scholarly journals An Outer Membrane Protein Involved in the Uptake of Glucose Is Essential for Cytophaga hutchinsonii Cellulose Utilization

2016 ◽  
Vol 82 (6) ◽  
pp. 1933-1944 ◽  
Author(s):  
Hong Zhou ◽  
Xia Wang ◽  
Tengteng Yang ◽  
Weixin Zhang ◽  
Guanjun Chen ◽  
...  
Keyword(s):  
Membrane Protein ◽  
Outer Membrane ◽  
Outer Membrane Protein ◽  
Cellulose Degradation ◽  
Expression Profiles ◽  
Transcriptional Profiling ◽  
Hydrolysis Product ◽  
Transcriptional Responses ◽  
Content Type ◽  
Cytophaga Hutchinsonii

ABSTRACTCytophaga hutchinsoniispecializes in cellulose digestion by employing a collection of novel cell-associated proteins. Here, we identified a novel gene locus, CHU_1276, that is essential forC. hutchinsoniicellulose utilization. Disruption of CHU_1276 inC. hutchinsoniiresulted in complete deficiency in cellulose degradation, as well as compromised assimilation of cellobiose or glucose at a low concentration. Further analysis showed that CHU_1276 was an outer membrane protein that could be induced by cellulose and low concentrations of glucose. Transcriptional profiling revealed that CHU_1276 exerted a profound effect on the genome-wide response to both glucose and Avicel and that the mutant lacking CHU_1276 displayed expression profiles very different from those of the wild-type strain under different culture conditions. Specifically, comparison of their transcriptional responses to cellulose led to the identification of a gene set potentially regulated by CHU_1276. These results suggest that CHU_1276 plays an essential role in cellulose utilization, probably by coordinating the extracellular hydrolysis of cellulose substrate with the intracellular uptake of the hydrolysis product inC. hutchinsonii.

scholarly journals Novel Outer Membrane Protein Involved in Cellulose and Cellooligosaccharide Degradation by Cytophaga hutchinsonii

2014 ◽  
Vol 80 (15) ◽  
pp. 4511-4518 ◽  
Author(s):  
Xiaofei Ji ◽  
Ying Wang ◽  
Cong Zhang ◽  
Xinfeng Bai ◽  
Weican Zhang ◽  
...  
Keyword(s):  
Membrane Protein ◽  
Cell Surface ◽  
Outer Membrane ◽  
Outer Membrane Protein ◽  
Cellulose Degradation ◽  
Outer Membrane Proteins ◽  
Cellulolytic Enzymes ◽  
Content Type ◽  
Cytophaga Hutchinsonii ◽  
Glucosidase Activity

ABSTRACTCytophaga hutchinsoniiis an aerobic cellulolytic soil bacterium which was reported to use a novel contact-dependent strategy to degrade cellulose. It was speculated that cellooligosaccharides were transported into the periplasm for further digestion. In this study, we reported that most of the endoglucanase and β-glucosidase activity was distributed on the cell surface ofC. hutchinsonii. Cellobiose and part of the cellulose could be hydrolyzed to glucose on the cell surface. However, the cell surface cellulolytic enzymes were not sufficient for cellulose degradation byC. hutchinsonii. An outer membrane protein, CHU_1277, was disrupted by insertional mutation. Although the mutant maintained the same endoglucanase activity and most of the β-glucosidase activity, it failed to digest cellulose, and its cellooligosaccharide utilization ability was significantly reduced, suggesting that CHU_1277 was essential for cellulose degradation and played an important role in cellooligosaccharide utilization. Further study of cellobiose hydrolytic ability of the mutant on the enzymatic level showed that the β-glucosidase activity in the outer membrane of the mutant was not changed. It revealed that CHU_1277 played an important role in assisting cell surface β-glucosidase to exhibit its activity sufficiently. Studies on the outer membrane proteins involved in cellulose and cellooligosaccharide utilization could shed light on the mechanism of cellulose degradation byC. hutchinsonii.

