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Objective: Campylobacter jejuni NCTC11168 is commonly used as a standard strain for flagellar biosynthesis research. In this report, two distinguished phenotypic isolates (CJ1Z, flhA mutant strain, lawn; CJ2S, flhA complemented strain, normal colony) appeared during laboratory passages for NCTC11168. Methods: Phenotypic assessments, including motility plates, transmission electron microscopy, biofilm formation assay, autoagglutination assay, and genome re-sequencing for these two isolates (CJ1Z, flhA mutant strain; CJ2S, flhA complemented strain) were carried out in this study. Results: Transmission electron microscopy revealed that the flagellum was lost in CJ1Z. Phenotypic assessments and genome sequencing of the two isolates were performed in this study. The capacity for biofilm formation, colony auto-agglutination, and isolate motility was reduced in the mutant CJ1Z. Comparative genomic analysis indicated a unique native nucleotide insertion in flhA (nt, 2154) that caused the I719Y and I720Y mutations and early truncation in flhA. Conclusion: FlhA has been found to influence the expression of flagella in C. jejuni. To the best of our knowledge, this is the first study to describe the function of the C-terminal of this protein.

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OBJECTIVE@#Campylobacter jejuni NCTC11168 is commonly used as a standard strain for flagellar biosynthesis research. In this report, two distinguished phenotypic isolates (CJ1Z, flhA mutant strain, lawn; CJ2S, flhA complemented strain, normal colony) appeared during laboratory passages for NCTC11168.@*METHODS@#Phenotypic assessments, including motility plates, transmission electron microscopy, biofilm formation assay, autoagglutination assay, and genome re-sequencing for these two isolates (CJ1Z, flhA mutant strain; CJ2S, flhA complemented strain) were carried out in this study.@*RESULTS@#Transmission electron microscopy revealed that the flagellum was lost in CJ1Z. Phenotypic assessments and genome sequencing of the two isolates were performed in this study. The capacity for biofilm formation, colony auto-agglutination, and isolate motility was reduced in the mutant CJ1Z. Comparative genomic analysis indicated a unique native nucleotide insertion in flhA (nt, 2154) that caused the I719Y and I720Y mutations and early truncation in flhA.@*CONCLUSION@#FlhA has been found to influence the expression of flagella in C. jejuni. To the best of our knowledge, this is the first study to describe the function of the C-terminal of this protein.

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Flagellar structural subunits are transported via the flagellar type III secretion system (fT3SS) and assemble at the distal end of the growing flagellar structure. The C-terminal cytoplasmic domain of FlhA (FlhAC) serves as a docking platform for export substrates and flagellar chaperones and plays an important role in hierarchical protein targeting and export. FlhAC consists of domains D1, D2, D3, and D4 and adopts open and closed conformations. Gly-368 of Salmonella FlhA is located within the highly conserved GYXLI motif and is critical for the dynamic domain motions of FlhAC. However, it remains unclear how it works. Here, we report that periodic conformational changes of the GYXLI motif induce a remodeling of hydrophobic side chain interaction networks in FlhAC and promote the cyclic open-close domain motions of FlhAC. The temperature-sensitive flhA(G368C) mutation stabilized a completely closed conformation at 42°C through strong hydrophobic interactions between Gln-498 of domain D1 and Pro-667 of domain D4 and between Phe-459 of domain D2 and Pro-646 of domain D4, thereby inhibiting flagellar protein export by the fT3SS. Its intragenic suppressor mutations reorganized the hydrophobic interaction networks in the closed FlhAC structure, restoring the protein export activity of the fT3SS to a significant degree. Furthermore, the conformational flexibility of the GYXLI motif was critical for flagellar protein export. We propose that the conserved GYXLI motif acts as a structural switch to induce the dynamic domain motions of FlhAC required for efficient and rapid protein export by the fT3SS. IMPORTANCE Many motile bacteria employ the flagellar type III secretion system (fT3SS) to construct flagella beyond the cytoplasmic membrane. The C-terminal cytoplasmic domain of FlhA (FlhAC), a transmembrane subunit of the fT3SS, provides binding sites for export substrates and flagellar export chaperones to coordinate flagellar protein export with assembly. FlhAC undergoes cyclic open-close domain motions. The highly conserved Gly-368 residue of FlhA is postulated to be critical for dynamic domain motions of FlhAC. However, it remains unknown how it works. Here, we carried out mutational analysis of FlhAC combined with molecular dynamics simulation and provide evidence that the conformational flexibility of FlhAC by Gly-368 is important for remodeling hydrophobic side chain interaction networks in FlhAC to facilitate its cyclic open-close domain motions, allowing the fT3SS to transport flagellar structural subunits for efficient and rapid flagellar assembly.

