The Two Chemotaxis Clusters in Caulobacter crescentus Play Different Roles in Chemotaxis and Biofilm Regulation

Chemosensory systems interact to shape relevant traits for bacterial plant pathogenesis

Chemosensory systems allow bacteria to respond and adapt to environmental conditions. Many bacteria contain more than one chemosensory system, but knowledge of their specific roles in regulating different functions remains scarce. Here, we address this issue by analyzing the function of the F6, F8, and alternative (non-motility) cellular functions (ACF) chemosensory systems of the model plant pathogen Pseudomonas syringae pv. tomato. In this work, we assign PsPto chemoreceptors to each chemosensory system, and we visualize for the first time the F6 and F8 chemosensory systems of PsPto using cryo-electron tomography. We confirm that chemotaxis and swimming motility are controlled by the F6 system, and we demonstrate how different components from the F8 and ACF systems also modulate swimming motility. We also determine how the kinase and response regulators from the F6 and F8 chemosensory systems do not work together in the regulation of biofilm, whereas both components from the ACF system contribute together to regulate these traits. Furthermore, we show how the F6, F8, and ACF kinases interact with the ACF response regulator WspR, supporting crosstalk among chemosensory systems. Finally, we reveal how all chemosensory systems play a role in regulating virulence.

IMPORTANCE

Chemoperception through chemosensory systems is an essential feature for bacterial survival, as it allows bacterial interaction with its surrounding environment. In the case of plant pathogens, it is especially relevant to enter the host and achieve full virulence. Multiple chemosensory systems allow bacteria to display a wider plasticity in their response to external signals. Here, we perform a deep characterization of the F6, F8, and alternative (non-motility) cellular functions chemosensory systems in the model plant pathogen Pseudomonas syringae pv. tomato DC3000. These chemosensory systems regulate key virulence-related traits, like motility and biofilm formation. Furthermore, we unveil an unexpected crosstalk among these chemosensory systems at the level of the interaction between kinases and response regulators. This work shows novel results that contribute to the knowledge of chemosensory systems and their role in functions alternative to chemotaxis.

A flagellar accessory protein links chemotaxis to surface sensing

Bacteria find suitable locations for colonization by sensing and responding to surfaces. Complex signaling repertoires control surface colonization, and surface contact sensing by the flagellum plays a central role in activating colonization programs. Caulobacter crescentus adheres to surfaces using a polysaccharide adhesin called the holdfast. In C. crescentus, disruption of the flagellum through interactions with a surface or mutation of flagellar genes increases holdfast production. Our group previously identified several C. crescentus genes involved in flagellar surface sensing. One of these, called fssF, codes for a protein with homology to the flagellar C-ring protein FliN. We show here that a fluorescently tagged FssF protein localizes to the flagellated pole of the cell and requires all components of the flagellar C-ring for proper localization, supporting the model that FssF associates with the C-ring. Deleting fssF results in a severe motility defect that we show is due to a disruption of chemotaxis. Epistasis experiments demonstrate that fssF promotes adhesion through a stator-dependent pathway when late-stage flagellar mutants are disrupted. Separately, we find that disruption of chemotaxis through deletion of fssF or other chemotaxis genes results in a hyperadhesion phenotype. Key genes in the surface sensing network (pleD, motB, and dgcB) contribute to both ∆flgH-dependent and ∆fssF-dependent hyperadhesion, but these genes affect adhesion differently in the two hyperadhesive backgrounds. Our results support a model in which the stator subunits of the flagella incorporate both mechanical and chemical signals to regulate adhesion.

Counterclockwise rotation of the flagellum promotes biofilm initiation in Helicobacter pylori

