Tetrameric architecture of an active phenol-bound form of the AAA+ transcriptional regulator DmpR

BrfA functions as a bacterial enhancer-binding protein to regulate functional amyloid Fap-dependent biofilm formation in Pseudomonas fluorescens by sensing cyclic diguanosine monophosphate

The functional amyloid of Pseudomonas (Fap) is essential for the formation of macrocolony biofilms, pellicles, and solid surface-associated (SSA) biofilms of Pseudomonas fluorescens PF07, an isolate from refrigerated marine fish. However, limited information on the expression regulation of fap genes is available. Herein, we found that a novel bacterial enhancer-binding protein (bEBP), BrfA, regulated Fap-dependent biofilm formation by directly sensing cyclic diguanosine monophosphate (c-di-GMP). Our in vivo data showed that the REC domain deletion of BrfA promoted fap gene expression and biofilm formation, and c-di-GMP positively regulated the transcription of fapA in a BrfA-dependent manner. In in vitro experiments, we found that the ATPase activity of BrfA was inhibited by the REC domain and was activated by c-di-GMP. BrfA and the sigma factor RpoN bound to the upstream region of fapA, and the binding ability of BrfA was not affected by either deletion of the REC domain or c-di-GMP. BrfA specifically bound to the three enhancer sites upstream of the fapA promoter, which contain the consensus sequence CA-(N4)-TGA(A/T)ACACC. In vivo experiments using a lacZ fusion reporter indicated that all three BrfA enhancer sites were essential for the activation of fapA transcription. Overall, these findings reveal that BrfA is a new type of c-di-GMP-responsive transcription factor that directly controls the transcription of Fap biosynthesis genes in P. fluorescens. Fap functional amyloids and BrfA-type transcription factors are widespread in Pseudomonas species. The novel insights into the c-di-GMP- and BrfA-dependent expression regulation of fap provided by this work will contribute to the development of antibiofilm strategies.

Sensitive and selective phenol sensing in denitrifying Aromatoleum aromaticum EbN1T

IMPORTANCE

Aromatic compounds are globally abundant organic molecules with a multitude of natural and anthropogenic sources, underpinning the relevance of their biodegradation. A. aromaticum EbN1T is a well-studied environmental betaproteobacterium specialized on the anaerobic degradation of aromatic compounds. The here studied responsiveness toward phenol in conjunction with the apparent high ligand selectivity (non-promiscuity) of its PheR sensor and those of the related p-cresol (PcrS) and p-ethylphenol (EtpR) sensors are in accord with the substrate-specificity and biochemical distinctiveness of the associated degradation pathways. Furthermore, the present findings advance our general understanding of the substrate-specific regulation of the strain's remarkable degradation network and of the concentration thresholds below which phenolic compounds become essentially undetectable and as a consequence should escape substantial biodegradation. Furthermore, the findings may inspire biomimetic sensor designs for detecting and quantifying phenolic contaminants in wastewater or environments.

Identifying Selectivity Filters in Protein Biosensor for Ligand Screening

Specialized sensing mechanisms in bacteria enable the identification of cognate ligands with remarkable selectivity in highly xenobiotic-polluted environments where these ligands are utilized as energy sources. Here, via integrating all-atom computer simulation, biochemical assay, and isothermal titration calorimetry measurements, we determine the molecular basis of MopR, a phenol biosensor's complex selection process of ligand entry. Our results reveal a set of strategically placed selectivity filters along the ligand entry pathway of MopR. These filters act as checkpoints, screening diverse aromatic ligands at the protein surface based on their chemical features and sizes. Ligands meeting specific criteria are allowed to enter the sensing site in an orientation-dependent manner. Sequence and structural analyses demonstrate the conservation of this ligand entry mechanism across the sensor class, with individual amino acids along the selectivity filter path playing a critical role in ligand selection. Together, this investigation highlights the importance of interactions with the ligand entry pathway, in addition to interactions within the binding pocket, in achieving ligand selectivity in biological sensing. The findings enhance our understanding of ligand selectivity in bacterial phenol biosensors and provide insights for rational expansion of the biosensor repertoire, particularly for the biotechnologically relevant class of aromatic pollutants.

A review on structural mechanisms of protein-persistent organic pollutant (POP) interactions

With the advent of the industrial revolution, the accumulation of persistent organic pollutants (POPs) in the environment has become ubiquitous. POPs are halogen-containing organic molecules that accumulate, and remain in the environment for a long time, thus causing toxic effects in living organisms. POPs exhibit a high affinity towards biological macromolecules such as nucleic acids, proteins and lipids, causing genotoxicity and impairment of homeostasis in living organisms. Proteins are essential members of the biological assembly, as they stipulate all necessary processes for the survival of an organism. Owing to their stereochemical features, POPs and their metabolites form energetically favourable complexes with proteins, as supported by biological and dose-dependent toxicological studies. Although individual studies have reported the biological aspects of protein-POP interactions, no comprehensive study summarizing the structural mechanisms, thermodynamics and kinetics of protein-POP complexes is available. The current review identifies and classifies protein-POP interaction according to the structural and functional basis of proteins into five major protein targets, including digestive and other enzymes, serum proteins, transcription factors, transporters, and G-protein coupled receptors. Further, analysis detailing the molecular interactions and structural mechanism evidenced that H-bonds, van der Waals, and hydrophobic interactions essentially mediate the formation of protein-POP complexes. Moreover, interaction of POPs alters the protein conformation through kinetic and thermodynamic processes like competitive inhibition and allostery to modulate the cellular signalling processes, resulting in various pathological conditions such as cancers and inflammations. In summary, the review provides a comprehensive insight into the critical structural/molecular aspects of protein-POP interactions.

