Cyclic Di-GMP-Mediated Repression of Swarming Motility by Pseudomonas aeruginosa PA14 Requires the MotAB Stator

The second messenger cyclic diguanylate (c-di-GMP) plays a critical role in the regulation of motility. In Pseudomonas aeruginosa PA14, c-di-GMP inversely controls biofilm formation and surface swarming motility, with high levels of this dinucleotide signal stimulating biofilm formation and repressing swarming. P. aeruginosa encodes two stator complexes, MotAB and MotCD, that participate in the function of its single polar flagellum. Here we show that the repression of swarming motility requires a functional MotAB stator complex. Mutating the motAB genes restores swarming motility to a strain with artificially elevated levels of c-di-GMP as well as stimulates swarming in the wild-type strain, while overexpression of MotA from a plasmid represses swarming motility. Using point mutations in MotA and the FliG rotor protein of the motor supports the conclusion that MotA-FliG interactions are critical for c-di-GMP-mediated swarming inhibition. Finally, we show that high c-di-GMP levels affect the localization of a green fluorescent protein (GFP)-MotD fusion, indicating a mechanism whereby this second messenger has an impact on MotCD function. We propose that when c-di-GMP level is high, the MotAB stator can displace MotCD from the motor, thereby affecting motor function. Our data suggest a newly identified means of c-di-GMP-mediated control of surface motility, perhaps conserved among Pseudomonas, Xanthomonas, and other organisms that encode two stator systems

Structural Insight into the Rotational Switching Mechanism of the Bacterial Flagellar Motor

Structural analysis of a clockwise-biased rotation mutant of the bacterial flagellar rotor protein FliG provides a new model for the arrangement of FliG subunits in the motor, and novel insights into rotation switching

Parts exchange: Tuning the flagellar motor to fit the conditions: MicroCommentary

Many cellular activities are driven by complex protein machines. By measuring the behaviour of fluorescent protein fusions in real time in living cells it has become apparent that many of these complexes are not fixed, but are dynamic. To some extent this might be expected, for example, for cell division complexes, as defining mid-cell is linked to growth and cell cycle, but perhaps comes as more of a surprise with a complex anchored machine like the bacterial flagellar motor. The assumption has been that once made it remains intact. However, the dynamics of this structure is strongly supported in two manuscripts in this issue of Molecular Microbiology. The stator units which form a peptioglycan anchored ring around the rotor, generating torque in response to the ion motive force, clearly disengage when conditions change. The driving ion is shown to be important in both engagement of the stator to the rotor and the selection of the type of stator unit. These new results provide an insight into the mechanisms underlying motor function, which might rely on dynamic processes, and clearly illustrate the need to move away from a static view of cellular structures. © 2008 Blackwell Publishing Ltd

The Microbial Olympics

Following the success of the inaugural games, the Microbial Olympics return with a new series of events and microbial competitors. The games may have moved to a new hosting venue, but the dedication to training, fitness, competition (and yes, education and humour) lives on. Four years have passed since the London games1, where phage burst through to take sprint glory and Rhodobacter dominated in the pool. Where Pseudomonas's disgrace made MRSA mighty and the common cold took relay gold. Winners have become legend, while valiant losers fade from memory. The next generation comes along rapidly in the world of microbial sport though, and a fresh cohort of competitors now rises. Training hard for selection, our new crop of microbial athletes have been honing their fitness and acquiring the skills needed to compete. With operons switched on and secretion systems sharpened, our heroes are ready to do battle once more. The drums are beating, the torch is lit, let the carnival begin. As the sun rises over the newly constructed Nature Microbiology stadium, we welcome you to the Microbial Olympics 2016

Signal-dependent turnover of the bacterial flagellar switch protein FliM

Most biological processes are performed by multiprotein complexes. Traditionally described as static entities, evidence is now emerging that their components can be highly dynamic, exchanging constantly with cellular pools. The bacterial flagellar motor contains ∼13 different proteins and provides an ideal system to study functional molecular complexes. It is powered by transmembrane ion flux through a ring of stator complexes that push on a central rotor. The Escherichia coli motor switches direction stochastically in response to binding of the response regulator CheY to the rotor switch component FliM. Much is known of the static motor structure, but we are just beginning to understand the dynamics of its individual components. Here we measure the stoichiometry and turnover of FliM in functioning flagellar motors, by using high-resolution fluorescence microscopy of E. coli expressing genomically encoded YPet derivatives of FliM at physiological levels. We show that the ∼30 FliM molecules per motor exist in two discrete populations, one tightly associated with the motor and the other undergoing stochastic turnover. This turnover of FliM molecules depends on the presence of active CheY, suggesting a potential role in the process of motor switching. In many ways the bacterial flagellar motor is as an archetype macromolecular assembly, and our results may have further implications for the functional relevance of protein turnover in other large molecular complexes