Motility promotes biofilm initiation during the early steps of this process: microbial surface association and attachment. Motility is controlled in part by chemotaxis signaling, so it seems reasonable that chemotaxis may also affect biofilm formation. There is a gap, however, in our understanding of the interactions between chemotaxis and biofilm formation, partly because most studies analyzed the phenotype of only a single chemotaxis signaling mutant, e.g., cheA. Here, we addressed the role of chemotaxis in biofilm formation using a full set of chemotaxis signaling mutants in Helicobacter pylori, a class I carcinogen that infects more than half the world's population and forms biofilms. Using mutants that lack each chemotaxis signaling protein, we found that chemotaxis signaling affected the biofilm initiation stage, but not mature biofilm formation. Surprisingly, some chemotaxis mutants elevated biofilm initiation, while others inhibited it in a manner that was not tied to chemotaxis ability or ligand input. Instead, the biofilm phenotype correlated with flagellar rotational bias. Specifically, mutants with a counterclockwise bias promoted biofilm initiation, e.g., ∆cheA, ∆cheW, or ∆cheV1; in contrast, those with a clockwise bias inhibited it, e.g., ∆cheZ, ∆chePep, or ∆cheV3. We tested this correlation using a counterclockwise bias-locked flagellum, which induced biofilm formation independent of the chemotaxis system. These CCW flagella, however, were not sufficient to induce biofilm formation, suggesting there are downstream players. Overall, our work highlights the new finding that flagellar rotational direction promotes biofilm initiation, with the chemotaxis signaling system operating as one mechanism to control flagellar rotation.

IMPORTANCE

Chemotaxis signaling systems have been reported to contribute to biofilm formation in many bacteria; however, how they regulate biofilm formation remains largely unknown. Chemotaxis systems are composed of many distinct kinds of proteins, but most previous work analyzed the biofilm effect of loss of only a few. Here, we explored chemotaxis' role during biofilm formation in the human-associated pathogenic bacterium Helicobacter pylori. We found that chemotaxis proteins are involved in biofilm initiation in a manner that correlated with how they affected flagellar rotation. Biofilm initiation was high in mutants with counterclockwise (CCW) flagellar bias and low in those with clockwise bias. We supported the idea that a major driver of biofilm formation is flagellar rotational direction using a CCW-locked flagellar mutant, which stays CCW independent of chemotaxis input and showed elevated biofilm initiation. Our data suggest that CCW-rotating flagella, independent of chemotaxis inputs, are a biofilm-promoting signal.

Physiological responses and molecular mechanisms of biofilm formation induced by extracellular metabolites of euglena in Pseudomonas aeruginosa LNR1 for diesel biodegradation based on transcriptomic and proteomic

Diesel, as a toxic and complex pollutant, is one of the main components in oily wastewater, and poses serious threats to the aquatic environment and the health of organisms. Employing environmentally friendly biostimulants to enhance the metabolic functions of microorganisms is currently the optimal choice to improve the biodegradation of oil-containing wastewater efficiency. This study takes Pseudomonas aeruginosa LNR1 as the target, analyzing the physiological responses and molecular mechanisms of biofilm formation when enhanced by the extracellular metabolites of euglena (EME) for diesel degradation. The results show that EME not only induces auto-aggregation behavior of strain LNR1, forming aerobic suspended granule biofilm, but also promotes the secretion of signaling molecules in the quorum sensing (QS) system. Transcriptomic and proteomic analyses indicate that the stimulatory effect of EME on strain LNR1 mainly manifests in biofilm formation, substance transmembrane transport, signal transduction, and other biological processes, especially the QS system in signal transduction, which plays a significant regulatory role in biofilm formation, chemotaxis, and two-component system (TCS). This study collectively unveils the molecular mechanisms of biostimulant EME inducing strain LNR1 to enhance diesel degradation from different aspects, providing theoretical guidance for the practical application of EME in oily wastewater pollution control.

Diverse domain architectures of CheA histidine kinase, a central component of bacterial and archaeal chemosensory systems

IMPORTANCE

We found that in contrast to the best-studied model organisms, such as Escherichia coli and Bacillus subtilis, most bacterial and archaeal species have a CheA protein with a different domain composition. We report variations in CheA architecture, such as domain duplication and acquisition as well as class-specific domain composition. Our results will be of interest to those working on signal transduction in bacteria and archaea and lay the foundation for experimental studies.

Chemotactic response of p-nitrophenol degrading Pseudomonas asiatica strain PNPG3 through phenotypic and genome sequence-based in silico studies