Engineering Modular and Highly Sensitive Cell-Based Biosensors for Aromatic Contaminant Monitoring and High-Throughput Enzyme Screening

Although a variety of whole-cell-based biosensors have been developed for different applications in recent years, most cannot meet practical requirements due to insufficient sensing performance. Here, we constructed two sets of modular genetic circuits by serial and parallel modes capable of significantly amplifying the input/output signal in Escherichia coli. The biosensors are engineered using σ⁵⁴-dependent phenol-responsive regulator DmpR as a sensor and enhanced green fluorescent protein as a reporter. Cells harboring serial and parallel genetic circuits displayed nearly 9- and 16-fold higher sensitivity than the general circuit. The genetic circuits enabled rapid detection of six phenolic contaminants in 12 h and showed the low limit of detection of 2.5 and 2.2 ppb for benzopyrene (BaP) and tetracycline (Tet), with a broad detection range of 0.01-1 and 0.005-5 μM, respectively. Furthermore, the positive rate was as high as 73% when the biosensor was applied to screen intracellular enzymes with ester-hydrolysis activity from soil metagenomic libraries using phenyl acetate as a phenolic substrate. Several novel enzymes were isolated, identified, and biochemically characterized, including serine peptidases and alkaline phosphatase family protein/metalloenzyme. Consequently, this study provides a new signal amplification method for cell-based biosensors that can be widely applied to environmental contaminant assessment and screening of intracellular enzymes.

Phenol sensing in nature is modulated via a conformational switch governed by dynamic allostery

The NtrC family of proteins senses external stimuli and accordingly stimulates stress and virulence pathways via activation of associated σ⁵⁴-dependent RNA polymerases. However, the structural determinants that mediate this activation are not well understood. Here, we establish using computational, structural, biochemical, and biophysical studies that MopR, an NtrC protein, harbors a dynamic bidirectional electrostatic network that connects the phenol pocket to two distal regions, namely the “G-hinge” and the “allosteric linker.” While the G-hinge influences the entry of phenol into the pocket, the allosteric linker passes the signal to the downstream ATPase domain. We show that phenol binding induces a rewiring of the electrostatic connections by eliciting dynamic allostery and demonstrates that perturbation of the core relay residues results in a complete loss of ATPase stimulation. Furthermore, we found a mutation of the G-hinge, ∼20 Å from the phenol pocket, promotes altered flexibility by shifting the pattern of conformational states accessed, leading to a protein with 7-fold enhanced phenol binding ability and enhanced transcriptional activation. Finally, we conducted a global analysis that illustrates that dynamic allostery-driven conserved community networks are universal and evolutionarily conserved across species. Taken together, these results provide insights into the mechanisms of dynamic allostery-mediated conformational changes in NtrC sensor proteins.

Cooperativity in ATP Hydrolysis by MopR Is Modulated by Its Signal Reception Domain and by Its Protein and Phenol Concentrations

The NtrC family of AAA+ proteins are bacterial transcriptional regulators that control σ54-dependent RNA polymerase transcription under certain stressful conditions. MopR, which is a member of this family, is responsive to phenol and stimulates its degradation. Biochemical studies to understand the role of ATP and phenol in oligomerization and allosteric regulation, which are described here, show that MopR undergoes concentration-dependent oligomerization in which dimers assemble into functional hexamers. The oligomerization occurs in a nucleation-dependent manner with a tetrameric intermediate. Additionally, phenol binding is shown to be responsible for shifting MopR's equilibrium from a repressed state (high affinity toward ATP) to a functionally active, derepressed state with low-affinity for ATP. Based on these findings, we propose a model for allosteric regulation of MopR. IMPORTANCE The NtrC family of bacterial transcriptional regulators are enzymes with a modular architecture that harbor a signal sensing domain followed by a AAA+ domain. MopR, a NtrC family member, responds to phenol and activates phenol adaptation pathways that are transcribed by σ54-dependent RNA polymerases. Our results show that for efficient ATP hydrolysis, MopR assembles as functional hexamers and that this activity of MopR is regulated by its effector (phenol), ATP, and protein concentration. Our findings, and the kinetic methods we employ, should be useful in dissecting the allosteric mechanisms of other AAA+ proteins, in general, and NtrC family members in particular.