scholarly journals An Extracytoplasmic Function Sigma Factor Controls Cellulose Utilization by Regulating the Expression of an Outer Membrane Protein inCytophaga hutchinsonii

2018 ◽  
Vol 85 (5) ◽  
Author(s):  
Xia Wang ◽  
Weixin Zhang ◽  
Hong Zhou ◽  
Guanjun Chen ◽  
Weifeng Liu
Keyword(s):  
Membrane Protein ◽  
Outer Membrane ◽  
Outer Membrane Protein ◽  
Regulatory Mechanism ◽  
Outer Membrane Proteins ◽  
Cellulolytic Bacterium ◽  
Content Type ◽  
Cytophaga Hutchinsonii ◽  
Σ Factor ◽  
The Common

ABSTRACTThe common soil cellulolytic bacterium known asCytophaga hutchinsoniimakes use of a unique but poorly understood strategy in order to utilize cellulose. While several genes have been identified as being an active part of the utilization of cellulose, the mechanism(s) by whichC. hutchinsoniiboth (i) senses its environment and (ii) regulates the expression of those genes are not as yet known. In this study, we identified and characterized the geneCHU_3097encoding an extracytoplasmic function (ECF) σ factor (σcel1), the disruption of which compromisedC. hutchinsoniicellulose assimilation to a large degree. The σcel1and its putative partner anti-σcel1, encoded by theCHU_3096gene found immediately downstream fromCHU_3097, copurifiedin vitro. The σcel1was discovered to be associated with inner membrane when cells were cultured on glucose and yet was partially released from the membrane in response to cellulose. This release was found to occur on glucose when the anti-σcel1was absent. Transcriptome analyses found a σcel1-regulated, cellulose-responsive gene regulon, within which an outer membrane protein encoding the geneCHU_1276, essential for cellulose utilization, was discovered to be significantly downregulated byCHU_3097disruption. The expression of CHU_1276 almost fully restored cellulose utilization to theCHU_3097mutant, demonstrating that CHU_1276 represents a critical regulatory target of σcel1. In this way, our study provided insights into the role of an ECF σ factor in coordinating the cellulolytic response ofC. hutchinsonii.IMPORTANCEThe common cellulolytic bacteriumCytophaga hutchinsoniiuses a unique but poorly understood strategy in order to make use of cellulose. Throughout the process of cellulosic biomass breakdown, outer membrane proteins are thought to play key roles; this is evidenced by CHU_1276, which is required for the utilization of cellulose. However, the regulatory mechanism of its expression is not yet known. We found and characterized an extracytoplasmic function σ factor that is involved in coordinating the cellulolytic response ofC. hutchinsoniiby directly regulating the expression ofCHU_1276. This study makes a contribution to our understanding of the regulatory mechanism used byC. hutchinsoniiin order to adjust its genetic programs and so deal with novel environmental cues.

scholarly journals Anaplasma phagocytophilum Outer Membrane Protein A Interacts with Sialylated Glycoproteins To Promote Infection of Mammalian Host Cells

2012 ◽  
Vol 80 (11) ◽  
pp. 3748-3760 ◽  
Author(s):  
Nore Ojogun ◽  
Amandeep Kahlon ◽  
Stephanie A. Ragland ◽  
Matthew J. Troese ◽  
Juliana E. Mastronunzio ◽  
...  
Keyword(s):  
Membrane Protein ◽  
Outer Membrane ◽  
Outer Membrane Protein ◽  
Anaplasma Phagocytophilum ◽  
Protein A ◽  
Host Cells ◽  
Mammalian Host ◽  
Content Type ◽  
Outer Membrane Protein A ◽  
Sialylated Glycoproteins