The Pseudomonas asiatica strain PNPG3 was documented to possess chemotactic potential toward p-nitrophenol (PNP), and other nitroaromatic compounds. Initial screening with drop plate and swarm plate assays demonstrated significant movement of the strain toward the test compounds. A quantitative capillary assay revealed the highest chemotactic potential of the strain toward 4-Aminophenol (4AP), (CI: 12.33); followed by p-benzoquinone (PBQ), (CI: 6.8); and PNP, (CI: 5.33). Gene annotation revealed the presence of chemotactic genes (Che), (Methyl-accepting Proteins) MCPs, rotary motor proteins, and flagellar proteins within the genome of strain PNPG3. The chemotactic machinery of the strain PNPG3 comprised of thirteen Che genes, twenty-two MCPs, eight rotary motors, and thirty-four flagellar proteins that are involved in sensing chemoattractant. Two chemotactic gene clusters were recorded in the genome, of which the major cluster consisted of two copies of CheW, one copy of CheA, CheY, CheZ, one MotD gene, and several Fli genes. Various conserved regions and motifs were documented in them using a standard bioinformatics tool. Genes involved in the chemotaxis of strain PNPG3 were compared with three closely related strains and one distantly related strain belonging to Burkholderia sp. Considering these phenotypic and genotypic data, it can be speculated that it is metabolism-dependent chemotaxis; and that test compound activated the Che. This study indicated that strain PNPG3 could be used as a model organism for the study of the molecular mechanism of chemotaxis and bioremediation of PNP.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03809-3.

A cytoplasmic chemoreceptor and reactive oxygen species mediate bacterial chemotaxis to copper

Chemotaxis is a widespread strategy used by unicellular and multicellular living organisms to maintain their fitness in stressful environments. We previously showed that bacteria can trigger a negative chemotactic response to a copper (Cu)-rich environment. Cu ion toxicity on bacterial cell physiology has been mainly linked to mismetallation events and reactive oxygen species (ROS) production, although the precise role of Cu-generated ROS remains largely debated. Here, using inductively coupled plasma optical emission spectrometry on cell fractionates, we found that the cytoplasmic Cu ion content mirrors variations of the extracellular Cu ion concentration. ROS-sensitive fluorescent probe and biosensor allowed us to show that the increase of cytoplasmic Cu ion content triggers a dose-dependent oxidative stress, which can be abrogated by superoxide dismutase and catalase overexpression. The inhibition of ROS production in the cytoplasm not only improves bacterial growth but also impedes Cu chemotaxis, indicating that ROS derived from cytoplasmic Cu ions mediate the control of bacterial chemotaxis to Cu. We also identified the Cu chemoreceptor McpR, which binds Cu ions with low affinity, suggesting a labile interaction. In addition, we demonstrate that the cysteine 75 and histidine 99 within the McpR sensor domain are key residues in Cu chemotaxis and Cu coordination. Finally, we discovered that in vitro both Cu(I) and Cu(II) ions modulate McpR conformation in a distinct manner. Overall, our study provides mechanistic insights on a redox-based control of Cu chemotaxis, indicating that the cellular redox status can play a key role in bacterial chemotaxis.

Diverse domain architectures of CheA histidine kinase, a central component of bacterial and archaeal chemosensory systems

Chemosensory systems in bacteria and archaea are complex, multi-protein pathways that enable rapid cellular responses to environmental changes. The CheA histidine kinase is a central component of chemosensory systems. In contrast to other histidine kinases, it lacks a sensor (input) domain and utilizes dedicated chemoreceptors for sensing. CheA is a multi-domain protein; in model organisms as diverse as Escherichia coli and Bacillus subtilis, it contains five single-copy domains. Deviations from this canonical domain architecture have been reported, however, a broad genome-wide analysis of CheA diversity is lacking. Here, we present results of a genomic survey of CheA domain composition carried out using an unbiased set of thousands of CheA sequences from bacteria and archaea. We found that four out of five canonical CheA domains comprise a minimal functional unit (core domains), as they are present in all surveyed CheA homologs. The most common deviations from a classical five-domain CheA architecture are the lack of a P2/CheY-binding domain, which is missing from more than a half of CheA homologs and the acquisition of a response regulator receiver (CheY-like) domain, which is present in ~35% of CheA homologs. We also document other deviations from classical CheA architecture, including bipartite CheA proteins, domain duplications and fusions, and reveal that phylogenetically defined CheA classes have pre-dominant domain architectures. This study lays a foundation for a better classification of CheA homologs and identifies targets for experimental investigations.

eDNA-stimulated cell dispersion from Caulobacter crescentus biofilms upon oxygen limitation is dependent on a toxin-antitoxin system

