The antiactivator FleN uses an allosteric mechanism to regulate σ54-dependent expression of flagellar genes in Pseudomonas aeruginosa

Transcription Regulation of Flagellins: A Structural Perspective

Bacterial flagella are complex molecular motors that are essential for locomotion and host colonization. They consist of 30 different proteins expressed in varying stoichiometries. Their assembly and function are governed by a hierarchical transcriptional regulatory network with multiple checkpoints primarily regulated by sigma factors. Expression of late flagellar genes requires the complete assembly of the flagellar basal body and hook. The extracellular segment of the flagellum, termed filament, is composed of self-assembling flagellin subunits encoded by the fliC gene and harbors potent antigenic epitopes. Structural studies have illuminated the molecular mechanisms underlying its assembly and its regulation at the transcription level. σ28, a key subunit of the RNA polymerase complex, binds to specific promoter sequences to initiate transcription of late flagellar genes, while its activity is controlled by the antisigma factor FlgM. This review summarizes current insights into the structural characterization of flagellins across various bacterial species, their transcription by σ28, and the structural mechanism controlling σ28 activity through FlgM. Additionally, we highlight the regulation of flagellin gene expression via transcription factors and their post-transcriptional regulation, providing a comprehensive overview of the intricate mechanisms that support bacterial motility and adaptation.

A conserved cell-pole determinant organizes proper polar flagellum formation

The coordination of cell cycle progression and flagellar synthesis is a complex process in motile bacteria. In γ-proteobacteria, the localization of the flagellum to the cell pole is mediated by the SRP-type GTPase FlhF. However, the mechanism of action of FlhF, and its relationship with the cell pole landmark protein HubP remain unclear. In this study, we discovered a novel protein called FipA that is required for normal FlhF activity and function in polar flagellar synthesis. We demonstrated that membrane-localized FipA interacts with FlhF and is required for normal flagellar synthesis in Vibrio parahaemolyticus, Pseudomonas putida, and Shewanella putrefaciens, and it does so independently of the polar localization mediated by HubP. FipA exhibits a dynamic localization pattern and is present at the designated pole before flagellar synthesis begins, suggesting its role in licensing flagellar formation. This discovery provides insight into a new pathway for regulating flagellum synthesis and coordinating cellular organization in bacteria that rely on polar flagellation and FlhF-dependent localization.

Synthesis and agricultural antimicrobial evaluation of new quinazoline derivatives containing both a piperazine linker and the N-acetyl moiety

BACKGROUND

To discover more efficient agricultural antimicrobial agents, a series of new quinazoline derivatives containing both a piperazine linker and the N-acetyl moiety were prepared and assessed for their antibacterial and antifungal activities.

RESULTS

All the target compounds were characterized by 1H and 13C NMR as well as high-resolution mass spectrometry (HRMS), and the chemical structure of the most potent compound E19 incorporating a 4-trifluoromethoxy substituent was clearly confirmed via single crystal X-ray diffraction measurements. The bioassay results indicated that some compounds possessed notable inhibitory effects in vitro against the bacterium Xanthomonas oryzae pv. oryzicola (Xoc). For example, compound E19 had an EC50 (effective concentration for 50% activity) value of 7.1 μg/mL towards this pathogen, approximately 15- and 10-fold more effective than the commercial bactericides thiodiazole copper and bismerthiazol (EC50 = 110.2 and 72.4 μg/mL, respectively). Subsequently, the mechanistic studies showed that compound E19 likely exerted its antibacterial efficacies by altering the cell morphology, increasing the permeability of bacterial cytoplasmic membrane, suppressing the production of bacterial extracellular polysaccharides and the extracellular enzyme activities (amylase and cellulase), and blocking the swimming motility of Xoc. Moreover, the proteomic analysis revealed that compound E19 could reduce the bacterial flagellar biosynthesis and decrease the flagellar motility by down-regulating the expression of the related differential proteins.

CONCLUSION

Compound E19 exhibited good potential for further development as a bactericide candidate for control of Xoc. © 2024 Society of Chemical Industry.