ABSTRACTAnaplasma phagocytophilumis the tick-transmitted obligate intracellular bacterium that causes human granulocytic anaplasmosis (HGA).A. phagocytophilumbinding to sialyl Lewis x (sLex) and other sialylated glycans that decorate P selectin glycoprotein 1 (PSGL-1) and other glycoproteins is critical for infection of mammalian host cells. Here, we demonstrate the importance ofA. phagocytophilumouter membrane protein A (OmpA) APH_0338 in infection of mammalian host cells. OmpA is transcriptionally induced during transmission feeding ofA. phagocytophilum-infected ticks on mice and is upregulated during invasion of HL-60 cells. OmpA is presented on the pathogen's surface. Sera from HGA patients and experimentally infected mice recognize recombinant OmpA. Pretreatment ofA. phagocytophilumorganisms with OmpA antiserum reduces their abilities to infect HL-60 cells. The OmpA N-terminal region is predicted to contain the protein's extracellular domain. GlutathioneS-transferase (GST)-tagged versions of OmpA and OmpA amino acids 19 to 74 (OmpA19-74) but not OmpA75-205bind to, and competitively inhibitA. phagocytophiluminfection of, host cells. Pretreatment of host cells with sialidase or trypsin reduces or nearly eliminates, respectively, GST-OmpA adhesion. Therefore, OmpA interacts with sialylated glycoproteins. This study identifies the firstA. phagocytophilumadhesin-receptor pair and delineates the region of OmpA that is critical for infection.

scholarly journals The Helicobacter pylori Autotransporter ImaA (HP0289) Modulates the Immune Response and Contributes to Host Colonization

2012 ◽  
Vol 80 (7) ◽  
pp. 2286-2296 ◽  
Author(s):  
William E. Sause ◽  
Andrea R. Castillo ◽  
Karen M. Ottemann
Keyword(s):  
Immune Response ◽  
Helicobacter Pylori ◽  
Membrane Protein ◽  
Outer Membrane ◽  
Outer Membrane Protein ◽  
Dependent Manner ◽  
Wild Type ◽  
Content Type ◽  
H Pylori ◽  
Murine Infections

ABSTRACTThe human pathogenHelicobacter pyloriemploys a diverse collection of outer membrane proteins to colonize, persist, and drive disease within the acidic gastric environment. In this study, we sought to elucidate the function of the host-induced geneHP0289, which encodes an uncharacterized outer membrane protein. We first generated an isogenicH. pylorimutant that lacksHP0289and found that the mutant has a colonization defect in single-strain infections and is greatly outcompeted in mouse coinfection experiments with wild-typeH. pylori. Furthermore, we used protease assays and biochemical fractionation coupled with an HP0289-targeted peptide antibody to verify that the HP0289 protein resides in the outer membrane. Our previous findings showed that theHP0289promoter is upregulated in the mouse stomach, and here we demonstrate thatHP0289expression is induced under acidic conditions in an ArsRS-dependent manner. Finally, we have shown that theHP0289mutant induces greater expression of the chemokine interleukin-8 (IL-8) and the cytokine tumor necrosis factor alpha (TNF-α) in gastric carcinoma cells (AGS). Similarly, transcription of the IL-8 homolog keratinocyte-derived chemokine (KC) is elevated in murine infections with the HP0289 mutant than in murine infections with wild-typeH. pylori. On the basis of this phenotype, we renamed HP0289 ImaA forimmunomodulatoryautotransporter protein. Our work has revealed that genes inducedin vivoplay an important role inH. pyloripathogenesis. Specifically, the outer membrane protein ImaA modulates a component of the host inflammatory response, and thus may allowH. pylorito fine tune the host immune response based on ImaA expression.

scholarly journals A Disulfide Oxidoreductase (CHU_1165) Is Essential for Cellulose Degradation by Affecting Outer Membrane Proteins in Cytophaga hutchinsonii