In their natural environment, most bacteria preferentially live as complex surface-attached multicellular colonies called biofilms. Biofilms begin with a few cells adhering to a surface, where they multiply to form a mature colony. When conditions deteriorate, cells can leave the biofilm. This dispersion is thought to be an important process that modifies the overall biofilm architecture and that promotes colonization of new environments. In Caulobacter crescentus biofilms, extracellular DNA (eDNA) is released upon cell death and prevents newborn cells from joining the established biofilm. Thus, eDNA promotes the dispersal of newborn cells and the subsequent colonization of new environments. These observations suggest that eDNA is a cue for sensing detrimental environmental conditions in the biofilm. Here, we show that the toxin-antitoxin system (TAS) ParDE₄ stimulates cell death in areas of a biofilm with decreased O₂ availability. In conditions where O₂ availability is low, eDNA concentration is correlated with cell death. Cell dispersal away from biofilms is decreased when parDE₄ is deleted, probably due to the lower local eDNA concentration. Expression of parDE₄ is positively regulated by O₂ and the expression of this operon is decreased in biofilms where O₂ availability is low. Thus, a programmed cell death mechanism using an O₂-regulated TAS stimulates dispersal away from areas of a biofilm with decreased O₂ availability and favors colonization of a new, more hospitable environment.

A cryptic transcription factor regulates Caulobacter adhesin development

Alphaproteobacteria commonly produce an adhesin that is anchored to the exterior of the envelope at one cell pole. In Caulobacter crescentus this adhesin, known as the holdfast, facilitates attachment to solid surfaces and cell partitioning to air-liquid interfaces. An ensemble of two-component signal transduction (TCS) proteins controls C. crescentus holdfast biogenesis by indirectly regulating expression of HfiA, a potent inhibitor of holdfast synthesis. We performed a genetic selection to discover direct hfiA regulators that function downstream of the adhesion TCS system and identified rtrC, a hypothetical gene. rtrC transcription is directly activated by the adhesion TCS regulator, SpdR. Though its primary structure bears no resemblance to any defined protein family, RtrC binds and regulates dozens of sites on the C. crescentus chromosome via a pseudo-palindromic sequence. Among these binding sites is the hfiA promoter, where RtrC functions to directly repress transcription and thereby activate holdfast development. Either RtrC or SpdR can directly activate transcription of a second hfiA repressor, rtrB. Thus, environmental regulation of hfiA transcription by the adhesion TCS system is subject to control by an OR-gated type I coherent feedforward loop; these regulatory motifs are known to buffer gene expression against fluctuations in regulating signals. We have further assessed the functional role of rtrC in holdfast-dependent processes, including surface adherence to a cellulosic substrate and formation of pellicle biofilms at air-liquid interfaces. Strains harboring insertional mutations in rtrC have a diminished adhesion profile in a competitive cheesecloth binding assay and a reduced capacity to colonize pellicle biofilms in select media conditions. Our results add to an emerging understanding of the regulatory topology and molecular components of a complex bacterial cell adhesion control system.

Phages against Pathogenic Bacterial Biofilms and Biofilm-Based Infections: A Review

Bacterial biofilms formed by pathogens are known to be hundreds of times more resistant to antimicrobial agents than planktonic cells, making it extremely difficult to cure biofilm-based infections despite the use of antibiotics, which poses a serious threat to human health. Therefore, there is an urgent need to develop promising alternative antimicrobial therapies to reduce the burden of drug-resistant bacterial infections caused by biofilms. As natural enemies of bacteria, bacteriophages (phages) have the advantages of high specificity, safety and non-toxicity, and possess great potential in the defense and removal of pathogenic bacterial biofilms, which are considered to be alternatives to treat bacterial diseases. This work mainly reviews the composition, structure and formation process of bacterial biofilms, briefly discusses the interaction between phages and biofilms, and summarizes several strategies based on phages and their derivatives against biofilms and drug-resistant bacterial infections caused by biofilms, serving the purpose of developing novel, safe and effective treatment methods against biofilm-based infections and promoting the application of phages in maintaining human health.

A multiscale 3D chemotaxis assay reveals bacterial navigation mechanisms

How motile bacteria navigate environmental chemical gradients has implications ranging from health to climate science, but the underlying behavioral mechanisms are unknown for most species. The well-studied navigation strategy of Escherichia coli forms a powerful paradigm that is widely assumed to translate to other bacterial species. This assumption is rarely tested because of a lack of techniques capable of bridging scales from individual navigation behavior to the resulting population-level chemotactic performance. Here, we present such a multiscale 3D chemotaxis assay by combining high-throughput 3D bacterial tracking with microfluidically created chemical gradients. Large datasets of 3D trajectories yield the statistical power required to assess chemotactic performance at the population level, while simultaneously resolving the underlying 3D navigation behavior for every individual. We demonstrate that surface effects confound typical 2D chemotaxis assays, and reveal that, contrary to previous reports, Caulobacter crescentus breaks with the E. coli paradigm.