Inhibition mechanism of crude lipopeptide from Bacillus subtilis against Aeromonas veronii growth, biofilm formation, and spoilage of channel catfish flesh

Aeromonas veronii is associated with food spoilage and some human diseases, such as diarrhea, gastroenteritis, hemorrhagic septicemia or asymptomatic and even death. This research investigated the mechanism of the growth, biofilm formation, virulence, stress resistance, and spoilage potential of Bacillus subtilis lipopeptide against Aeromonas veronii. Lipopeptides suppressed the transmembrane transport of Aeromonas veronii by changing the cell membrane's permeability, the structure of membrane proteins, and Na+/K+-ATPase. Lipopeptide significantly reduced the activities of succinate dehydrogenase (SDH) and malate dehydrogenase (MDH) by 86.03% and 56.12%, respectively, ultimately slowing Aeromonas veronii growth. Lipopeptides also restrained biofilm formation by inhibiting Aeromonas veronii motivation and extracellular polysaccharide secretion. Lipopeptides downregulated gene transcriptional levels related to the virulence and stress tolerance of Aeromonas veronii. Furthermore, lipopeptides treatment resulted in a considerable decrease in the extracellular protease activity of Aeromonas veronii, which restrained the decomposing of channel catfish flesh. This research provides new insights into lipopeptides for controlling Aeromonas veronii and improving food safety.

Flip the switch: the role of FleQ in modulating the transition between the free-living and sessile mode of growth in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a Gram-negative, opportunistic pathogen causing chronic infections that are associated with the sessile/biofilm mode of growth rather than the free-living/planktonic mode of growth. The transcriptional regulator FleQ contributes to both modes of growth by functioning both as an activator and repressor and inversely regulating flagella genes associated with the planktonic mode of growth and genes contributing to the biofilm mode of growth. Here, we review findings that enhance our understanding of the molecular mechanism by which FleQ enables the transition between the two modes of growth. We also explore recent advances in the mechanism of action of FleQ to both activate and repress gene expression from a single promoter. Emphasis will be on the role of sigma factors, cyclic di-GMP, and the transcriptional regulator AmrZ in inversely regulating flagella and biofilm-associated genes and converting FleQ from a repressor to an activator.

The global regulation of c-di-GMP and cAMP in bacteria

Nucleotide second messengers are highly versatile signaling molecules that regulate a variety of key biological processes in bacteria. The best-studied examples are cyclic AMP (cAMP) and bis-(3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP), which both act as global regulators. Global regulatory frameworks of c-di-GMP and cAMP in bacteria show several parallels but also significant variances. In this review, we illustrate the global regulatory models of the two nucleotide second messengers, compare the different regulatory frameworks between c-di-GMP and cAMP, and discuss the mechanisms and physiological significance of cross-regulation between c-di-GMP and cAMP. c-di-GMP responds to numerous signals dependent on a great number of metabolic enzymes, and it regulates various signal transduction pathways through its huge number of effectors with varying activities. In contrast, due to the limited quantity, the cAMP metabolic enzymes and its major effector are regulated at different levels by diverse signals. cAMP performs its global regulatory function primarily by controlling the transcription of a large number of genes via cAMP receptor protein (CRP) in most bacteria. This review can help us understand how bacteria use the two typical nucleotide second messengers to effectively coordinate and integrate various physiological processes, providing theoretical guidelines for future research.

REC domain stabilizes the active heptamer of σ54-dependent transcription factor, FleR from Pseudomonas aeruginosa

Motility in Pseudomonas aeruginosa is mediated through a single, polar flagellum, which is essential for virulence, colonization, and biofilm formation. FleSR, a two-component system (TCS), serves as a critical checkpoint in flagellar assembly. FleR is a σ54-dependent response regulator that undergoes phosphorylation via cognate sensor kinase FleS for the assembly of the functionally active form. The active form remodels the σ54-RNAP complex to initiate transcription. Small-angle X-ray scattering, crystallography, and negative staining electron microscopy reconstructions of FleR revealed that it exists predominantly as a dimer in the inactive form with low ATPase activity and assembles into heptamers upon phosphorylation with amplified ATPase activity. We establish that receiver (REC) domain stabilizes the heptamers and is indispensable for assembly of the functional phosphorylated form of FleR. The structural, biochemical, and in vivo complementation assays provide details of the phosphorylation-mediated assembly of FleR to regulate the expression of flagellar genes.

