Nitric oxide controls c-di-GMP turnover in Dinoroseobacter shibae.

The ubiquitous bacterial second messenger bis-(3'-5')-cyclic dimeric GMP (c-di-GMP) is involved in the regulation of numerous processes including biofilm formation, motility, virulence, cell cycle and differentiation. In this study, we searched the genome of the ecologically important marine alphaproteobacterium Dinoroseobacter shibae DFL12T for genes encoding putative c-di-GMP-modulating enzymes. Overall, D. shibae was found to possess two diguanylate cyclases (Dshi_2814 and Dshi_2820) as well as two c-di-GMP-specific phosphodiesterases (Dhi_0329 and Dshi_3065). Recombinant expression and purification followed by enzymatic analysis revealed that all four proteins exhibit their proposed activity. Furthermore, adjacent to Dshi_2814 we identified a gene encoding a heme nitric oxide/oxygen binding (H-NOX) protein. These proteins are often found in association with c-di-GMP signal transduction pathways and modulate their function through binding of diatomic gases such as nitric oxide. Here, we demonstrate that H-NOX constitutes a functional unit together with the diguanylate cyclase Dshi_2814. NO-bound H-NOX strongly inhibits DGC activity. Based on these results, and with respect to previously published data including micro-array analysis, we propose an interlinkage of c-di-GMP signalling with cell-cell communication and differentiation in D. shibae.

The Sinorhizobium fredii HH103 MucR1 Global Regulator Is Connected With the nod Regulon and Is Required for Efficient Symbiosis With Lotus burttii and Glycine max cv. Williams.

Sinorhizobium fredii HH103 is a rhizobial strain showing a broad host range of nodulation. In addition to the induction of bacterial nodulation genes, transition from a free-living to a symbiotic state requires complex genetic expression changes with the participation of global regulators. We have analyzed the role of the zinc-finger transcriptional regulator MucR1 from S. fredii HH103 under both free-living conditions and symbiosis with two HH103 host plants, Glycine max and Lotus burttii. Inactivation of HH103 mucR1 led to a severe decrease in exopolysaccharide (EPS) biosynthesis but enhanced production of external cyclic glucans (CG). This mutant also showed increased cell aggregation capacity as well as a drastic reduction in nitrogen-fixation capacity with G. max and L. burttii. However, in these two legumes, the number of nodules induced by the mucR1 mutant was significantly increased and decreased, respectively, with respect to the wild-type strain, indicating that MucR1 can differently affect nodulation depending on the host plant. RNA-Seq analysis carried out in the absence and the presence of flavonoids showed that MucR1 controls the expression of hundreds of genes (including some related to EPS production and CG transport), some of them being related to the nod regulon.

The Sinorhizobium fredii HH103 Genome: A Comparative Analysis With S. fredii Strains Differing in Their Symbiotic Behavior With Soybean.

Sinorhizobium fredii HH103 is a fast-growing rhizobial strain infecting a broad range of legumes including both American and Asiatic soybeans. In this work, we present the sequencing and annotation of the HH103 genome (7.25 Mb), consisting of one chromosome and six plasmids and representing the structurally most complex sinorhizobial genome sequenced so far. Comparative genomic analyses of S. fredii HH103 with strains USDA257 and NGR234 showed that the core genome of these three strains contains 4,212 genes (61.7% of the HH103 genes). Synteny plot analysis revealed that the much larger chromosome of USDA257 (6.48 Mb) is colinear to the HH103 (4.3 Mb) and NGR324 chromosomes (3.9 Mb). An additional region of the USDA257 chromosome of about 2 Mb displays similarity to plasmid pSfHH103e. Remarkable differences exist between HH103 and NGR234 concerning nod genes, flavonoid effect on surface polysaccharide production, and quorum-sensing systems. Furthermore a number of protein secretion systems have been found. Two genes coding for putative type III-secreted effectors not previously described in S. fredii, nopI and gunA, have been located on the HH103 genome. These differences could be important to understand the different symbiotic behavior of S. fredii strains HH103, USDA257, and NGR234 with soybean.

The Sinorhizobium meliloti EmrAB efflux system is regulated by flavonoids through a TetR-like regulator (EmrR).

