Precise Measurement of the Stoichiometry of the Adaptive Bacterial Flagellar Switch

The cytoplasmic ring (C-ring) of the bacterial flagellar motor controls the motor rotation direction, thereby controlling bacterial run-and-tumble behavior. The C-ring has been shown to undergo adaptive remodeling in response to changes in motor directional bias. However, the stoichiometry and arrangement of the C-ring is still unclear due to contradiction between the results from fluorescence studies and cryo-electron microscopy (cryo-EM) structural analysis. Here, by using the copy number of FliG molecules (34) in the C-ring as a reference, we precisely measured the copy numbers of FliM molecules in motors rotating exclusively counterclockwise (CCW) and clockwise (CW). We surprisingly found that there are on average 45 and 58 FliM molecules in CW and CCW rotating motors, respectively, which are much higher than previous estimates. Our results suggested a new mechanism of C-ring adaptation, that is, extra FliM molecules could be bound to the primary C-ring with probability depending on the motor rotational direction. We further confirmed that all of the FliM molecules in the C-ring function in chemotaxis signaling transduction because all of them could be bound by the chemotactic response regulator CheY-P. Our measurements provided new insights into the structure and arrangement of the flagellar switch. IMPORTANCE The bacterial flagellar switch can undergo adaptive remodeling in response to changes in motor rotation direction, thereby shifting its operating point to match the output of the chemotaxis signaling pathway. However, it remains unclear how the flagellar switch accomplishes this adaptive remodeling. Here, via precise fluorescence studies, we measured the absolute copy numbers of the critical component in the switch for motors rotating counterclockwise and clockwise, obtaining much larger numbers than previous relative estimates. Our results suggested a new mechanism of adaptive remodeling of the flagellar switch and provided new insights for updating the conformation spread model of the switch.

Mutations involved in the emergence of Yersinia ruckeri biotype 2 in France

Yersina ruckeri is an enterobacteria responsible for Enteric redmouth disease (ERM), which causes significant economic losses in the aquaculture industry worldwide. Two biotypes have been described within Y. ruckeri: biotype 1 (BT1) and biotype 2 (BT2). Unlike BT1, BT2 is negative for motility and lipase secretion. The emergence of BT2 Y. ruckeri has been associated with disease outbreaks in vaccinated fish in several countries, notably France in the early 2000s. In this study, 15 BT2 strains (14 BT2 strains isolated in France and the BT2 reference strain EX5) were studied to compare the phenotypic characters of the BT1 and BT2 strains and to determine the genetic origin of the emergence of BT2 in France. BT1 bacteria are significantly longer in size than BT2 bacteria (a difference of 0.222 µm). The loss of motility of some French BT2 strains could be due to the loss of their ability to produce flagella caused by three mutations within the fliG, flhC and flgA genes. In the light of these results, the emergence of BT2 Yersinia ruckeri in France is discussed.

[Construction and characterization of a recombinant mutant homolog of the CheW protein from Thermotoga petrophila RKU-1]

In the work a recombinant chemotaxis protein CheW from Thermotoga petrophila RKU-1 (TpeCheW) and its mutant homolog (TpeCheW-mut) were created. It was shown that, despite the low homology with CheW prototypes from intestinal bacteria, these proteins didn't cause metabolic overload and were well expressed by cells of E. coli laboratory strains. We have discovered a broad spectrum of industrial valuable properties of the TpeCheW-mut protein such as stability in a wide range of temperatures and pH, high expression level, solubility and possibility of the application of a simple low-stage purification methodology with the use of preliminary heat treatment. Possible directions of the scientific and industrial application of this protein were claimed.

Adaptive remodelling by FliN in the bacterial rotary motor

Sensory adaptation in the Escherichia coli chemosensory pathway has been the subject of interest for decades, with investigation focusing on the receptors that process extracellular inputs. Recent studies demonstrate that the flagellar motors responsible for cell locomotion also play a role, adding or subtracting FliM subunits to maximise sensitivity to pathway signals. It is difficult to reconcile this FliM remodelling with the observation that partner FliN subunits are relatively static fixtures in the motor. By fusing a fluorescent protein internally to FliN, we show that there is in fact significant FliN remodelling. The kinetics and stoichiometry of FliN in steady state and in adapting motors are investigated and found to match the behaviour of FliM in all respects except for timescale where FliN rates are about 4 times slower. We notice that motor adaptation is slower in the presence of the fluorescent protein, indicating a possible source for the difference. The behaviour of FliM and FliN is consistent with a kinetic and stoichiometric model that contradicts the traditional view of a packed, rigid motor architecture.

Mechanism for adaptive remodeling of the bacterial flagellar switch

The bacterial flagellar motor has been shown in previous work to adapt to changes in the steady-state concentration of the chemotaxis signaling molecule, CheY-P, by changing the FliM content. We show here that the number of FliM molecules in the motor and the fraction of FliM molecules that exchange depend on the direction of flagellar rotation, not on CheY-P binding per se. Our results are consistent with a model in which the structural differences associated with the direction of rotation modulate the strength of FliM binding. When the motor spins counterclockwise, FliM binding strengthens, the fraction of FliM molecules that exchanges decreases, and the ring content increases. The larger number of CheY-P binding sites enhances the motor's sensitivity, i.e., the motor adapts. An interesting unresolved question is how additional copies of FliM might be accommodated.

Structure of flagellar motor proteins in complex allows for insights into motor structure and switching

The flagellar motor is one type of propulsion device of motile bacteria. The cytoplasmic ring (C-ring) of the motor interacts with the stator to generate torque in clockwise and counterclockwise directions. The C-ring is composed of three proteins, FliM, FliN, and FliG. Together they form the "switch complex" and regulate switching and torque generation. Here we report the crystal structure of the middle domain of FliM in complex with the middle and C-terminal domains of FliG that shows the interaction surface and orientations of the proteins. In the complex, FliG assumes a compact conformation in which the middle and C-terminal domains (FliG(MC)) collapse and stack together similarly to the recently published structure of a mutant of FliG(MC) with a clockwise rotational bias. This intramolecular stacking of the domains is distinct from the intermolecular stacking seen in other structures of FliG. We fit the complex structure into the three-dimensional reconstructions of the motor and propose that the cytoplasmic ring is assembled from 34 FliG and FliM molecules in a 1:1 fashion.

A molecular brake, not a clutch, stops the Rhodobacter sphaeroides flagellar motor

Many bacterial species swim by employing ion-driven molecular motors that power the rotation of helical filaments. Signals are transmitted to the motor from the external environment via the chemotaxis pathway. In bidirectional motors, the binding of phosphorylated CheY (CheY-P) to the motor is presumed to instigate conformational changes that result in a different rotor-stator interface, resulting in rotation in the alternative direction. Controlling when this switch occurs enables bacteria to accumulate in areas favorable for their survival. Unlike most species that swim with bidirectional motors, Rhodobacter sphaeroides employs a single stop-start flagellar motor. Here, we asked, how does the binding of CheY-P stop the motor in R. sphaeroides--using a clutch or a brake? By applying external force with viscous flow or optical tweezers, we show that the R. sphaeroides motor is stopped using a brake. The motor stops at 27-28 discrete angles, locked in place by a relatively high torque, approximately 2-3 times its stall torque.