2020 ◽  
Vol 86 (8) ◽  
Author(s):  
Dong Zhao ◽  
Ying Wang ◽  
Sen Wang ◽  
Weican Zhang ◽  
Qingsheng Qi ◽  
...  
Keyword(s):  
Membrane Proteins ◽  
Outer Membrane ◽  
Disulfide Bond ◽  
Cellulose Degradation ◽  
Outer Membrane Proteins ◽  
Hypothetical Protein ◽  
Pleiotropic Effect ◽  
Content Type ◽  
Cytophaga Hutchinsonii ◽  
Disulfide Oxidoreductase

ABSTRACT Cytophaga hutchinsonii cells can bind to the surface of insoluble cellulose and degrade it by utilizing a novel cell contact-dependent mechanism, in which the outer membrane proteins may play important roles. In this study, the deletion of a gene locus, chu_1165, which encodes a hypothetical protein with 32% identity with TlpB, a disulfide oxidoreductase in Flavobacterium psychrophilum, caused a complete cellulolytic defect in C. hutchinsonii. Further study showed that cells of the Δ1165 strain could not bind to cellulose, and the levels of many outer membrane proteins that can bind to cellulose were significantly decreased. The N-terminal region of CHU_1165 is anchored to the cytoplasmic membrane with five predicted transmembrane helices, and the C-terminal region is predicted to stretch to the periplasm and has a similar thioredoxin (Trx) fold containing a Cys-X-X-Cys motif that is conserved in disulfide oxidoreductases. Recombinant CHU_1165His containing the Cys-X-X-Cys motif was able to reduce the disulfide bonds of insulin in vitro. Site-directed mutation showed that the cysteines in the Cys-X-X-Cys motif and at residues 106 and 108 were indispensable for the function of CHU_1165. Western blotting showed that CHU_1165 was in an oxidized state in vivo, suggesting that it may act as an oxidase to catalyze disulfide bond formation. However, many of the decreased outer membrane proteins that were essential for cellulose degradation contained no or one cysteine, and mutation of the cysteine in these proteins did not affect cellulose degradation, indicating that CHU_1165 may have an indirect or pleiotropic effect on the function of these outer membrane proteins. IMPORTANCE Cytophaga hutchinsonii can rapidly digest cellulose in a contact-dependent manner, in which the outer membrane proteins may play important roles. In this study, a hypothetical protein, CHU_1165, characterized as a disulfide oxidoreductase, is essential for cellulose degradation by affecting the cellulose binding ability of many outer membrane proteins in C. hutchinsonii. Disulfide oxidoreductases are involved in disulfide bond formation. However, our studies show that many of the decreased outer membrane proteins that were essential for cellulose degradation contained no or one cysteine, and mutation of cysteine did not affect their function, indicating that CHU_1165 did not facilitate the formation of a disulfide bond in these proteins. It may have an indirect or pleiotropic effect on the function of these outer membrane proteins. Our study provides an orientation for exploring the proteins that assist in the appropriate conformation of many outer membrane proteins essential for cellulose degradation, which is important for exploring the novel mechanism of cellulose degradation in C. hutchinsonii.

scholarly journals Estimation of Full-Length TprK Diversity in Treponema pallidum subsp. pallidum

mBio ◽  
2020 ◽  
Vol 11 (5) ◽  
Author(s):  
Amin Addetia ◽  
Michelle J. Lin ◽  
Quynh Phung ◽  
Hong Xie ◽  
Meei-Li Huang ◽  
...  
Keyword(s):  
Immune System ◽  
Membrane Protein ◽  
Outer Membrane ◽  
Deep Sequencing ◽  
Outer Membrane Protein ◽  
Treponema Pallidum ◽  
The United States ◽  
Full Length ◽  
Variable Regions ◽  
Content Type

