FlhE functions as a chaperone to prevent formation of periplasmic flagella in Gram-negative bacteria.

The bacterial flagellum, which facilitates motility, is composed of ~20 structural proteins organized into a long extracellular filament connected to a cytoplasmic rotor-stator complex via a periplasmic rod. Flagellum assembly is regulated by multiple checkpoints that ensure an ordered gene expression pattern coupled to the assembly of the various building blocks. Here, we use epifluorescence, super-resolution, and transmission electron microscopy to show that the absence of a periplasmic protein (FlhE) prevents proper flagellar morphogenesis and results in the formation of periplasmic flagella in Salmonella enterica. The periplasmic flagella disrupt cell wall synthesis, leading to a loss of normal cell morphology resulting in cell lysis. We propose that FlhE functions as a periplasmic chaperone to control assembly of the periplasmic rod, thus preventing formation of periplasmic flagella.

FlhE functions as a chaperone to prevent formation of periplasmic flagella in Gram-negative bacteria.

The bacterial flagellum is an organelle utilized by many Gram-negative bacteria to facilitate motility. The flagellum is composed of a several µm long, extracellular filament that is connected to a cytoplasmic rotor-stator complex via a periplasmic rod. Composed of ∼20 structural proteins, ranging from a few subunits to several thousand building blocks, the flagellum is a paradigm of a complex macromolecular structure that utilizes a highly regulated assembly process. This process is governed by multiple checkpoints that ensure an ordered gene expression pattern coupled to the assembly of the various flagellar building blocks in order to produce a functional flagellum. Using epifluorescence, super-resolution STED and transmission electron microscopy, we discovered that in Salmonella , the absence of one periplasmic protein, FlhE, prevents proper flagellar morphogenesis and results in the formation of periplasmic flagella. The periplasmic flagella disrupt cell wall synthesis, leading to a loss of the standard cell morphology resulting in cell lysis. We propose a model where FlhE functions as a periplasmic chaperone to control assembly of the periplasmic rod to prevent formation of periplasmic flagella. Our results highlight that bacteria evolved sophisticated regulatory mechanisms to control proper flagellar assembly and minor deviations from this highly regulated process can cause dramatic physiological consequences.

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.

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.

Evolutionary history of MEK1 illuminates the nature of deleterious mutations.

Mutations in signal transduction pathways lead to various diseases including cancers. MEK1 kinase, encoded by the human MAP2K1 gene, is one of the central components of the MAPK pathway and more than a hundred somatic mutations in the MAP2K1 gene were identified in various tumors. Germline mutations deregulating MEK1 also lead to congenital abnormalities, such as the cardiofaciocutaneous syndrome and arteriovenous malformation. Evaluating variants associated with a disease is a challenge, and computational genomic approaches aid in this process. Establishing evolutionary history of a gene improves computational prediction of disease-causing mutations; however, the evolutionary history of MEK1 is not well understood. Here, by revealing a precise evolutionary history of MEK1, we construct a well-defined dataset of MEK1 metazoan orthologs, which provides sufficient depth to distinguish between conserved and variable amino acid positions. We matched known and predicted disease-causing and benign mutations to evolutionary changes observed in corresponding amino acid positions and found that all known and many suspected disease-causing mutations are evolutionarily intolerable. We selected several variants that cannot be unambiguously assessed by automated prediction tools but that are confidently identified as "damaging" by our approach, for experimental validation in Drosophila. In all cases, evolutionary intolerant variants caused increased mortality and severe defects in fruit fly embryos confirming their damaging nature. We anticipate that our analysis will serve as a blueprint to help evaluate known and novel missense variants in MEK1 and that our approach will contribute to improving automated tools for disease-associated variant interpretation.

Two disulfide-reducing pathways are required for the maturation of plastid c-type cytochromes in Chlamydomonas reinhardtii.