Nutrient complexity triggers transitions between solitary and colonial growth in bacterial populations

Microbial populations often experience fluctuations in nutrient complexity in their natural environment such as between high molecular weight polysaccharides and simple monosaccharides. However, it is unclear if cells can adopt growth behaviors that allow individuals to optimally respond to differences in nutrient complexity. Here, we directly control nutrient complexity and use quantitative single-cell analysis to study the growth dynamics of individuals within populations of the aquatic bacterium Caulobacter crescentus. We show that cells form clonal microcolonies when growing on the polysaccharide xylan, which is abundant in nature and degraded using extracellular cell-linked enzymes; and disperse to solitary growth modes when the corresponding monosaccharide xylose becomes available or nutrients are exhausted. We find that the cellular density required to achieve maximal growth rates is four-fold higher on xylan than on xylose, indicating that aggregating is advantageous on polysaccharides. When collectives on xylan are transitioned to xylose, cells start dispersing, indicating that colony formation is no longer beneficial and solitary behaviors might serve to reduce intercellular competition. Our study demonstrates that cells can dynamically tune their behaviors when nutrient complexity fluctuates, elucidates the quantitative advantages of distinct growth behaviors for individual cells and indicates why collective growth modes are prevalent in microbial populations.

Diversity of bacterial chemosensory systems

Chemosensory system is the most complex, specialized mode of signal transduction in bacteria and archaea. It is composed of several core and auxiliary protein components that are highly organized in order to deliver a fast response to changing environmental conditions. Chemosensory pathways were studied in-depth in a handful of model organisms and experimentally characterized at least to some degree in approximately thirty other species. However, genome-wide analyses have revealed their presence in thousands of sequenced microbial genomes. Both experimental and computational studies uncovered substantial diversity in system design, functional regulation, cellular localization and phyletic distribution of chemosensory pathways. Here, we summarize advances and expose gaps in our current understanding of the diversity of chemosensory systems.

The phosphorylated regulator of chemotaxis is crucial throughout biofilm biogenesis in Shewanella oneidensis

The core of the chemotaxis system of Shewanella oneidensis is made of the CheA3 kinase and the CheY3 regulator. When appropriated, CheA3 phosphorylates CheY3, which, in turn, binds to the rotor of the flagellum to modify the swimming direction. In this study, we showed that phosphorylated CheY3 (CheY3-P) also plays an essential role during biogenesis of the solid-surface-associated biofilm (SSA-biofilm). Indeed, in a ΔcheY3 strain, the formation of this biofilm is abolished. Using the phospho-mimetic CheY3D56E mutant, we showed that CheY-P is required throughout the biogenesis of the biofilm but CheY3 phosphorylation is independent of CheA3 during this process. We have recently found that CheY3 interacts with two diguanylate cyclases (DGCs) and with MxdA, the c-di-GMP effector, probably triggering exopolysaccharide synthesis by the Mxd machinery. Here, we discovered two additional DGCs involved in SSA-biofilm development and showed that one of them interacts with CheY3. We therefore propose that CheY3-P acts together with DGCs to control SSA-biofilm formation. Interestingly, two orthologous CheY regulators complement the biofilm defect of a ΔcheY3 strain, supporting the idea that biofilm formation could involve CheY regulators in other bacteria.

Comparative proteomic analysis reveals novel potential virulence factors of Aeromonas veronii

Aeromonas veronii is an important zoonotic and aquatic pathogen. An increasing number of reports indicate that it has caused substantial economic losses in the aquaculture industry, in addition to threatening human health. However, little is known about its pathogenesis. Exploration of new virulence factors of A. veronii would be helpful for further understanding its pathogenesis. Hence, we comparatively analyzed the proteomes of virulent, attenuated, and avirulent strains of A. veronii using tandem mass tag (TMT) protein labeling and found numerous proteins either up- or downregulated in the virulent strain. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses showed that these differentially expressed proteins (DEPs) were involved mainly in pathways associated with bacterial chemotaxis and microbial metabolism in diverse environments. Furthermore, the expression levels of lysine decarboxylase, endoribonuclease, maltoporin, pullulanase, and aerolysin were positively correlated with the virulence of the strains, suggesting that their function may be closely related to the virulence of A. veronii. The results of qRT-PCR and multiple reaction monitoring for some DEPs were consistent with the results of TMT protein labeling. These results suggest that these DEPs may be novel potential virulence factors and will help to further understand the pathogenesis of A. veronii.

