELISA-based experiment confirmed the most active peptide P07 to inhibit the σ(70):core interaction. Furthermore, an abortive transcription assay revealed that P07 impedes transcription initiation. In order to study the mechanism of action of P07 in more detail, molecular dynamics simulations and a rational amino acid replacement study were performed, leading to the conclusion that P07 binds to the coiled-coil region in β' and that its flexible N-terminus inhibits the enzyme by interaction with the β' lid-rudder-system (LRS). This work revisits the β' coiled-coil as a hot spot for the protein-protein interaction inhibition and expands it by introduction of the LRS as target site.

Influence of DNA template choice on transcription and inhibition of Escherichia coli RNA polymerase

In recent decades, quantitative transcription assays using bacterial RNA polymerase (RNAP) have been performed under widely diverse experimental conditions. We demonstrate that the template choice can influence the inhibitory potency of RNAP inhibitors. Furthermore, we illustrate that the sigma factor (σ(70)) surprisingly increases the transcription efficiency of templates with nonphysiological nonprokaryotic promoters. Our results might be a useful guideline in the early stages of using RNAP for drug discovery.

Chemical modulators working at pharmacological interface of target proteins

For last few decades, the active site cleft and substrate-binding site of enzymes as well as ligand-binding site of the receptors have served as the main pharmacological space for drug discovery. However, rapid accumulation of proteome and protein network analysis data has opened a new therapeutic space that is the interface between the interacting proteins. Due to the complexity of the interaction modes and the numbers of the participating components, it is still challenging to identify the chemicals that can accurately control the protein-protein interactions at desire. Nonetheless, the number of chemical drugs and candidates working at the interface of the interacting proteins are rapidly increasing. This review addresses the current case studies and state-of-the-arts in the development of small chemical modulators controlling the interactions of the proteins that have pathological implications in various human diseases such as cancer, immune disorders, neurodegenerative and infectious diseases.

A screen to identify small molecule inhibitors of protein-protein interactions in mycobacteria

Despite extensive efforts in tuberculosis (TB) drug research, very few novel inhibitors have been discovered. This issue emphasizes the need for innovative methods to discover new anti-TB drugs. In this study, we established a new high-throughput screen (HTS) platform technology that differs from traditional TB drug screens because it utilizes Mycobacterial-Protein Fragment Complementation (M-PFC) to identify small molecule inhibitors of protein-protein interactions in mycobacteria. Several examples of protein-protein interactions were tested with M-PFC to highlight the diversity of selectable drug targets that could be used for screening. These included interactions of essential regulators (IdeR dimerization), enzymatic complexes (LeuCD), secretory antigens (Cfp10-Esat6), and signaling pathways (DevR dimerization). The feasibility of M-PFC in a HTS platform setting was tested by performing a proof-of-concept quantitative HTS of 3,600 small molecule compounds on DevR-DevR interaction, which was chosen because of its strong implications in Mycobacterium tuberculosis persistence and the need for effective drugs against latent TB. The calculated Z'-factor was consistently ≥0.8, indicating a robust and reproducible assay. Completion of the proof-of-concept screen allowed for the identification of advantages and disadvantages in the current assay design, where improvements made will further pioneer M-PFC-based applications in a large-scale HTS format.

Challenges of antibacterial discovery

The discovery of novel small-molecule antibacterial drugs has been stalled for many years. The purpose of this review is to underscore and illustrate those scientific problems unique to the discovery and optimization of novel antibacterial agents that have adversely affected the output of the effort. The major challenges fall into two areas: (i) proper target selection, particularly the necessity of pursuing molecular targets that are not prone to rapid resistance development, and (ii) improvement of chemical libraries to overcome limitations of diversity, especially that which is necessary to overcome barriers to bacterial entry and proclivity to be effluxed, especially in Gram-negative organisms. Failure to address these problems has led to a great deal of misdirected effort.

Essential biological processes of an emerging pathogen: DNA replication, transcription, and cell division in Acinetobacter spp

Within the last 15 years, members of the bacterial genus Acinetobacter have risen from relative obscurity to be among the most important sources of hospital-acquired infections. The driving force for this has been the remarkable ability of these organisms to acquire antibiotic resistance determinants, with some strains now showing resistance to every antibiotic in clinical use. There is an urgent need for new antibacterial compounds to combat the threat imposed by Acinetobacter spp. and other intractable bacterial pathogens. The essential processes of chromosomal DNA replication, transcription, and cell division are attractive targets for the rational design of antimicrobial drugs. The goal of this review is to examine the wealth of genome sequence and gene knockout data now available for Acinetobacter spp., highlighting those aspects of essential systems that are most suitable as drug targets. Acinetobacter spp. show several key differences from other pathogenic gammaproteobacteria, particularly in global stress response pathways. The involvement of these pathways in short- and long-term antibiotic survival suggests that Acinetobacter spp. cope with antibiotic-induced stress differently from other microorganisms.