The divergently oriented Sinorhizobium meliloti emrAB (SMc03168 and SMc03167) and emrR (SMc03169) genes are predicted to encode an efflux system of the major facilitator superfamily and a TetR-like transcriptional regulator, respectively. The transcription of the emrA gene was found to be inducible by flavonoids, including luteolin and apigenin, which are known inducers of the nodulation genes in S. meliloti. Interestingly, quercetin, which does not induce nodulation genes, was also a potent inducer of emrA, indicating that NodD is not directly involved in regulation of emrA. The likely regulator of emrAB is EmrR, which binds to palindrome-like sequences in the intergenic region. Several modifications of the palindromes, including an increase of the spacing between the two half sites, prevented binding of EmrR. Binding was also impaired by the presence of luteolin. Mutations in emrA had no obvious effect on symbiosis. This was in contrast to the emrR mutant, which exhibited a symbiotic deficiency with Medicago sativa. Conserved binding sites for TetR-like regulators within the intergenic regions between the emrAB and emrR genes were identified in many symbiotic and pathogenic members of the order Rhizobiales.

Identification of Shemin pathway genes for tetrapyrrole biosynthesis in bacteriophage sequences from aquatic environments.

Tetrapyrroles such as heme, chlorophyll, and vitamin B12 are essential for various metabolic pathways. They derive from 5-aminolevulinic acid (5-ALA), which can be synthesized by a single enzyme (5-ALA synthase or AlaS, Shemin pathway) or by a two-enzyme pathway. The genomes of some bacteriophages from aquatic environments carry various tetrapyrrole biosynthesis genes. Here, we analyze available metagenomic datasets and identify alaS homologs (viral alaS, or valaS) in sequences corresponding to marine and freshwater phages. The genes are found individually or as part of complete or truncated three-gene loci encoding heme-catabolizing enzymes. Amino-acid sequence alignments and three-dimensional structure prediction support that the valaS sequences likely encode functional enzymes. Indeed, we demonstrate that is the case for a freshwater phage valaS sequence, as it can complement an Escherichia coli 5-ALA auxotroph, and an E. coli strain overexpressing the gene converts the typical AlaS substrates glycine and succinyl-CoA into 5-ALA. Thus, our work identifies valaS as an auxiliary metabolic gene in phage sequences from aquatic environments, further supporting the importance of tetrapyrrole metabolism in bacteriophage biology.

Characterization of a soluble library of the Pseudomonas aeruginosa PAO1 membrane proteome with emphasis on c-di-GMP turnover enzymes.

Studies of protein-protein interactions in membranes are very important to fully understand the biological function of a cell. The extraction of proteins from the native membrane environment is a critical step in the preparation of membrane proteins that might affect the stability of protein complexes. In this work, we used the amphiphilic diisobutylene/maleic acid copolymer to extract the membrane proteome of the opportunistic pathogen Pseudomonas aeruginosa, thereby creating a soluble membrane-protein library within a native-like lipid-bilayer environment. Size fractionation of nanodisc-embedded proteins and subsequent mass spectrometry enabled the identification of 3358 proteins. The native membrane-protein library showed a very good overall coverage compared to previous proteome data. The pattern of size fractionation indicated that protein complexes were preserved in the library. More than 20 previously described complexes, e.g. the SecYEG and Pili complexes, were identified and analyzed for coelution. Although the mass-spectrometric dataset alone did not reveal new protein complexes, combining pulldown assays with mass spectrometry was successful in identifying new protein interactions in the native membrane-protein library. Thus, we identified several candidate proteins for interactions with the membrane phosphodiesterase NbdA, a member of the c-di-GMP network. We confirmed the candidate proteins CzcR, PA4200, SadC, and PilB as novel interaction partners of NbdA using the bacterial adenylate cyclase two-hybrid assay. Taken together, this work demonstrates the usefulness of the native membrane-protein library of P. aeruginosa for the investigation of protein interactions and membrane-protein complexes. Data are available via ProteomeXchange with identifiers PXD039702 and PXD039700.

Genome sequence of the soybean symbiont Sinorhizobium fredii HH103.

