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In Escherichia coli and Salmonella , the c-di-GMP effector YcgR inhibits flagellar motility by interacting directly with the motor to alter both its bias and speed. Here, we demonstrate that in both of these bacteria, YcgR acts sequentially, altering motor bias first and then decreasing motor speed. We show that when c-di-GMP levels are high, deletion of ycgR restores wild-type motor behavior in E. coli , indicating that YcgR is the only motor effector in this bacterium. Yet, motility and chemotaxis in soft agar do not return to normal, suggesting that there is a second mechanism that inhibits motility under these conditions. In Salmonella , c-di-GMP-induced synthesis of extracellular cellulose has been reported to entrap flagella and to be responsible for the YcgR-independent motility defect. We found that this is not the case in E. coli Instead, we found through reversion analysis that deletion of rssB , which codes for a response regulator/adaptor protein that normally directs ClpXP protease to target σ S for degradation, restored wild-type motility in the ycgR mutant. Our data suggest that high c-di-GMP levels may promote altered interactions between these proteins to downregulate flagellar gene expression. IMPORTANCE Flagellum-driven motility has been studied in E. coli and Salmonella for nearly half a century. Over 60 genes control flagellar assembly and function. The expression of these genes is regulated at multiple levels in response to a variety of environmental signals. Cues that elevate c-di-GMP levels, however, inhibit motility by direct binding of the effector YcgR to the flagellar motor. In this study conducted mainly in E. coli , we show that YcgR is the only effector of motor control and tease out the order of YcgR-mediated events. In addition, we find that the σ S regulator protein RssB contributes to negative regulation of flagellar gene expression when c-di-GMP levels are elevated., (Copyright © 2019 American Society for Microbiology.)

A sensory adaptation system that tunes chemoreceptor sensitivity enables motile Escherichia coli cells to track chemical gradients with high sensitivity over a wide dynamic range. Sensory adaptation involves feedback control of covalent receptor modifications by two enzymes: CheR, a methyltransferase, and CheB, a methylesterase. This study describes a CheR function that opposes the signaling consequences of its catalytic activity. In the presence of CheR, a variety of mutant serine chemoreceptors displayed up to 40-fold enhanced detection sensitivity to chemoeffector stimuli. This response enhancement effect did not require the known catalytic activity of CheR, but did involve a binding interaction between CheR and receptor molecules. Response enhancement was maximal at low CheR:receptor stoichiometry and quantitative analyses argued against a reversible binding interaction that simply shifts the ON-OFF equilibrium of receptor signaling complexes. Rather, a short-lived CheR binding interaction appears to promote a long-lasting change in receptor molecules, either a covalent modification or conformation that enhances their response to attractant ligands., Competing Interests: The authors declare no conflict of interest.