Substratum-associated microbiota

Highlights of new, interesting, and emerging research findings on substratum-associated microbiota covered from a survey of 2019 literature from primarily freshwaters provide insight into research trends of interest to the Water Environment Federation and others interested in benthic, aquatic environments. Coverage of topics on bottom-associated or attached algae and cyanobacteria, though not comprehensive, includes new methods, taxa new-to-science, nutrient dynamics, auto- and heterotrophic interactions, grazers, bioassessment, herbicides and other pollutants, metal contaminants, and nuisance, and bloom-forming and harmful algae. Coverage of bacteria, also not comprehensive, focuses on the ecology of benthic biofilms and microbial communities, along with the ecology of microbes like Caulobacter crescentus, Rhodobacter, and other freshwater microbial species. Bacterial topics covered also include metagenomics and metatranscriptomics, toxins and pollutants, bacterial pathogens and bacteriophages, and bacterial physiology. Readers may use this literature review to learn about or renew their interest in the recent advances and discoveries regarding substratum-associated microbiota. PRACTITIONER POINTS: This review of literature from 2019 on substratum-associated microbiota presents highlights of findings on algae, cyanobacteria, and bacteria from primarily freshwaters. Coverage of algae and cyanobacteria includes findings on new methods, taxa new to science, nutrient dynamics, auto- and heterotrophic interactions, grazers, bioassessment, herbicides and other pollutants, metal contaminants, and nuisance, bloom-forming and harmful algae. Coverage of bacteria includes findings on ecology of benthic biofilms and microbial communities, the ecology of microbes, metagenomics and metatranscriptomics, toxins and pollutants, bacterial pathogens and bacteriophages, and bacterial physiology. Highlights of new, noteworthy and emerging topics build on those from 2018 and will be of relevance to the Water Environment Federation and others interested in benthic, aquatic environments.

Programmed Proteolysis of Chemotaxis Proteins in Sinorhizobium meliloti: Features in the C-Terminal Region Control McpU Degradation

Chemotaxis and motility are important traits that support bacterial survival in various ecological niches and in pathogenic and symbiotic host interaction. Chemotactic stimuli are sensed by chemoreceptors or methyl-accepting chemotaxis proteins (MCPs), which direct the swimming behavior of the bacterial cell. In this study, we present evidence that the cellular abundance of chemoreceptors in the plant symbiont Sinorhizobium meliloti can be altered by the addition of several to as few as one amino acid residues and by including common epitope tags such as 3×FLAG and 6×His at their C termini. To further dissect this phenomenon and its underlying molecular mechanism, we focused on a detailed analysis of the amino acid sensor McpU. Controlled proteolysis is important for the maintenance of an appropriate stoichiometry of chemoreceptors and between chemoreceptors and chemotactic signaling proteins, which is essential for an optimal chemotactic response. We hypothesized that enhanced stability is due to interference with protease binding, thus affecting proteolytic efficacy. Location of the protease recognition site was defined through McpU stability measurements in a series of deletion and amino acid substitution mutants. Deletions in the putative protease recognition site had similar effects on McpU abundance, as did extensions at the C terminus. Our results provide evidence that the programmed proteolysis of chemotaxis proteins in S. meliloti is cell cycle regulated. This posttranslational control, together with regulatory pathways on the transcriptional level, limits the chemotaxis machinery to the early exponential growth phase. Our study identified parallels to cell cycle-dependent processes during asymmetric cell division in Caulobacter crescentus IMPORTANCE The symbiotic bacterium Sinorhizobium meliloti contributes greatly to growth of the agriculturally valuable host plant alfalfa by fixing atmospheric nitrogen. Chemotaxis of S. meliloti cells toward alfalfa roots mediates this symbiosis. The present study establishes programmed proteolysis as a factor in the maintenance of the S. meliloti chemotaxis system. Knowledge about cell cycle-dependent, targeted, and selective proteolysis in S. meliloti is important to understand the molecular mechanisms of maintaining a suitable chemotaxis response. While the role of regulated protein turnover in the cell cycle progression of Caulobacter crescentus is well understood, these pathways are just beginning to be characterized in S. meliloti In addition, our study should alert about the cautionary use of epitope tags for protein quantification.