Structures of the P. aeruginosa FleQ-FleN master regulators reveal large-scale conformational switching in motility and biofilm control

Pseudomonas aeruginosa can cause a wide array of chronic and acute infections associated with its ability to rapidly switch between planktonic, biofilm, and dispersed lifestyles, each with a specific arsenal for bacterial survival and virulence. At the cellular level, many of the physiological transitions are orchestrated by the intracellular second messenger c-di-GMP and its receptor-effector FleQ. A bacterial enhancer binding protein, FleQ acts as a master regulator of both flagellar motility and adherence factor secretion and uses remarkably different transcription activation mechanisms depending on its dinucleotide loading state, adenosine triphosphatase (ATPase) activity, interactions with polymerase sigma (σ) factors, and complexation with a second ATPase, FleN. How the FleQ-FleN tandem can exert diverse effects through recognition of a conserved FleQ binding consensus has remained enigmatic. Here, we provide cryogenic electron microscopy (cryo-EM) structures of both c-di-GMP-bound and c-di-GMP-free FleQ-FleN complexes which deepen our understanding of the proteins' (di)nucleotide-dependent conformational switching and fine-tuned roles in gene expression regulation.

Structural analysis of novel drug targets for mitigation of Pseudomonas aeruginosa biofilms

Pseudomonas aeruginosa is an opportunistic human pathogen responsible for acute and chronic, hard to treat infections. Persistence of P. aeruginosa is due to its ability to develop into biofilms, which are sessile bacterial communities adhered to substratum and encapsulated in layers of self-produced exopolysaccharides. These biofilms provide enhanced protection from the host immune system and resilience towards antibiotics, which poses a challenge for treatment. Various strategies have been expended for combating biofilms, which involve inhibiting biofilm formation or promoting their dispersal. The current remediation approaches offer some hope for clinical usage, however, treatment and eradication of preformed biofilms is still a challenge. Thus, identifying novel targets and understanding the detailed mechanism of biofilm regulation becomes imperative. Structure-based drug discovery (SBDD) provides a powerful tool that exploits the knowledge of atomic resolution details of the targets to search for high affinity ligands. This review describes the available structural information on the putative target protein structures that can be utilized for high throughput in silico drug discovery against P. aeruginosa biofilms. Integrating available structural information on the target proteins in readily accessible format will accelerate the process of drug discovery.

Identification of determinants for entering into a viable but nonculturable state in Vibrio alginolyticus by Tn-seq

The viable but nonculturable (VBNC) state is a dormant state of nonsporulating bacteria that enhances survival in adverse environments. Systematic genome-wide research on the genetic basis of VBNC formation is warranted. In this study, we demonstrated that the marine bacterium Vibrio alginolyticus lost culturability but remained viable and entered into the VBNC state when exposed to low nutrient concentrations for prolonged periods of time. Using transposon-insertion sequencing (Tn-seq), we identified 635 determinants governing the formation of the VBNC state, including 322 genes with defective effects on VBNC formation and 313 genes contributing to entry into the VBNC state. Tn-seq analysis revealed that genes involved in various metabolic pathways were shown to have an inhibitory effect on VBNC formation, while genes related to chemotaxis or folate biosynthesis promoted entry into the VBNC state. Moreover, the effects of these genes on the formation of VBNC were validated with the growth of deletion mutants of eight selected genes under nutrient-limited conditions. Interestingly, fleQ and pyrI were identified as essential for entry into the VBNC state, and they affected the formation of the VBNC state independent of RpoE or ToxR regulation. Collectively, these results provide new insights into the mechanism of VBNC formation. KEY POINTS: • Vibrio alginolyticus has the ability to enter into the VBNC state under low nutrient conditions at low temperature. • The 635 determinants for entry into the VBNC state were systematically identified by transposon-insertion sequencing. • PyrI and FleQ were validated to play significant roles in the formation of the VBNC state.

The Vibrio Polar Flagellum: Structure and Regulation

Here we discuss the structure and regulation of the Vibrio flagellum and its role in the virulence of pathogenic species. We will cover some of the novel insights into the structure of this nanomachine that have recently been enabled by cryoelectron tomography. We will also highlight the recent genetic studies that have increased our understanding in flagellar synthesis specifically at the bacterial cell pole, temporal regulation of flagellar genes, and how the flagellum enables directional motility through Run-Reverse-Flick cycles.