The interaction of Bacillus subtilis sigmaA with RNA polymerase

RNA polymerase (RNAP) is an essential and highly conserved enzyme in all organisms. The process of transcription initiation is fundamentally different between prokaryotes and eukaryotes. In prokaryotes, initiation is regulated by sigma factors, making the essential interaction between sigma factors and RNAP an attractive target for antimicrobial agents. Our objective was to achieve the first step in the process of developing novel antimicrobial agents, namely to prove experimentally that the interaction between a bacterial RNAP and an essential sigma factor can be disrupted by introducing carefully designed mutations into sigma(A) of Bacillus subtilis. This disruption was demonstrated qualitatively by Far-Western blotting. Design of mutant sigmas was achieved by computer-aided visualization of the RNAP-sigma interface of the B. subtilis holoenzyme (RNAP + sigma) constructed using a homology modeling approach with published crystal structures of bacterial RNAPs. Models of the holoenzyme and the core RNAP were rigorously built, evaluated, and validated. To allow a high-quality RNAP-sigma interface model to be constructed for the design of mutations, a crucial error in the B. subtilis sigma(A) sequence in published databases at amino acid 165 had to be corrected first. The new model was validated through determination of RNAP-sigma interactions using targeted mutations.

Structure−Activity Relationships of Phenyl-Furanyl-Rhodanines as Inhibitors of RNA Polymerase with Antibacterial Activity on Biofilms

The dramatic rise of antibiotic-resistant bacteria over the past two decades has stressed the need for completely novel classes of antibacterial agents. Accordingly, recent advances in the study of prokaryotic transcription open new opportunities for such molecules. This paper reports the structure-activity relationships of a series of phenyl-furanyl-rhodanines (PFRs) as antibacterial inhibitors of RNA polymerase (RNAP). The molecules have been evaluated for their ability to inhibit transcription and affect growth of bacteria living in suspension or in a biofilm and for their propensity to interact with serum albumin, a critical parameter for antibacterial drug discovery. The most active of these molecules inhibit Escherichia coli RNAP transcription at concentrations of </=10 microM and have promising activities against various Gram-positive pathogens including Staphylococcus epidermidis biofilms, a major cause of nosocomial infection.

In vitro activities of different inhibitors of bacterial transcription against Staphylococcus epidermidis biofilm

Staphylococcus epidermidis is a major cause of nosocomial infections because of its ability to form biofilms on the surface of medical devices. Only a few antibacterial agents are relatively active against biofilms, and rifampin, a transcription inhibitor, ranks among the most effective molecules against biofilm-related infections. Whether this efficacy is due to advantageous structural properties of rifampin or to the fact that the RNA polymerase is a favorable target remains unclear. In an attempt to answer this question, we investigated the action of different transcription inhibitors against S. epidermidis biofilm, including the newest synthetic transcription inhibitors. This comparison suggests that most of the antibiotics that target the RNA polymerase are active on S. epidermidis biofilms at concentrations close to their MICs. One of these compounds, CBR703, despite its high MIC ranks among the best antibiotics to eradicate biofilm-embedded bacteria.

Progress in targeting bacterial transcription

The bacterial RNA polymerase (RNAP) is an essential enzyme that is responsible for making RNA from a DNA template and is targeted by several antibiotics. Rifampicin was the first of such antibiotics to be described and is one of the most efficient anti-tuberculosis drugs in use. In the past five years, structural studies of bacterial RNAP and the resolution of several complexes of drugs bound to RNAP subunits have revealed molecular details of the drug-binding sites and the mechanism of drug action. This knowledge opens avenues for the development of antibiotics. Here these drugs are reviewed, together with their mechanisms and their potential interest for therapeutic applications.

Competition of bacteriophage polypeptides with native replicase proteins for binding to the DNA sliding clamp reveals a novel mechanism for DNA replication arrest in Staphylococcus aureus

Bacteriophages have evolved specific mechanisms that redirect bacterial metabolic pathways to the bacteriophage reproduction cycle. In this study, we characterized the bactericidal mechanism of two polypeptides from bacteriophages Twort and G1 that target the DNA sliding clamp of Staphylococcus aureus. The DNA sliding clamp, which tethers DNA polymerase to its template and thereby confers processivity upon the enzyme, was found to be essential for the viability of S. aureus. Expression of polypeptides TwortORF168 and G1ORF240 in S. aureus selectively inhibited DNA replication which in turn resulted in cell death. Both polypeptides specifically inhibited the S. aureus DNA replicase that was reconstituted in vitro but not the corresponding replicase of Streptococcus pyogenes. We demonstrated that inhibition of DNA synthesis is multifaceted and occurs via binding the DNA sliding clamp: TwortORF168 and G1ORF240 bound tightly to the DNA sliding clamp and prevented both its loading onto DNA and its interaction with DNA polymerase C. These results elucidate the impact of bacteriophage polypeptide expression upon DNA replication in the growing cell.

A new class of small molecule RNA polymerase inhibitors with activity against Rifampicin-resistant Staphylococcus aureus1

The RNA polymerase holoenzyme is a proven target for antibacterial agents. A high-throughput screening program based on this enzyme from Staphylococcus aureus had previously identified a 2-ureidothiophene-3-carboxylate as a low micromolar inhibitor. An investigation of the relationships between the structures of this class of compounds and their inhibitory- and antibacterial activities is described here, leading to a set of potent RNA polymerase inhibitors with antibacterial activity. Characterization of this bioactivity, including studies of the mechanism of action, is provided, highlighting the power of the reverse chemical genetics approach in providing tools to inhibit the bacterial RNA polymerase.