ABSTRACT Immune evasion and disease progression of Treponema pallidum subsp. pallidum are associated with sequence diversity in the hypervariable outer membrane protein TprK. Previous attempts to study variation within TprK have sequenced at depths insufficient to fully appreciate the hypervariable nature of the protein, failed to establish linkage between the protein’s seven variable regions, or were conducted on isolates passed through rabbits. As a consequence, a complete profile of tprK during infection in the human host is still lacking. Furthermore, prior studies examining how T. pallidum subsp. pallidum uses its repertoire of genomic donor sites to generate diversity within the variable regions of the tprK have yielded a partial understanding of this process due to the limited number of tprK alleles examined. In this study, we used short- and long-read deep sequencing to directly characterize full-length tprK alleles from T. pallidum subsp. pallidum collected from early lesions of patients attending two sexually transmitted infection clinics in Italy. We demonstrate that strains collected from cases of secondary syphilis contain significantly more unique variable region sequences and full-length TprK sequences than those from cases of primary syphilis. Our data, combined with recent data available on Chinese T. pallidum subsp. pallidum specimens, show the near-complete absence of overlap in TprK sequences among the 41 specimens profiled to date. We further estimate that the potential antigenic variability carried by TprK rivals that of current estimates of the human adaptive immune system. These data underscore the immunoevasive ability of TprK that allows T. pallidum subsp. pallidum to establish lifelong infection. IMPORTANCE Syphilis continues to be a significant public health issue in both low- and high-income countries, including the United States where the rate of syphilis infection has increased over the past 5 years. Treponema pallidum subsp. pallidum, the causative agent of syphilis, carries the outer membrane protein TprK that undergoes segmental gene conversion to constantly create new sequences. We performed full-length deep sequencing of TprK to examine TprK diversity in clinical T. pallidum subsp. pallidum strains. We then combined our results with data from all samples for which TprK deep sequencing results were available. We found almost no overlap in TprK sequences between different patients. Moreover, our data allowed us to estimate the total number of TprK variants that T. pallidum subsp. pallidum can potentially generate. Our results support how the T. pallidum subsp. pallidum TprK antigenic variation system is an equal adversary of the human immune system leading to pathogen persistence in the host.

scholarly journals Outer Membrane Protein Complex of Meningococcus Enhances the Antipolysaccharide Antibody Response to Pneumococcal Polysaccharide–CRM197Conjugate Vaccine

2011 ◽  
Vol 18 (5) ◽  
pp. 724-729 ◽  
Author(s):  
Zengzu Lai ◽  
John R. Schreiber
Keyword(s):  
T Cell ◽  
Membrane Protein ◽  
Outer Membrane ◽  
Conjugate Vaccine ◽  
Outer Membrane Protein ◽  
Protein Complex ◽  
Carrier Protein ◽  
Conjugate Vaccines ◽  
Membrane Protein Complex ◽  
Content Type

ABSTRACTBacterial polysaccharides (PS) are T cell-independent antigens that do not induce immunologic memory and are poor immunogens in infants. Conjugate vaccines in which the PS is covalently linked to a carrier protein have enhanced immunogenicity that resembles that of T cell-dependent antigens. TheHaemophilus influenzaetype b (Hib) conjugate vaccine, which uses the outer membrane protein complex (OMPC) from meningococcus as a carrier protein, elicits protective levels of anti-capsular PS antibody (Ab) after a single dose, in contrast to other conjugate vaccines, which require multiple doses. We have previously shown that OMPC robustly engages Toll-like receptor 2 (TLR2) and enhances the early anti-Hib PS Ab titer associated with an increase in TLR2-mediated induction of cytokines. We now show that the addition of OMPC to the 7-valent pneumococcal PS-CRM197conjugate vaccine during immunization significantly increases the anti-PS IgG and IgM responses to most serotypes of pneumococcus contained in the vaccine. The addition of OMPC also increased the likelihood of anti-PS IgG3 production against serotypes 4, 6B, 9V, 18C, 19F, and 23F. Splenocytes from mice who had received OMPC with the pneumococcal conjugate vaccine produced significantly more interleukin-2 (IL-2), IL-4, IL-6, IL-10, tumor necrosis factor alpha (TNF-α), and gamma interferon (IFN-γ) than splenocytes from mice who received phosphate-buffered saline (PBS) plus the conjugate vaccine. We conclude that OMPC enhances the anti-PS Ab response to pneumococcal PS-CRM197conjugate vaccine, an effect associated with a distinct change in cytokine profile. It may be possible to reduce the number of conjugate vaccine doses required to achieve protective Ab levels by priming with adjuvants that are TLR2 ligands.