In plastids, conversion of light energy into ATP relies on cytochrome f, a key electron carrier with a heme covalently attached to a CXXCH motif. Covalent heme attachment requires reduction of the disulfide-bonded CXXCH by CCS5 and CCS4. CCS5 receives electrons from the oxidoreductase CCDA, while CCS4 is a protein of unknown function. In Chlamydomonas reinhardtii, loss of CCS4 or CCS5 yields a partial cytochrome f assembly defect. Here, we report that the ccs4ccs5 double mutant displays a synthetic photosynthetic defect characterized by a complete loss of holocytochrome f assembly. This defect is chemically corrected by reducing agents, confirming the placement of CCS4 and CCS5 in a reducing pathway. CCS4-like proteins occur in the green lineage, and we show that HCF153, a distant ortholog from Arabidopsis thaliana, can substitute for Chlamydomonas CCS4. Dominant suppressor mutations mapping to the CCS4 gene were identified in photosynthetic revertants of the ccs4ccs5 mutants. The suppressor mutations yield changes in the stroma-facing domain of CCS4 that restore holocytochrome f assembly above the residual levels detected in ccs5. Because the CCDA protein accumulation is decreased specifically in the ccs4 mutant, we hypothesize the suppressor mutations enhance the supply of reducing power through CCDA in the absence of CCS5. We discuss the operation of a CCS5-dependent and a CCS5-independent pathway controlling the redox status of the heme-binding cysteines of apocytochrome f.

Diverse Sensory Repertoire of Paralogous Chemoreceptors Tlp2, Tlp3, and Tlp4 in Campylobacter jejuni.

Campylobacter jejuni responds to extracellular stimuli via transducer-like chemoreceptors (Tlps). Here, we describe receptor-ligand interactions of a unique paralogue family of dCache_1 (double Calcium channels and chemotaxis) chemoreceptors: Tlp2, Tlp3, and Tlp4. Phylogenetic analysis revealed that Tlp2, Tlp3, and Tlp4 receptors may have arisen through domain duplications, followed by a divergent evolutionary drift, with Tlp3 emerging more recently, and unexpectedly, responded to glycans, as well as multiple organic and amino acids with overlapping specificities. All three Tlps interacted with five monosaccharides and complex glycans, including Lewis's antigens, P antigens, and fucosyl GM1 ganglioside, indicating a potential role in host-pathogen interactions. Analysis of chemotactic motility of single, double, and triple mutants indicated that these chemoreceptors are likely to work together to balance responses to attractants and repellents to modulate chemotaxis in C. jejuni. Molecular docking experiments, in combination with saturation transfer difference nuclear magnetic resonance spectroscopy and competition surface plasmon resonance analysis, illustrated that the ligand-binding domain of Tlp3 possess one major binding pocket with two overlapping, but distinct binding sites able to interact with multiple ligands. A diverse sensory repertoire could provide C. jejuni with the ability to modulate responses to attractant and repellent signals and allow for adaptation in host-pathogen interactions. IMPORTANCE Campylobacter jejuni responds to extracellular stimuli via transducer-like chemoreceptors (Tlps). This remarkable sensory perception mechanism allows bacteria to sense environmental changes and avoid unfavorable conditions or to maneuver toward nutrient sources and host cells. Here, we describe receptor-ligand interactions of a unique paralogue family of chemoreceptors, Tlp2, Tlp3, and Tlp4, that may have arisen through domain duplications, followed by a divergent evolutionary drift, with Tlp3 emerging more recently. Unlike previous reports of ligands interacting with sensory proteins, Tlp2, Tlp3, and Tlp4 responded to many types of chemical compounds, including simple and complex sugars such as those present on human blood group antigens and gangliosides, indicating a potential role in host-pathogen interactions. Diverse sensory repertoire could provide C. jejuni with the ability to modulate responses to attractant and repellent signals and allow for adaptation in host-pathogen interactions.

Phyletic Distribution and Diversification of the Phage Shock Protein Stress Response System in Bacteria and Archaea.

