Click-chemistry mediated synthesis of OTBN-1,2,3-Triazole derivatives exhibiting STK33 inhibition with diverse anti-cancer activities.

There is a continuous and pressing need to establish new brain-penetrant bioactive compounds with anti-cancer properties. To this end, a new series of 4'-((4-substituted-4,5-dihydro-1H-1,2,3-triazol-1-yl)methyl)-[1,1'-biphenyl]-2-carbonitrile (OTBN-1,2,3-triazole) derivatives were synthesized by click chemistry. The series of bioactive compounds were designed and synthesized from diverse alkynes and N3-OTBN, using copper (II) acetate monohydrate in aqueous dimethylformamide at room temperature. Besides being highly cost-effective and significantly reducing synthesis, the reaction yielded 91-98 % of the target products without the need of any additional steps or chromatographic techniques. Two analogues exhibit promising anti-cancer biological activities. Analogue 4l shows highly specific cytostatic activity against lung cancer cells, while analogue 4k exhibits pan-cancer anti-growth activity. A kinase screen suggests compound 4k has single-digit micromolar activity against kinase STK33. High STK33 RNA expression correlates strongly with poorer patient outcomes in both adult and pediatric glioma. Compound 4k potently inhibits cell proliferation, invasion, and 3D neurosphere formation in primary patient-derived glioma cell lines. The observed anti-cancer activity is enhanced in combination with specific clinically relevant small molecule inhibitors. Herein we establish a novel biochemical kinase inhibitory function for click-chemistry-derived OTBN-1,2,3-triazole analogues and further report their anti-cancer activity in vitro for the first time.

Alkoxy-functionalised dihydropyrimido[4,5-b]quinolinones enabling anti-proliferative and anti-invasive agents.

In this communication, we explored the synthesis of novel alkoxy-functionalised dihydropyrimido[4,5-b]quinolinones using a microwave-assisted multicomponent reaction. All the synthesized molecules were screened for anti-proliferative and anti-invasive activity against glioblastoma cells. 5c shows the most potent anti-proliferative activity with a half maximal effective concentration of less than 3 µM against primary patient-derived glioblastoma cells. 5c effectively inhibited invasion and tumor growth of 3D primary glioma cultures in a basement membrane matrix. This suggests that the novel compounds could inhibit both the proliferation and invasive spread of glioma and they were selected for further study.

A green bio-organic catalyst (taurine) promoted one-pot synthesis of (R/S)-2-thioxo-3,4-dihydropyrimidine(TDHPM)-5-carboxanilides: chiral investigations using circular dichroism and validation by computational approaches.

Owing to the massive importance of dihydropyrimidine (DHPMs) scaffolds in the pharmaceutical industry and other areas, we developed an effective and sustainable one-pot reaction protocol for the synthesis of (R/S)-2-thioxo-DHPM-5-carboxanilides via the Biginelli-type cyclo-condensation reaction of aryl aldehydes, thiourea and various acetoacetanilide derivatives in ethanol at 100 °C. In this protocol, taurine was used as a green and reusable bio-organic catalyst. Twenty-three novel derivatives of (R/S)-TDHPM-5-carboxanilides and their structures were confirmed by various spectroscopy techniques. The aforementioned compounds were synthesized via the formation of one asymmetric centre, one new C-C bond, and two new C-N bonds in the final product. All the newly synthesized compounds were obtained in their racemic form with up to 99% yield. In addition, the separation of the racemic mixture of all the newly synthesized compounds was carried out by chiral HPLC (Prep LC), which provided up to 99.99% purity. The absolute configuration of all the enantiomerically pure isomers was determined using a circular dichroism study and validated by a computational approach. With up to 99% yield of 4d, this one-pot synthetic approach can also be useful for large-scale industrial production. One of the separated isomers (4R)-(+)-4S developed as a single crystal, and it was found that this crystal structure was orthorhombic.

Indole clubbed 2,4-thiazolidinedione linked 1,2,3-triazole as a potent antimalarial and antibacterial agent against drug-resistant strain and molecular modeling studies.