scholarly journals Vaccination with the Recombinant Major Outer Membrane Protein Elicits Antibodies to the Constant Domains and Induces Cross-Serovar Protection against Intranasal Challenge with Chlamydia trachomatis

2013 ◽  
Vol 81 (5) ◽  
pp. 1741-1750 ◽  
Author(s):  
Delia F. Tifrea ◽  
Pooja Ralli-Jain ◽  
Sukumar Pal ◽  
Luis M. de la Maza
Keyword(s):  
Chlamydia Trachomatis ◽  
Membrane Protein ◽  
Outer Membrane ◽  
Outer Membrane Protein ◽  
Median Number ◽  
Negative Control ◽  
Major Outer Membrane Protein ◽  
Control Groups ◽  
Content Type ◽  
Chlamydia Muridarum

ABSTRACTTo determine the ability of the major outer membrane protein (MOMP) to elicit cross-serovar protection, groups of mice were immunized by the intramuscular (i.m.) and subcutaneous (s.c.) routes with recombinant MOMP (rMOMP) fromChlamydia trachomatisserovars D (UW-3/Cx), E (Bour), or F (IC-Cal-3) orChlamydia muridarumstrain Nigg II using CpG-1826 and Montanide ISA 720 VG as adjuvants. Negative-control groups were immunized i.m. and s.c. withNeisseria gonorrhoeaerecombinant porin B (Ng-rPorB) or i.n. with Eagle's minimal essential medium (MEM-0). Following vaccination, the mice developed antibodies not only against the homologous serovar but also against heterologous serovars. The rMOMP-vaccinated animals also mounted cell-mediated immune responses as assessed by a lymphoproliferative assay. Four weeks after the last immunization, mice were challenged i.n. with 104inclusion-forming units (IFU) ofC. muridarum. The mice were weighed for 10 days and euthanized, and the number of IFU in their lungs was determined. At 10 days postinfection (p.i.), mice immunized with the rMOMP ofC. muridarumorC. trachomatisD, E, or F had lost 4%, 6%, 8%, and 8% of their initial body weight, respectively, significantly different from the negative-control groups (Ng-rPorB, 13%; MEM-0, 19%;P< 0.05). The median number of IFU recovered from the lungs of mice immunized withC. muridarumrMOMP was 0.13 × 106. The median number of IFU recovered from mice immunized with rMOMP from serovars D, E, and F were 0.38 × 106, 7.56 × 106, and 11.94 × 106IFU, respectively. All the rMOMP-immunized animals had significantly less IFU than theNg-rPorB (40 × 106)- or MEM-0 (70 × 106)-immunized mice (P< 0.05). In conclusion, vaccination with rMOMP can elicit protection against homologous and heterologousChlamydiaserovars.

scholarly journals NlpI Facilitates Deposition of C4bp on Escherichia coli by Blocking Classical Complement-Mediated Killing, Which Results in High-Level Bacteremia

2012 ◽  
Vol 80 (10) ◽  
pp. 3669-3678 ◽  
Author(s):  
Yu-ting Tseng ◽  
Shainn-Wei Wang ◽  
Kwang Sik Kim ◽  
Ying-Hsiang Wang ◽  
Yufeng Yao ◽  
...  
Keyword(s):  
Escherichia Coli ◽  
Membrane Protein ◽  
Outer Membrane ◽  
Outer Membrane Protein ◽  
Neonatal Meningitis ◽  
Content Type ◽  
E Coli ◽  
Classical Complement Pathway ◽  
High Level ◽  
Classical Complement