Maintaining cell envelope integrity is of vital importance for all microorganisms. Not surprisingly, evolution has shaped conserved protein protection networks that connect stress perception, transmembrane signal transduction, and mediation of cellular responses upon cell envelope stress. The phage shock protein (Psp) stress response is one such conserved protection network. Most knowledge about the Psp response derives from studies in the Gram-negative model bacterium Escherichia coli, where the Psp system consists of several well-defined protein components. Homologous systems were identified in representatives of the Proteobacteria, Actinobacteria, and Firmicutes. However, the Psp system distribution in the microbial world remains largely unknown. By carrying out a large-scale, unbiased comparative genomics analysis, we found components of the Psp system in many bacterial and archaeal phyla and describe that the predicted Psp systems deviate dramatically from the known prototypes. The core proteins PspA and PspC have been integrated into various (often phylum-specifically) conserved protein networks during evolution. Based on protein domain-based and gene neighborhood analyses of pspA and pspC homologs, we built a natural classification system for Psp networks in bacteria and archaea. We validate our approach by performing a comprehensive in vivo protein interaction study of Psp domains identified in the Gram-positive model organism Bacillus subtilis and found a strong interconnected protein network. Our study highlights the diversity of Psp domain organizations and potentially diverse functions across the plethora of the microbial landscape, thus laying the ground for studies beyond known Psp functions in underrepresented organisms. IMPORTANCE The PspA protein domain is found in all domains of life, highlighting its central role in Psp networks. To date, all insights into the core functions of Psp responses derive mainly from protein network blueprints representing only three bacterial phyla. Despite large overlaps in function and regulation, the evolutionary diversity of Psp networks remains largely elusive. Here, we present an unbiased protein domain- and genomic context-centered approach that describes and classifies Psp systems. Our results suggest so-far-unknown Psp-associated roles with other protein networks giving rise to new functions. We demonstrate the applicability of our approach by dissecting the Psp protein network present in Bacillus subtilis and demonstrate Psp domains working in concert with other cell envelope stress response systems. We find that the Psp-like protein universe reflects a surprising diversity within the bacterial and archaeal microbial world.

Amino acid sensor conserved from bacteria to humans.

SignificanceAmino acids are the building blocks of life and important signaling molecules. Despite their common structure, no universal mechanism for amino acid recognition by cellular receptors is currently known. We discovered a simple motif, which binds amino acids in various receptor proteins from all major life-forms. In humans, this motif is found in subunits of calcium channels that are implicated in pain and neurodevelopmental disorders. Our findings suggest that γ-aminobutyric acid-derived drugs bind to the same motif in human proteins that binds natural ligands in bacterial receptors, thus enabling future improvement of important drugs.

A species-specific functional module controls formation of pollen apertures.

Pollen apertures are an interesting model for the formation of specialized plasma-membrane domains. The plant-specific protein INP1 serves as a key aperture factor in such distantly related species as Arabidopsis, rice and maize. Although INP1 orthologues probably play similar roles throughout flowering plants, they show substantial sequence divergence and often cannot substitute for each other, suggesting that INP1 might require species-specific partners. Here, we present a new aperture factor, INP2, which satisfies the criteria for being a species-specific partner for INP1. Both INP proteins display similar structural features, including the plant-specific DOG1 domain, similar patterns of expression and mutant phenotypes, as well as signs of co-evolution. These proteins interact with each other in a species-specific manner and can restore apertures in a heterologous system when both are expressed but not when expressed individually. Our findings suggest that the INP proteins form a species-specific functional module that underlies formation of pollen apertures.

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 Campylobacter jejuni chemoreceptor Tlp10 has a bimodal ligand-binding domain and specificity for multiple classes of chemoeffectors.

Campylobacter jejuni is a bacterial pathogen that is a common cause of enteritis in humans. We identified a previously uncharacterized type of sensory domain in the periplasmic region of the C. jejuni chemoreceptor Tlp10, termed the DAHL domain, that is predicted to have a bimodular helical architecture. Through two independent ligand-binding sites in this domain, Tlp10 responded to molecular aspartate, isoleucine, fumarate, malate, fucose, and mannose as attractants and to arginine, galactose, and thiamine as repellents. Tlp10 also recognized glycan ligands when present as terminal and intermediate residues of complex structures, such as the fucosylated human ganglioside GM1 and Lewisa antigen. A tlp10 mutant strain lacking the ligand-binding sites was attenuated in its ability to colonize avian caeca and to adhere to cultured human intestinal cells, indicating the potential involvement of the DAHL domain in host colonization and disease. The Tlp10 intracellular signaling domain interacted with the scaffolding proteins CheV and CheW, which couple chemoreceptors to intracellular signaling machinery, and with the signaling domains of other chemoreceptors, suggesting a key role for Tlp10 in signal transduction and incorporation into sensory arrays. We identified the DAHL domain in other bacterial signal transduction proteins, including the essential virulence induction protein VirA from the plant pathogen Agrobacterium tumefaciens Together, these results suggest a potential link between Tlp10 and C. jejuni virulence.