In the face of escalating challenges of microbial resistance strains, this study describes the design and synthesis of 5-({1-[(1H-1,2,3-triazol-4-yl)methyl]-1H-indol-3-yl}methylene)thiazolidine-2,4-dione derivatives, which have demonstrated significant antimicrobial properties. Compared with the minimum inhibitory concentrations (MIC) values of ciprofloxacin on the respective strains, compounds 5a, 5d, 5g, 5l, and 5m exhibited potent antibacterial activity with MIC values ranging from 16 to 25 µM. Almost all the synthesized compounds showed lower MIC compared to standards against vancomycin-resistant enterococcus and methicillin-resistant Staphylococcus aureus strains. Additionally, the majority of the synthesized compounds demonstrated remarkable antifungal activity, against Candida albicans and Aspergillus niger, as compared to nystatin, griseofulvin, and fluconazole. Furthermore, the majority of compounds exhibited notable inhibitory effects against the Plasmodium falciparum strain, having IC50 values ranging from 1.31 to 2.79 µM as compared to standard quinine (2.71 µM). Cytotoxicity evaluation of compounds 5a-q on SHSY-5Y cells at up to 100 µg/mL showed no adverse effects. Comparison with control groups highlights their noncytotoxic characteristics. Molecular docking confirmed compound binding to target active sites, with stable protein-ligand complexes displaying drug-like molecules. Molecular dynamics simulations revealed dynamic stability and interactions. Rigorous tests and molecular modeling unveil the effectiveness of the compounds against drug-resistant microbes, providing hope for new antimicrobial compounds with potential safety.

Development and optimisation of in vitro sonodynamic therapy for glioblastoma.

Sonodynamic therapy (SDT) is currently on critical path for glioblastoma therapeutics. SDT is a non-invasive approach utilising focused ultrasound to activate photosensitisers like 5-ALA to impede tumour growth. Unfortunately, the molecular mechanisms underlying the therapeutic functions of SDT remain enigmatic. This is primarily due to the lack of intricately optimised instrumentation capable of modulating SDT delivery to glioma cells in vitro. Consequently, very little information is available on the effects of SDT on glioma stem cells which are key drivers of gliomagenesis and recurrence. To address this, the current study has developed and validated an automated in vitro SDT system to allow the application and mapping of focused ultrasound fields under varied exposure conditions and setup configurations. The study optimizes ultrasound frequency, intensity, plate base material, thermal effect, and the integration of live cells. Indeed, in the presence of 5-ALA, focused ultrasound induces apoptotic cell death in primary patient-derived glioma cells with concurrent upregulation of intracellular reactive oxygen species. Intriguingly, primary glioma stem neurospheres also exhibit remarkably reduced 3D growth upon SDT exposure. Taken together, the study reports an in vitro system for SDT applications on tissue culture-based disease models to potentially benchmark the novel approach to the current standard-of-care.

One-pot two-step catalytic synthesis of 6-amino-2-pyridone-3,5-dicarbonitriles enabling anti-cancer bioactivity.

We report a one-pot two-step synthesis of a bioactive 6-amino-2-pyridone-3,5-dicarbonitrile derivative using natural product catalysts betaine and guanidine carbonate. Anti-cancer bioactivity was observed in specific molecules within the library of 16 derivatives. Out of the compounds, 5o had the most potent anti-cancer activity against glioblastoma cells and was selected for further study. Compound 5o showed anti-cancer properties against liver, breast, lung cancers as well as primary patient-derived glioblastoma cell lines. Furthermore, 5o in combination with specific clinically relevant small molecule inhibitors induced enhanced cytotoxicity in glioblastoma cells. Through our current work, we establish a promising 6-amino-2-pyridone-3,5-dicarbonitrile based lead compound with anti-cancer activity either on its own or in combination with specific clinically relevant small molecule kinase and proteasome inhibitors.

Exploring the expression and functionality of the rsm sRNAs in Pseudomonas syringae pv. tomato DC3000.

The Gac-rsm pathway is a global regulatory network that governs mayor lifestyle and metabolic changes in gamma-proteobacteria. In a previous study, we uncovered the role of CsrA proteins promoting growth and repressing motility, alginate production and virulence in the model phytopathogen Pseudomonas syringae pv. tomato (Pto) DC3000. Here, we focus on the expression and regulation of the rsm regulatory sRNAs, since Pto DC3000 exceptionally has seven variants (rsmX1-5, rsmY and rsmZ). The presented results offer further insights into the functioning of the complex Gac-rsm pathway and the interplay among its components. Overall, rsm expressions reach maximum levels at high cell densities, are unaffected by surface detection, and require GacA for full expression. The rsm levels of expression and GacA-dependence are determined by the sequences found in their -35/-10 promoter regions and GacA binding boxes, respectively. rsmX5 stands out for being the only rsm in Pto DC3000 whose high expression does not require GacA, constituting the main component of the total rsm pool in a gacA mutant. The deletion of rsmY and rsmZ had minor effects on Pto DC3000 motility and virulence phenotypes, indicating that rsmX1-5 can functionally replace them. On the other hand, rsmY or rsmZ overexpression in a gacA mutant did not revert its phenotype. Additionally, a negative feedback regulatory loop in which the CsrA3 protein promotes its own titration by increasing the levels of several rsm RNAs in a GacA-dependent manner has been disclosed as part of this work.