ABSTRACTNeonatal meningitisEscherichia coli(NMEC) is the most common Gram-negative organism that is associated with neonatal meningitis, which usually develops as a result of hematogenous spread of the bacteria. There are two key pathogenesis processes for NMEC to penetrate into the brain, the essential step for the development ofE. colimeningitis: a high-level bacteremia and traversal of the blood-brain barrier (BBB). Our previous study has shown that the bacterial outer membrane protein NlpI contributes to NMEC binding to and invasion of brain microvascular endothelial cells, the major component cells of the BBB, suggesting a role for NlpI in NMEC crossing of the BBB. In this study, we showed that NlpI is involved in inducing a high level of bacteremia. In addition, NlpI contributed to the recruitment of the complement regulator C4bp to the surface of NMEC to evade serum killing, which is mediated by the classical complement pathway. NlpI may be involved in the interaction between C4bp and OmpA, which is an outer membrane protein that directly interacts with C4bp on the bacterial surface. The involvement of NlpI in two key pathogenesis processes of NMEC meningitis may make this bacterial factor a potential target for prevention and therapy ofE. colimeningitis.

scholarly journals NsrA, a Predicted β-Barrel Outer Membrane Protein Involved in Plant Signal Perception and the Control of Secondary Infection inSinorhizobium meliloti

2018 ◽  
Vol 200 (11) ◽  
Author(s):  
Anne-Marie Garnerone ◽  
Fernando Sorroche ◽  
Lan Zou ◽  
Céline Mathieu-Demazière ◽  
Chang Fu Tian ◽  
...  
Keyword(s):  
Membrane Protein ◽  
Outer Membrane ◽  
Outer Membrane Protein ◽  
Secondary Infection ◽  
Transmembrane Signaling ◽  
Root Infection ◽  
Content Type ◽  
Signal Perception ◽  
Symbiotic Interactions ◽  
Signal Exchange

ABSTRACTAn ongoing signal exchange fine-tunes the symbiotic interactions between rhizobia and legumes, ensuring the establishment and maintenance of mutualism. In a recently identified regulatory loop, endosymbioticSinorhizobium melilotiexerts negative feedback on root infection in response to unknown plant cues. Upon signal perception, three bacterial adenylate cyclases (ACs) of the inner membrane, namely, CyaD1, CyaD2, and CyaK, synthesize the second messenger cAMP, which, together with the cAMP-dependent Clr transcriptional activator, activates the expression of genes involved in root infection control. The pathway that links signal perception at the surface of the cell to cytoplasmic cAMP production by ACs was thus far unknown. Here we first show that CyaK is the cognate AC for the plant signal, called signal 1, that was observed previously in mature nodule and shoot extracts. We also show that inactivation of the gene immediately upstream ofcyaK,nsrA(smb20775), which encodes a β-barrel protein of the outer membrane, abolished signal 1 perceptionex planta, whereasnsrAoverexpression increased signal 1 responsiveness. Inactivation of thensrAgene abolished all Clr-dependent gene expression in nodules and led to a marked hyperinfection phenotype on plants, similar to that of acyaD1 cyaD2 cyaKtriple mutant. We suggest that the NsrA protein acts as the (co)receptor for two signal molecules, signal 1 and a hypothetical signal 1′, in mature and young nodules that cooperate in controlling secondary infection inS. meliloti-Medicagosymbiosis. The predicted topology and domain composition of the NsrA protein hint at a mechanism of transmembrane signaling.IMPORTANCESymbiotic interactions, especially mutualistic ones, rely on a continuous signal exchange between the symbionts. Here we report advances regarding a recently discovered signal transduction pathway that fine-tunes the symbiotic interaction betweenS. melilotiand itsMedicagohost plant. We have identified an outer membrane protein ofS. meliloti, called NsrA, that transducesMedicagoplant signals to adenylate cyclases in the inner membrane, thereby triggering a cAMP signaling cascade that controls infection. Besides their relevance for the rhizobium-legume symbiosis, these findings shed light on the mechanisms of signal perception and transduction by adenylate cyclases and transmembrane signaling in bacteria.

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