Diverse Partners of the Partitioning ParB Protein in Pseudomonas aeruginosa

In the majority of bacterial species, the tripartite ParAB-parS system, composed of an ATPase (ParA), a DNA-binding protein (ParB), and its target parS sequence(s), assists in the chromosome partitioning. ParB forms large nucleoprotein complexes at parS(s), located in the vicinity of origin of chromosomal replication (oriC), which after replication are subsequently positioned by ParA in cell poles. Remarkably, ParA and ParB participate not only in the chromosome segregation but through interactions with various cellular partners they are also involved in other cell cycle-related processes, in a species-specific manner. In this work, we characterized Pseudomonas aeruginosa ParB interactions with the cognate ParA, showing that the N-terminal motif of ParB is required for these interactions, and demonstrated that ParAB-parS-mediated rapid segregation of newly replicated ori domains prevented structural maintenance of chromosome (SMC)-mediated cohesion of sister chromosomes. Furthermore, using proteome-wide techniques, we have identified other ParB partners in P. aeruginosa, which encompass a number of proteins, including the nucleoid-associated proteins NdpA(PA3849) and NdpA2, MinE (PA3245) of Min system, and transcriptional regulators and various enzymes, e.g., CTP synthetase (PA3637). Among them are also NTPases PA4465, PA5028, PA3481, and FleN (PA1454), three of them displaying polar localization in bacterial cells. Overall, this work presents the spectrum of P. aeruginosa ParB partners and implicates the role of this protein in the cross-talk between chromosome segregation and other cellular processes. IMPORTANCE In Pseudomonas aeruginosa, a Gram-negative pathogen causing life-threatening infections in immunocompromised patients, the ParAB-parS system is involved in the precise separation of newly replicated bacterial chromosomes. In this work, we identified and characterized proteins interacting with partitioning protein ParB. We mapped the domain of interactions with its cognate ParA partner and showed that ParB-ParA interactions are crucial for the chromosome segregation and for proper SMC action on DNA. We also demonstrated ParB interactions with other DNA binding proteins, metabolic enzymes, and NTPases displaying polar localization in the cells. Overall, this study uncovers novel players cooperating with the chromosome partition system in P. aeruginosa, supporting its important regulatory role in the bacterial cell cycle.

FlrA-independent production of flagellar proteins is required for proper flagellation in Shewanella putrefaciens

Flagella are multiprotein complexes whose assembly and positioning require complex spatiotemporal control. Flagellar assembly is thought to be controlled by several transcriptional tiers, which are mediated through various master regulators. Here, we revisited the regulation of flagellar genes in polarly flagellated gammaproteobacteria by the regulators FlrA, RpoN (σ⁵⁴ ) and FliA (σ²⁸ ) in Shewanella putrefaciens CN-32 at the transcript and protein level. We found that a number of regulatory and structural proteins were present in the absence of the main regulators, suggesting that initiation of flagella assembly and motor activation relies on the abundance control of only a few structural key components that are required for the formation of the MS- and C-ring and the flagellar type III secretion system. We identified FlrA-independent promoters driving expression of the regulators of flagellar number and positioning, FlhF and FlhG. Reduction of the gene expression levels from these promoters resulted in the emergence of hyperflagellation. This finding indicates that basal expression is required to adjust the flagellar counter in Shewanella. This is adding a deeper layer to the regulation of flagellar synthesis and assembly.

Function and Structure of FlaK, a Master Regulator of the Polar Flagellar Genes in Marine Vibrio