Role of Sinorhizobium meliloti and Escherichia coli Long-Chain Acyl-CoA Synthetase FadD in Long-Term Survival.

FadD is an acyl-coenzyme A (CoA) synthetase specific for long-chain fatty acids (LCFA). Strains mutated in fadD cannot produce acyl-CoA and thus cannot grow on exogenous LCFA as the sole carbon source. Mutants in the fadD (smc02162) of Sinorhizobium meliloti are unable to grow on oleate as the sole carbon source and present an increased surface motility and accumulation of free fatty acids at the entry of the stationary phase of growth. In this study, we found that constitutive expression of the closest FadD homologues of S. meliloti, encoded by sma0150 and smb20650, could not revert any of the mutant phenotypes. In contrast, the expression of Escherichia coli fadD could restore the same functions as S. meliloti fadD. Previously, we demonstrated that FadD is required for the degradation of endogenous fatty acids released from membrane lipids. Here, we show that absence of a functional fadD provokes a significant loss of viability in cultures of E. coli and of S. meliloti in the stationary phase, demonstrating a crucial role of fatty acid degradation in survival capacity.

2-Tridecanone impacts surface-associated bacterial behaviours and hinders plant-bacteria interactions.

Surface motility and biofilm formation are behaviours which enable bacteria to infect their hosts and are controlled by different chemical signals. In the plant symbiotic alpha-proteobacterium Sinorhizobium meliloti, the lack of long-chain fatty acyl-coenzyme A synthetase activity (FadD) leads to increased surface motility, defects in biofilm development and impaired root colonization. In this study, analyses of lipid extracts and volatiles revealed that a fadD mutant accumulates 2-tridecanone (2-TDC), a methylketone (MK) known as a natural insecticide. Application of pure 2-TDC to the wild-type strain phenocopies the free-living and symbiotic behaviours of the fadD mutant. Structural features of the MK determine its ability to promote S. meliloti surface translocation, which is mainly mediated by a flagella-independent motility. Transcriptomic analyses showed that 2-TDC induces differential expression of iron uptake, redox and stress-related genes. Interestingly, this MK also influences surface motility and impairs biofilm formation in plant and animal pathogenic bacteria. Moreover, 2-TDC not only hampers alfalfa nodulation but also the development of tomato bacterial speck disease. This work assigns a new role to 2-TDC as an infochemical that affects important bacterial traits and hampers plant-bacteria interactions by interfering with microbial colonization of plant tissues.

Multiple CsrA Proteins Control Key Virulence Traits in Pseudomonas syringae pv. tomato DC3000.

The phytopathogenic bacterium Pseudomonas syringae pv. tomato DC3000 has a complex Gac-rsm global regulatory pathway that controls virulence, motility, production of secondary metabolites, carbon metabolism, and quorum sensing. However, despite the fact that components of this pathway are known, their physiological roles have not yet been established. Regarding the CsrA/RsmA type proteins, five paralogs, three of which are well conserved within the Pseudomonas genus (csrA1, csrA2, and csrA3), have been found in the DC3000 genome. To decipher their function, mutants lacking the three most conserved CsrA proteins have been constructed and their physiological outcomes examined. We show that they exert nonredundant functions and demonstrate that CsrA3 and, to a lesser extent, CsrA2 but not CsrA1 alter the expression of genes involved in a variety of pathways and systems important for motility, exopolysaccharide synthesis, growth, and virulence. Particularly, alginate synthesis, syringafactin production, and virulence are considerably de-repressed in a csrA3 mutant, whereas growth in planta is impaired. We propose that the linkage of growth and symptom development is under the control of CsrA3, which functions as a pivotal regulator of the DC3000 life cycle, repressing virulence traits and promoting cell division in response to environmental cues.

