Domain-based biophysical characterization of the structural and thermal stability of FliG, an essential rotor component of the Na+-driven flagellar motor

The bio-decolorization of methyl orange by S.putrefaciens CN32 and responding mechanism to salinity stress based on transcriptomic analysis

Salinity poses a significant obstacle to the bio-decolorization of azo dyes. However, the underlying responding mechanisms of bio-decolorization bacteria remain unclear. Shewanella putrefaciens CN32 (S.P CN32) can reduce azo dyes through special electron transfer pathways. Hence, this study used methyl orange (MO) as a representative of azo dyes to investigate azo dye decolorization by S.P CN32 and to explore the responding mechanism of S.P CN32 to salinity stress in the decolorization process. More than 95 % of MO was decolorized by S.P CN32 under the conditions of pH 6-10, MO concentration high than 850 mg/L, and salinity 0-2 %. Lactate was the optimal electron donor in MO decolorization process by S.P CN32. The complete decolorization time was lagged by 30 h under 2 % salinity. FTIR and LC-MS were utilized to identify metabolites and analysis possible metabolic pathways in MO decolorization process. The expression changes of genes involved in MO bio-decolorization and response to salinity stress were characterized by transcriptome analysis. The salinity inhibition on MO decolorization was linked to the down-regulation of azoreductase-associated electron transport pathways and electron transfer chains including Mtr pathway, NADH dehydrogenase, mequinones and heme synthesis. But the up-regulation of flavins synthesis could slightly remit the inhibition. When exposed to salinity, bacteria up-regulated genes expression associated with Na+/K+ transport, the transfer and biosynthesis of proline and glutamate. Most of genes related to energy production by lactate metabolism, TCA cycle and fatty acid metabolism also up-regulated, while most genes related to cell motility down-regulated. This is conducive to supply energy for adapting salinity stress to survive. This study demonstrated the potential role of S.P CN32 in MO decolorization process, and may provide some new insights into the bio-treatment of textile wastewater with high salinity from the perspective of the molecular mechanism.

Changes in the hydrophobic network of the FliGMC domain induce rotational switching of the flagellar motor

The FliG protein plays a pivotal role in switching the rotational direction of the flagellar motor between clockwise and counterclockwise. Although we previously showed that mutations in the Gly-Gly linker of FliG induce a defect in switching rotational direction, the detailed molecular mechanism was not elucidated. Here, we studied the structural changes in the FliG fragment containing the middle and C-terminal regions, named FliG_MC, and the switch-defective FliG_MC-G215A, using nuclear magnetic resonance (NMR) and molecular dynamics simulations. NMR analysis revealed multiple conformations of FliG_MC, and the exchange process between these conformations was suppressed by the G215A residue substitution. Furthermore, changes in the intradomain orientation of FliG were induced by changes in hydrophobic interaction networks throughout FliG. Our finding applies to FliG in a ring complex in the flagellar basal body, and clarifies the switching mechanism of the flagellar motor.

Site-Specific Isotope Labeling of FliG for Studying Structural Dynamics Using Nuclear Magnetic Resonance Spectroscopy

To understand flagella-driven motility of bacteria, it is important to understand the structure and dynamics of the flagellar motor machinery. We have conducted structural dynamics analyses using solution nuclear magnetic resonance (NMR) to elucidate the detailed functions of flagellar motor proteins. Here, we introduce the analysis of the FliG protein, which is a flagellar motor protein, focusing on the preparation method of the original stable isotope-labeled protein.

Rotational direction of flagellar motor from the conformation of FliG middle domain in marine Vibrio

FliG, which is composed of three distinctive domains, N-terminal (N), middle (M), and C-terminal (C), is an essential rotor component that generates torque and determines rotational direction. To determine the role of FliG in determining flagellar rotational direction, we prepared rotational biased mutants of fliG in Vibrio alginolyticus. The E144D mutant, whose residue is belonging to the EHPQR-motif in FliG_M, exhibited an increased number of switching events. This phenotype generated a response similar to the phenol-repellent response in chemotaxis. To clarify the effect of E144D mutation on the rotational switching, we combined the mutation with other che mutations (G214S, G215A and A282T) in FliG. Two of the double mutants suppressed the rotational biased phenotype. To gain structural insight into the mutations, we performed molecular dynamic simulations of the FliG_MC domain, based on the crystal structure of Thermotoga maritima FliG and nuclear magnetic resonance analysis. Furthermore, we examined the swimming behavior of the fliG mutants lacking CheY. The results suggested that the conformation of FliG in E144D mutant was similar to that in the wild type. However, that of G214S and G215A caused a steric hindrance in FliG. The conformational change in FliG_M triggered by binding CheY may lead to a rapid change of direction and may occur in both directional states.

Flagellar Basal Body Structural Proteins FlhB, FliM, and FliY Are Required for Flagellar-Associated Protein Expression in Listeria monocytogenes

Listeria monocytogenes is a food-associated bacterium that is responsible for food-related illnesses worldwide. In the L. monocytogenes EGD-e genome, FlhB, FliM, and FliY (encoded by lmo0679, lmo0699, and lmo0700, respectively) are annotated as putative flagella biosynthesis factors, but their functions remain unknown. To explore whether FlhB, FliM, and FliY are involved in Listeria flagella synthesis, we constructed flhB, fliM, fliY, and other flagellar-related gene deletion mutants using a homologous recombination strategy. Then, we analyzed the motility, flagella synthesis, and protein expression of these mutant strains. Motility and flagella synthesis were completely abolished in the absence of flhB, fliM, or fliY. These impaired phenotypes were fully restored in the complemented strains CΔflhB, CΔfliM, and CΔfliY. The transcriptional levels of flagellar-related genes, including flaA, fliM, fliY, lmo0695, lmo0698, fliI, and fliS, were downregulated markedly in the absence of flhB, fliM, or fliY. Deletion of flhB resulted in the complete abolishment of FlaA expression, while it decreased FliM and FliY expression. The expression of FlaA was abolished completely in the absence of fliM or fliY. No significant changes were found in the expression of FlhF and two flagella synthesis regulatory factors, MogR and GmaR. We demonstrate for the first time that FlhB, FliM, and FliY not only mediate Listeria motility, but also are involved in regulating flagella synthesis. This study provides novel insights that increase our understanding of the roles played by FlhB, FliM, and FliY in the flagellar type III secretion system in L. monocytogenes.