The maximum number of torque-generating units in the flagellar motor of Escherichia coli is at least 11

Abstract

Torque is generated in the rotary motor at the base of the bacterial flagellum by ion translocating stator units anchored to the peptidoglycan cell wall. Stator units are composed of the proteins MotA and MotB in proton-driven motors, and they are composed of PomA and PomB in sodium-driven motors. Strains of Escherichia coli lacking functional stator proteins produce flagella that do not rotate, and induced expression of the missing proteins leads to restoration of motor rotation in discrete speed increments, a process known as ''resurrection.'' Early work suggested a maximum of eight units. More recent indications that WT motors may contain more than eight units, based on recovery of disrupted motors, are inconclusive. Here we demonstrate conclusively that the maximum number of units in a motor is at least 11. Using back-focal-plane interferometry of 1-m polystyrene beads attached to flagella, we observed at least 11 distinct speed increments during resurrection with three different combinations of stator proteins in E. coli. The average torques generated by a single unit and a fully induced motor were lower than previous estimates. Speed increments at high numbers of units are smaller than those at low numbers, indicating that not all units in a fully induced motor are equivalent.T he flagellar motor is the mechanism of propulsion for most swimming bacteria (1-5). In Escherichia coli Ϸ40 gene products are required for motor assembly, with Ϸ20 of them being present in the final structure. Each motor drives a helical filament up to Ϸ10 m long. The flagellar motor spans the inner and outer bacterial membranes (Fig. 1). Torque is generated by interactions between the rotor protein FliG, located at the intersection between the MS and C rings, and the stator units attached to the cell wall (5). Each unit is believed to contain two copies of MotB and four copies of MotA in proton-driven motors of E. coli, two copies of PomB and four copies of PomA in sodium-driven motors of Vibrio alginolyticus (6, 7), and function as an ion channel (8, 9). Ion flux through these channels powers the motor (10, 11). Functional chimeras have been engineered containing components of proton-and sodium-driven motors (12), indicating that the structure and mechanisms of both types of motor are very similar. The torque-speed relationship of the motor has been measured by using electrorotation of tethered cells (13) and attaching varying viscous loads (14-16). A notable feature is a regime between stall and a speed of Ϸ175 Hz in WT E. coli at room temperature, over which torque falls linearly with increasing speed to Ϸ90% of the value at stall. At higher speeds torque falls more steeply, eventually to zero at a speed of Ϸ350 Hz.Successive incorporation of torque-generating units to restore rotation in paralyzed motors is known as resurrection (17). The maximum number of speed increments previously seen during resurrection of a single motor, using inducible plasmids to express missing stator proteins, was eight (18). However, in experiments w...

Evidence for symmetry in the elementary process of bidirectional torque generation by the bacterial flagellar motor

“…S3). Because the torque at high load is proportional to the number of stators (10,25,26), we estimated the number of functional stators in the motor by measuring the rotation rates of 1.0-μm beads attached to partially sheared, sticky filaments of SJW46. Fig.…”

“…MotA and MotB form the MotA 4 MotB 2 complex within the cytoplasmic membrane (7,8) and act as the stator that conducts proton (9). A fully functional motor contains at least 11 stator units around the rotor (10). FliF, FliG, FliM, and FliN form the rotor called the MS-C ring; FliF forms the MS ring, which spans through the cytoplasmic membrane with the M ring portion, and the remaining three proteins form the C ring on the cytoplasmic side of the MS ring (11).…”

Effect of gamma-ray irradiation on Escherichia coli motility

“…Currently, it is not known how the presence of active and inactive motors in a single cell affects the swimming behavior of bacterial cells. According to a well-accepted model for motor construction [15][16][17][18][19][20], each flagellar motor in E. coli has about 8 ~ 11 force-generating units composed of MotAB complexes, which are thought to be the reaction center of the molecular motor. These force-generating units may work independently and additively.…”

A 3D Motile Rod-Shaped Monotrichous Bacterial Model

“…Stall torque estimates vary from 350 pN nm for Caulobacter crescentus [69], 1260 pN nm for E. coli [87] and 4000 pN nm for V. alginolyticus [95]. For comparison, we see that the computed flagellar torque shown in Table 4 for the 0.2 micron amplitude model bacterial cell converts to 5640 pN nm.…”