Genome-Wide Identification of Medicago Peptides Involved in Macronutrient Responses and Nodulation.

Growing evidence indicates that small, secreted peptides (SSPs) play critical roles in legume growth and development, yet the annotation of SSP-coding genes is far from complete. Systematic reannotation of the Medicago truncatula genome identified 1,970 homologs of established SSP gene families and an additional 2,455 genes that are potentially novel SSPs, previously unreported in the literature. The expression patterns of known and putative SSP genes based on 144 RNA sequencing data sets covering various stages of macronutrient deficiencies and symbiotic interactions with rhizobia and mycorrhiza were investigated. Focusing on those known or suspected to act via receptor-mediated signaling, 240 nutrient-responsive and 365 nodulation-responsive Signaling-SSPs were identified, greatly expanding the number of SSP gene families potentially involved in acclimation to nutrient deficiencies and nodulation. Synthetic peptide applications were shown to alter root growth and nodulation phenotypes, revealing additional regulators of legume nutrient acquisition. Our results constitute a powerful resource enabling further investigations of specific SSP functions via peptide treatment and reverse genetics.

The NtrY/NtrX System of Sinorhizobium meliloti GR4 Regulates Motility, EPS I Production, and Nitrogen Metabolism but Is Dispensable for Symbiotic Nitrogen Fixation.

Sinorhizobium meliloti can translocate over surfaces. However, little is known about the regulatory mechanisms that control this trait and its relevance for establishing symbiosis with alfalfa plants. To gain insights into this field, we isolated Tn5 mutants of S. meliloti GR4 with impaired surface motility. In mutant strain GRS577, the transposon interrupted the ntrY gene encoding the sensor kinase of the NtrY/NtrX two-component regulatory system. GRS577 is impaired in flagella synthesis and overproduces succinoglycan, which is responsible for increased biofilm formation. The mutant also shows altered cell morphology and higher susceptibility to salt stress. GRS577 induces nitrogen-fixing nodules in alfalfa but exhibits decreased competitive nodulation. Complementation experiments indicate that both ntrY and ntrX account for all the phenotypes displayed by the ntrY::Tn5 mutant. Ectopic overexpression of VisNR, the motility master regulator, was sufficient to rescue motility and competitive nodulation of the transposant. A transcriptome profiling of GRS577 confirmed differential expression of exo and flagellar genes, and led to the demonstration that NtrY/NtrX allows for optimal expression of denitrification and nifA genes under microoxic conditions in response to nitrogen compounds. This study extends our knowledge of the complex role played by NtrY/NtrX in S. meliloti.

FleQ coordinates flagellum-dependent and -independent motilities in Pseudomonas syringae pv. tomato DC3000.

Motility plays an essential role in bacterial fitness and colonization in the plant environment, since it favors nutrient acquisition and avoidance of toxic substances, successful competition with other microorganisms, the ability to locate the preferred hosts, access to optimal sites within them, and dispersal in the environment during the course of transmission. In this work, we have observed that the mutation of the flagellar master regulatory gene, fleQ, alters bacterial surface motility and biosurfactant production, uncovering a new type of motility for Pseudomonas syringae pv. tomato DC3000 on semisolid surfaces. We present evidence that P. syringae pv. tomato DC3000 moves over semisolid surfaces by using at least two different types of motility, namely, swarming, which depends on the presence of flagella and syringafactin, a biosurfactant produced by this strain, and a flagellum-independent surface spreading or sliding, which also requires syringafactin. We also show that FleQ activates flagellum synthesis and negatively regulates syringafactin production in P. syringae pv. tomato DC3000. Finally, it was surprising to observe that mutants lacking flagella or syringafactin were as virulent as the wild type, and only the simultaneous loss of both flagella and syringafactin impairs the ability of P. syringae pv. tomato DC3000 to colonize tomato host plants and cause disease.

Plant flavonoids target Pseudomonas syringae pv. tomato DC3000 flagella and type III secretion system.