Origins and Molecular Evolution of the NusG Paralog RfaH.

The only universally conserved family of transcription factors comprises housekeeping regulators and their specialized paralogs, represented by well-studied NusG and RfaH. Despite their ubiquity, little information is available on the evolutionary origins, functions, and gene targets of the NusG family members. We built a hidden Markov model profile of RfaH and identified its homologs in sequenced genomes. While NusG is widespread among bacterial phyla and coresides with genes encoding RNA polymerase and ribosome in all except extremely reduced genomes, RfaH is mostly limited to Proteobacteria and lacks common gene neighbors. RfaH activates only a few xenogeneic operons that are otherwise silenced by NusG and Rho. Phylogenetic reconstructions reveal extensive duplications and horizontal transfer of rfaH genes, including those borne by plasmids, and the molecular evolution pathway of RfaH, from "early" exclusion of the Rho terminator and tightened RNA polymerase binding to "late" interactions with the ops DNA element and autoinhibition, which together define the RfaH regulon. Remarkably, NusG is not only ubiquitous in Bacteria but also common in plants, where it likely modulates the transcription of plastid genes.IMPORTANCE In all domains of life, NusG-like proteins make contacts similar to those of RNA polymerase and promote pause-free transcription yet may play different roles, defined by their divergent interactions with nucleic acids and accessory proteins, in the same cell. This duality is illustrated by Escherichia coli NusG and RfaH, which silence and activate xenogenes, respectively. We combined sequence analysis and recent functional and structural insights to envision the evolutionary transformation of NusG, a core regulator that we show is present in all cells using bacterial RNA polymerase, into a virulence factor, RfaH. Our results suggest a stepwise conversion of a NusG duplicate copy into a sequence-specific regulator which excludes NusG from its targets but does not compromise the regulation of housekeeping genes. We find that gene duplication and lateral transfer give rise to a surprising diversity within the only ubiquitous family of transcription factors.

Cross Talk between Chemosensory Pathways That Modulate Chemotaxis and Biofilm Formation.

Complex chemosensory systems control multiple biological functions in bacteria, such as chemotaxis, gene regulation, and cell cycle progression. Many species contain more than one chemosensory system per genome, but little is known about their potential interplay. In this study, we reveal cross talk between two chemosensory pathways that modulate chemotaxis and biofilm formation in Comamonas testosteroni We demonstrate that some chemoreceptors that govern chemotaxis also contribute to biofilm formation and these chemoreceptors can physically interact with components of both pathways. Finally, we show that the chemotaxis histidine kinase CheA can phosphorylate not only its cognate response regulator CheY2 but also one of the response regulators from the pathway mediating biofilm formation, FlmD. The phosphoryl group transfer from CheA to CheY2 is much faster than that from CheA to FlmD, which is consistent with chemotaxis being a fast response and biofilm formation being a much slower developmental process. We propose that cross talk between chemosensory pathways may play a role in coordination of complex behaviors in bacteria.IMPORTANCE In many bacteria, two or more homologous chemosensory pathways control several cellular functions, such as motility and gene regulation, in response to changes in the cell's microenvironment. Cross talk between signal transduction systems is poorly understood; while generally it is considered to be undesired, in some instances it might be beneficial for coregulation of complex behaviors. We demonstrate that several receptors from the pathway controlling motility can physically interact with downstream components of the pathway controlling biofilm formation. We further show that a kinase from the pathway controlling motility can also phosphorylate a response regulator from the pathway controlling biofilm formation. We propose that cross talk between two chemosensory pathways might be involved in coordination of two types of cell behavior-chemotaxis and biofilm formation.