Vibrio alginolyticus has a flagellum at the cell pole, and the fla genes, involved in its formation, are hierarchically regulated in several classes. FlaK (also called FlrA) is an ortholog of Pseudomonas aeruginosa FleQ, an AAA+ ATPase that functions as a master regulator for all later fla genes. In this study, we conducted mutational analysis of FlaK to examine its ATPase activity, ability to form a multimeric structure, and function in flagellation. We cloned flaK and confirmed that its deletion caused a nonflagellated phenotype. We substituted amino acids at the ATP binding/hydrolysis site and at the putative subunit interfaces in a multimeric structure. Mutations in these sites abolished both ATPase activity and the ability of FlaK to induce downstream flagellar gene expression. The L371E mutation, at the putative subunit interface, abolished flagellar gene expression but retained ATPase activity, suggesting that ATP hydrolysis is not sufficient for flagellar gene expression. We also found that FlhG, a negative flagellar biogenesis regulator, suppressed the ATPase activity of FlaK. The 20 FlhG C-terminal residues are critical for reducing FlaK ATPase activity. Chemical cross-linking and size exclusion chromatography revealed that FlaK mostly exists as a dimer in solution and can form multimers, independent of ATP. However, ATP induced the interaction between FlhG and FlaK to form a large complex. The in vivo effects of FlhG on FlaK, such as multimer formation and/or DNA binding, are important for gene regulation. IMPORTANCE FlaK is an NtrC-type activator of the AAA+ ATPase subfamily of σ⁵⁴-dependent promoters of flagellar genes. FlhG, a MinD-like ATPase, negatively regulates the polar flagellar number by collaborating with FlhF, an FtsY-like GTPase. We found that FlaK and FlhG interact in the presence of ATP to form a large complex. Mutational analysis revealed the importance of FlaK ATPase activity in flagellar gene expression and provided a model of the Vibrio molecular mechanism that regulates the flagellar number.

Formation of multiple flagella caused by a mutation of the flagellar rotor protein FliM in Vibrio alginolyticus

Marine bacterium Vibrio alginolyticus forms a single flagellum at a cell pole. In Vibrio, two proteins (GTPase FlhF and ATPase FlhG) regulate the number of flagella. We previously isolated the NMB155 mutant that forms multiple flagella despite the absence of mutations in flhF and flhG. Whole-genome sequencing of NMB155 identified an E9K mutation in FliM that is a component of C-ring in the flagellar rotor. Mutations in FliM result in defects in flagellar formation (fla) and flagellar rotation (che or mot); however, there are a few reports indicating that FliM mutations increase the number of flagella. Here, we determined that the E9K mutation confers the multi-flagellar phenotype and also the che phenotype. The co-expression of wild-type FliM and FliM-E9K indicated that they were competitive in regard to determining the flagellar number. The ATPase activity of FlhG has been correlated with the number of flagella. We observed that the ATPase activity of FlhG was increased by the addition of FliM but not by the addition of FliM-E9K in vitro. This indicates that FliM interacts with FlhG to increase its ATPase activity, and the E9K mutation may inhibit this interaction. FliM may control the ATPase activity of FlhG to properly regulate the number of the polar flagellum at the cell pole. This article is protected by copyright. All rights reserved.

The alternative sigma factors, rpoN1 and rpoN2 are required for mycophagous activity of Burkholderia gladioli strain NGJ1

Bacteria utilize RpoN, an alternative sigma factor (σ54) to grow in diverse habitats, including nitrogen-limiting conditions. Here, we report that a rice-associated mycophagous bacterium Burkholderia gladioli strain NGJ1 encodes two paralogues of rpoN viz. rpoN1 and rpoN2. Both of them are upregulated during 24 h of mycophagous interaction with Rhizoctonia solani, a polyphagous fungal pathogen. Disruption of either one of rpoNs renders the mutant NGJ1 bacterium defective in mycophagy, whereas ectopic expression of respective rpoN genes restores mycophagy in the complementing strains. NGJ1 requires rpoN1 and rpoN2 for efficient biocontrol to prevent R. solani to establish disease in rice and tomato. Further, we have identified 17 genes having RpoN regulatory motif in NGJ1, majority of them encode potential type III secretion system (T3SS) effectors, nitrogen assimilation, and cellular transport-related functions. Several of these RpoN regulated genes as well as certain previously reported T3SS apparatus (hrcC and hrcN) and effector (Bg_9562 and endo-β-1,3-glucanase) encoding genes are upregulated in NGJ1 but not in ΔrpoN1 or ΔrpoN2 mutant bacterium, during mycophagous interaction with R. solani. This highlights that RpoN1 and RpoN2 modulate T3SS, nitrogen assimilation as well as cellular transport systems in NGJ1 and thereby promote bacterial mycophagy.