Flavonoids are among the most abundant plant secondary metabolites involved in plant protection against pathogens, but micro-organisms have developed resistance mechanisms to those compounds. We previously demonstrated that the MexAB-OprM efflux pump mediates resistance of Pseudomonas syringae pv. tomato (Pto) DC3000 to flavonoids, facilitating its survival and the colonization of the host. Here, we have shown that tomato plants respond to Pto infection producing flavonoids and other phenolic compounds. The effects of flavonoids on key traits of this model plant-pathogen bacterium have also been investigated observing that they reduce Pto swimming and swarming because of the loss of flagella, and also inhibited the expression and assembly of a functional type III secretion system. Those effects were more severe in a mutant lacking the MexAB-OprM pump. Our results suggest that flavonoids inhibit the function of the GacS/GacA two-component system, causing a depletion of rsmY RNA, therefore affecting the synthesis of two important virulence factors in Pto DC3000, flagella and the type III secretion system. These data provide new insights into the flavonoid role in the molecular dialog between host and pathogen.

Conjugal transfer of the Sinorhizobium meliloti 1021 symbiotic plasmid is governed through the concerted action of one- and two-component signal transduction regulators.

Conjugal transfer of Sinorhizobium meliloti and Rhizobium etli symbiotic plasmids are repressed by the transcriptional regulator RctA. Here we report on new key players in the signal transduction cascade towards S. meliloti pSym conjugation. We have identified S. meliloti pSymA gene SMa0974 as an orthologue of the R. etli rctB gene which is required to antagonize repression by RctA. In S. meliloti two additional genes, rctR and rctC participate in control of rctB expression. rctR (SMa0955) encodes a protein of the GntR family of transcriptional regulators involved in repression of rctB. A rctR mutant promotes pSymA conjugal transfer and displays increased transcription of tra, virB and rctB genes even in presence of wild-type rctA gene. Among genes repressed by RctR, rctC (SMa0961) encodes a response regulator required to activate rctB transcription and therefore for derepression of plasmid conjugative functions. We conclude that in both R. etli and S. meliloti pSym conjugal transfer is derepressed via rctB, however the regulatory cascades to achieve activation of rctB are probably different. Upstream of rctB, the S. meliloti pSym conjugal transfer is regulated through the concerted action of genes representing one- (rctR) and two-component (rctC) signal transduction systems in response to yet unidentified signals.

ExpR is not required for swarming but promotes sliding in Sinorhizobium meliloti.

Swarming is a mode of translocation dependent on flagellar activity that allows bacteria to move rapidly across surfaces. In several bacteria, swarming is a phenotype regulated by quorum sensing. It has been reported that the swarming ability of the soil bacterium Sinorhizobium meliloti Rm2011 requires a functional ExpR/Sin quorum-sensing system. However, our previous published results demonstrate that strains Rm1021 and Rm2011, both known to have a disrupted copy of expR, are able to swarm on semisolid minimal medium. In order to clarify these contradictory results, the role played by the LuxR-type regulator ExpR has been reexamined. Results obtained in this work revealed that S. meliloti can move over semisolid surfaces using at least two different types of motility. One type is flagellum-independent surface spreading or sliding, which is positively influenced by a functional expR gene mainly through the production of exopolysaccharide II (EPS II). To a lesser extent, EPS II-deficient strains can also slide on surfaces by a mechanism that is at least dependent on the siderophore rhizobactin 1021. The second type of surface translocation shown by S. meliloti is swarming, which is greatly dependent on flagella and rhizobactin 1021 but does not require ExpR. We have extended our study to demonstrate that the production of normal amounts of succinoglycan (EPS I) does not play a relevant role in surface translocation but that its overproduction facilitates both swarming and sliding motilities.

FadD is required for utilization of endogenous fatty acids released from membrane lipids.

FadD is an acyl coenzyme A (CoA) synthetase responsible for the activation of exogenous long-chain fatty acids (LCFA) into acyl-CoAs. Mutation of fadD in the symbiotic nitrogen-fixing bacterium Sinorhizobium meliloti promotes swarming motility and leads to defects in nodulation of alfalfa plants. In this study, we found that S. meliloti fadD mutants accumulated a mixture of free fatty acids during the stationary phase of growth. The composition of the free fatty acid pool and the results obtained after specific labeling of esterified fatty acids with a Δ5-desaturase (Δ5-Des) were in agreement with membrane phospholipids being the origin of the released fatty acids. Escherichia coli fadD mutants also accumulated free fatty acids released from membrane lipids in the stationary phase. This phenomenon did not occur in a mutant of E. coli with a deficient FadL fatty acid transporter, suggesting that the accumulation of fatty acids in fadD mutants occurs inside the cell. Our results indicate that, besides the activation of exogenous LCFA, in bacteria FadD plays a major role in the activation of endogenous fatty acids released from membrane lipids. Furthermore, expression analysis performed with S. meliloti revealed that a functional FadD is required for the upregulation of genes involved in fatty acid degradation and suggested that in the wild-type strain, the fatty acids released from membrane lipids are degraded by ß-oxidation in the stationary phase of growth.

pSymA-dependent mobilization of the Sinorhizobium meliloti pSymB megaplasmid.