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In some free-living and pathogenic bacteria, problems in the synthesis and assembly of early flagellar components can cause cell-division defects. However, the mechanism that couples cell division with the flagellar biogenesis has remained elusive. Herein, we discover the regulator MadA that controls transcription of flagellar and cell-division genes in Caulobacter crescentus. We demonstrate that MadA, a small soluble protein, binds the type III export component FlhA to promote activation of FliX, which in turn is required to license the conserved σ54-dependent transcriptional activator FlbD. While in the absence of MadA, FliX and FlbD activation is crippled, bypass mutations in FlhA restore flagellar biogenesis and cell division. Furthermore, we demonstrate that MadA safeguards the divisome stoichiometry to license cell division. We propose that MadA has a sentinel-type function that senses an early flagellar biogenesis event and, through cell-division control, ensures that a flagellated offspring emerges.

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FlhA and FlhB are transmembrane proteins of the flagellar type III protein export apparatus, and their C-terminal cytoplasmic domains (FlhAC and FlhBC) coordinate flagellar protein export with assembly. FlhBC undergoes autocleavage between Asn-269 and Pro-270 in a well-conserved NPTH loop located between FlhBCN and FlhBCC polypeptides and interacts with the C-terminal domain of the FliK ruler when the length of the hook has reached about 55 nm in Salmonella As a result, the flagellar protein export apparatus switches its substrate specificity, thereby terminating hook assembly and initiating filament assembly. The mechanism of export switching remains unclear. Here, we report the role of FlhBC cleavage in the switching mechanism. Photo-cross-linking experiments revealed that the flhB(N269A) and flhB(P270A) mutations did not affect the binding affinity of FlhBC for FliK. Genetic analysis of the flhB(P270A) mutant revealed that the P270A mutation affects a FliK-dependent conformational change of FlhBC, thereby inhibiting the substrate specificity switching. The flhA(A489E) mutation in FlhAC suppressed the flhB(P270A) mutation, suggesting that an interaction between FlhBC and FlhAC is critical for the export switching. We propose that the interaction between FliKC and a cleaved form of FlhBC promotes a conformational change in FlhBC responsible for the termination of hook-type protein export and a structural remodeling of the FlhAC ring responsible for the initiation of filament-type protein export.IMPORTANCE The flagellar type III protein export apparatus coordinates protein export with assembly, which allows the flagellum to be efficiently built at the cell surface. Hook completion is an important morphological checkpoint for the sequential flagellar assembly process. The protein export apparatus switches its substrate specificity from the hook protein to the filament protein upon hook completion. FliK, FlhB, and FlhA are involved in the export-switching process, but the mechanism remains a mystery. By analyzing a slow-cleaving flhB(P270A) mutant, we provide evidence that an interaction between FliK and FlhB induces conformational rearrangements in FlhB, followed by a structural remodeling of the FlhA ring structure that terminates hook assembly and initiates filament formation.

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The bacterial flagellar type III export apparatus consists of a cytoplasmic ATPase complex and a transmembrane export gate complex, which are powered by ATP and proton motive force (PMF) across the cytoplasmic membrane, respectively, and transports flagellar component proteins from the cytoplasm to the distal end of the growing flagellar structure where their assembly occurs (Minamino, 2014). The export gate complex can utilize sodium motive force in addition to PMF when the cytoplasmic ATPase complex does not work properly. A transmembrane export gate protein FlhA acts as a dual ion channel to conduct both H+ and Na+ ( Minamino et al., 2016 ). Here, we describe how to measure the intracellular Na+ concentrations in living Escherichia coli cells using a sodium-sensitive fluorescent dye, CoroNa Green ( Minamino et al., 2016 ). Fluorescence intensity measurements of CoroNa Green by epi-fluorescence microscopy allows us to measure the intracellular Na+ concentration quantitatively.

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For construction of the bacterial flagellum, flagellar proteins are exported via its specific export apparatus from the cytoplasm to the distal end of the growing flagellar structure. The flagellar export apparatus consists of a transmembrane (TM) export gate complex and a cytoplasmic ATPase complex consisting of FliH, FliI, and FliJ. FlhA is a TM export gate protein and plays important roles in energy coupling of protein translocation. However, the energy coupling mechanism remains unknown. Here, we performed a cross-complementation assay to measure robustness of the energy transduction system of the export apparatus against genetic perturbations. Vibrio FlhA restored motility of a Salmonella ΔflhA mutant but not that of a ΔfliH-fliI flhB(P28T) ΔflhA mutant. The flgM mutations significantly increased flagellar gene expression levels, allowing Vibrio FlhA to exert its export activity in the ΔfliH-fliI flhB(P28T) ΔflhA mutant. Pull-down assays revealed that the binding affinities of Vibrio FlhA for FliJ and the FlgN-FlgK chaperone-substrate complex were much lower than those of Salmonella FlhA. These suggest that Vibrio FlhA requires the support of FliH and FliI to efficiently and properly interact with FliJ and the FlgN-FlgK complex. We propose that FliH and FliI ensure robust and efficient energy coupling of protein export during flagellar assembly.

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