Design of stable and self-regulated microbial consortia for chemical synthesis

Microbial coculture engineering has emerged as a promising strategy for biomanufacturing. Stability and self-regulation pose a significant challenge for the generation of intrinsically robust cocultures for large-scale applications. Here, we introduce the use of multi-metabolite cross-feeding (MMCF) to establish a close correlation between the strains and the design rules for selecting the appropriate metabolic branches. This leads to an intrinicially stable two-strain coculture where the population composition and the product titer are insensitive to the initial inoculation ratios. With an intermediate-responsive biosensor, the population of the microbial coculture is autonomously balanced to minimize intermediate accumulation. This static-dynamic strategy is extendable to three-strain cocultures, as demonstrated with de novo biosynthesis of silybin/isosilybin. This strategy is generally applicable, paving the way to the industrial application of microbial cocultures.

Stability and tunability are two desirable properties of microbial consortia-based bioproduction. Here, the authors integrate a caffeate-responsive biosensor into two and three strains coculture system to achieve autonomous regulation of strain ratios for coniferol and silybin/isosiltbin production, respectively.

Next generation single-molecule techniques: Imaging, labeling, and manipulation in vitro and in cellulo

Owing to their unique abilities to manipulate, label, and image individual molecules in vitro and in cellulo, single-molecule techniques provide previously unattainable access to elementary biological processes. In imaging, single-molecule fluorescence resonance energy transfer (smFRET) and protein-induced fluorescence enhancement in vitro can report on conformational changes and molecular interactions, single-molecule pull-down (SiMPull) can capture and analyze the composition and function of native protein complexes, and single-molecule tracking (SMT) in live cells reveals cellular structures and dynamics. In labeling, the abilities to specifically label genomic loci, mRNA, and nascent polypeptides in cells have uncovered chromosome organization and dynamics, transcription and translation dynamics, and gene expression regulation. In manipulation, optical tweezers, integration of single-molecule fluorescence with force measurements, and single-molecule force probes in live cells have transformed our mechanistic understanding of diverse biological processes, ranging from protein folding, nucleic acids-protein interactions to cell surface receptor function.

Structural analyses of the AAA+ ATPase domain of the transcriptional regulator GtrR in the BDSF quorum-sensing system in Burkholderia cenocepacia

Global transcriptional regulator downstream RpfR (GtrR) is a key downstream regulator for quorum-sensing signaling molecule cis-2-dodecenoic acid (BDSF). As a bacterial enhancer-binding protein (bEBP), GtrR is composed of an N-terminal receiver domain, a central ATPases associated with diverse cellular activities (AAA+) ATPase σ54 -interaction domain, and a C-terminal helix-turn-helix DNA-binding domain. In this work, we solved its AAA+ ATPase domain in both apo and GTP-bound forms. The structure revealed how GtrR specifically recognizes GTP. In addition, we also revealed that GtrR has moderate GTPase activity in vitro in the absence of its activation signal. Finally, we found the residues K170, D236, R311, and R357 in GtrR that are crucial to its biological function, any single mutation leading to completely abolishing GtrR activity.

An updated structural model of the A domain of the Pseudomonas putida XylR regulator poses an atypical interplay with aromatic effectors

A revised model of the aromatic binding A domain of the σ⁵⁴ -dependent regulator XylR of Pseudomonas putida mt-2 was produced based on the known 3D structures of homologous regulators PoxR, MopR and DmpR. The resulting frame was instrumental for mapping a number of mutations known to alter effector specificity, which were then reinterpreted under a dependable spatial reference. Some of these changes involved the predicted aromatic binding pocket but others occurred in distant locations, including dimerization interfaces and putative zinc binding site. The effector pocket was buried within the protein structure and accessible from the outside only through a narrow tunnel. Yet, several loop regions of the A domain could provide the flexibility required for widening such a tunnel for passage of aromatic ligands. The model was experimentally validated by treating the cells in vivo and the purified protein in vitro with benzyl bromide, which reacts with accessible nucleophilic residues on the protein surface. Structural and proteomic analyses confirmed the predicted in/out distribution of residues but also supported two additional possible scenarios of interaction of the A domain with aromatic effectors: a dynamic interaction of the fully structured yet flexible protein with the aromatic partner and/or inducer-assisted folding of the A domain.

Antagonistic Control of Genetic Circuit Performance for Rapid Analysis of Targeted Enzyme Activity in Living Cells

Genetic circuits have been developed for quantitative measurement of enzyme activity, metabolic engineering of strain development, and dynamic regulation of microbial cells. A genetic circuit consists of several bio-elements, including enzymes and regulatory cassettes, that can generate the desired output signal, which is then used as a precise criterion for enzyme screening and engineering. Antagonists and inhibitors are small molecules with inhibitory effects on regulators and enzymes, respectively. In this study, an antagonist and an inhibitor were applied to a genetic circuit for a dynamic detection range. We developed a genetic circuit relying on regulators and enzymes, allowing for straightforward control of its output signal without additional genetic modification. We used para-nitrophenol and alanine as an antagonist of DmpR and inhibitor of tyrosine phenol-lyase, respectively. We show that the antagonist resets the detection range of the genetic circuit similarly to a resistor in an electrical logic circuit. These biological resistors in genetic circuits can be used as a rapid and precise controller of variable outputs with minimal circuit configuration.