Sinorhizobium meliloti 1021 carries two megaplasmids, pSymA of 1,354 kb and pSymB of 1,683 kb, which are essential in establishing symbiosis with its legume hosts and important for bacterial fitness in the rhizosphere. We have previously shown that pSymA is self-transmissible and that its conjugal functions are regulated by the transcriptional repressor RctA. Here, we show conjugal transfer of pSymB as an in trans mobilization event that requires the type IV secretion system encoded by pSymA. pSymB carries a functional oriT and an adjacent relaxase gene, traA2, that is also transcriptionally repressed by rctA. Both symbiotic megaplasmids would require the relaxase genes in cis with their respective oriTs to achieve the highest transfer efficiencies.

Transcriptome profiling of a Sinorhizobium meliloti fadD mutant reveals the role of rhizobactin 1021 biosynthesis and regulation genes in the control of swarming.

BACKGROUND: Swarming is a multicellular phenomenom characterized by the coordinated and rapid movement of bacteria across semisolid surfaces. In Sinorhizobium meliloti this type of motility has been described in a fadD mutant. To gain insights into the mechanisms underlying the process of swarming in rhizobia, we compared the transcriptome of a S. meliloti fadD mutant grown under swarming inducing conditions (semisolid medium) to those of cells grown under non-swarming conditions (broth and solid medium). RESULTS: More than a thousand genes were identified as differentially expressed in response to growth on agar surfaces including genes for several metabolic activities, iron uptake, chemotaxis, motility and stress-related genes. Under swarming-specific conditions, the most remarkable response was the up-regulation of iron-related genes. We demonstrate that the pSymA plasmid and specifically genes required for the biosynthesis of the siderophore rhizobactin 1021 are essential for swarming of a S. meliloti wild-type strain but not in a fadD mutant. Moreover, high iron conditions inhibit swarming of the wild-type strain but not in mutants lacking either the iron limitation response regulator RirA or FadD. CONCLUSIONS: The present work represents the first transcriptomic study of rhizobium growth on surfaces including swarming inducing conditions. The results have revealed major changes in the physiology of S. meliloti cells grown on a surface relative to liquid cultures. Moreover, analysis of genes responding to swarming inducing conditions led to the demonstration that iron and genes involved in rhizobactin 1021 synthesis play a role in the surface motility shown by S. meliloti which can be circumvented in a fadD mutant. This work opens a way to the identification of new traits and regulatory networks involved in swarming by rhizobia.

Genetic characterization of oligopeptide uptake systems in Sinorhizobium meliloti.

The genetic characterization of three ABC transport systems involved in oligopeptide uptake by Sinorhizobium meliloti is reported. Oligopeptide permease (Opp) encoded by the pSymB oppABCD operon, is required for uptake of tetrapeptides and certain tripeptides like 3Ala and bialaphos. The chromosomally encoded dipeptide permease (Dpp1), also able to import the toxic tripeptide bialaphos, is required for utilization of dipeptides and tripeptides like 3Gly and GlyGlyAla, with minor importance for utilization of 3Ala and tetrapeptides. The ttp (tri and tetrapeptide uptake) operon, encodes a third ABC system (Ttp) unable of transporting bialaphos and with minor role in the utilization of tetrapeptides and tripeptides like 3-Ala. Despite the overlapping substrate specificities of these ABC transporters, the corresponding gene operons displayed distinct expression profiles: dpp1 showed high constitutive expression levels under all conditions tested, in contrast to the low expression levels of ttp, whereas opp was maximally expressed upon entry into stationary phase. Nevertheless, complex interactions among the three systems at the transcriptional level were observed: opp was negatively autoregulated via OppA and positively regulated via DppA1, whereas dpp1 seems negatively autoregulated via DppA1. The expression of both opp and dppl was reduced in Ttp mutants. The ABC transport systems characterized in this work are not essential for the establishment of nitrogen-fixing symbiosis with alfalfa.