Sinorhizobium fredii HH103 is a fast-growing rhizobial strain that is able to nodulate legumes that develop determinate nodules, e.g., soybean, and legumes that form nodules of the indeterminate type. Here we present the genome of HH103, which consists of one chromosome and five plasmids with a total size of 7.22 Mb.

Characterization of the self-cleaving effector protein NopE1 of Bradyrhizobium japonicum.

NopE1 is a type III-secreted protein of the symbiont Bradyrhizobium japonicum which is expressed in nodules. In vitro it exhibits self-cleavage in a duplicated domain of unknown function (DUF1521) but only in the presence of calcium. Here we show that either domain is self-sufficient for cleavage. An exchange of the aspartic acid residue at the cleavage site with asparagine prevented cleavage; however, cleavage was still observed with glutamic acid at the same position, indicating that a negative charge at the cleavage site is sufficient. Close to each cleavage site, an EF-hand-like motif is present. A replacement of one of the conserved aspartic acid residues with alanine prevented cleavage at the neighboring site. Except for EDTA, none of several protease inhibitors blocked cleavage, suggesting that a known protease-like mechanism is not involved in the reaction. In line with this, the reaction takes place within a broad pH and temperature range. Interestingly, magnesium, manganese, and several other divalent cations did not induce cleavage, indicating a highly specific calcium-binding site. Based on results obtained by blue-native gel electrophoresis, it is likely that the uncleaved protein forms a dimer and that the fragments of the cleaved protein oligomerize. A database search reveals that the DUF1521 domain is present in proteins encoded by Burkholderia phytofirmans PsNJ (a plant growth-promoting betaproteobacterium) and Vibrio coralliilyticus ATCC BAA450 (a pathogenic gammaproteobacterium). Obviously, this domain is more widespread in proteobacteria, and it might contribute to the interaction with hosts.

The type III-secreted protein NopE1 affects symbiosis and exhibits a calcium-dependent autocleavage activity.

The type III-secreted proteins NopE1 and NopE2 of Bradyrhizobium japonicum contain a repeated domain of unknown function (DUF1521), which is present in a few uncharacterized proteins. A nopE1/nopE2 double mutant strain exhibited higher nodulation efficiency on Vigna radiata KPS2 than the wild type or single nopE1 or nopE2 mutants. This indicates that both proteins are effectors that functionally overlap. To test translocation into the plant cell compartment during symbiosis, NopE1 and NopE2 were fused with adenylate cyclase (cya) as reporter. A fusion with the full-length proteins or N-terminal peptides resulted in increased cAMP levels in nodules, indicating translocation. Purified NopE1 exhibited self-cleavage in the presence of Ca(2+). Two identical cleavage sites (GD'PHVD) were identified inside the DUF1521 domains. The C-terminal cleavage site was analyzed by alanine scanning. Protein variants in which aspartate or proline next to the cleavage sites was substituted displayed no cleavage. A noncleavable protein was obtained by exchange of the aspartate residues preceding both cleavage sites. Complementation analysis with the noncleavable NopE1 variant did not restore wild-type phenotype on Vigna radiata KPS2, indicating a physiological role of NopE1 cleavage in effector function.

Calcium binding to a disordered domain of a type III-secreted protein from a coral pathogen promotes secondary structure formation and catalytic activity.

Strains of the Gram-negative bacterium Vibrio coralliilyticus cause the bleaching of corals due to decomposition of symbiotic microalgae. The V. coralliilyticus strain ATCC BAA-450 (Vc450) encodes a type III secretion system (T3SS). The gene cluster also encodes a protein (locus tag VIC_001052) with sequence homology to the T3SS-secreted nodulation proteins NopE1 and NopE2 of Bradyrhizobium japonicum (USDA110). VIC_001052 has been shown to undergo auto-cleavage in the presence of Ca2+ similar to the NopE proteins. We have studied the hitherto unknown secondary structure, Ca2+-binding affinity and stoichiometry of the "metal ion-inducible autocleavage" (MIIA) domain of VIC_001052 which does not possess a classical Ca2+-binding motif. CD and fluorescence spectroscopy revealed that the MIIA domain is largely intrinsically disordered. Binding of Ca2+ and other di- and trivalent cations induced secondary structure and hydrophobic packing after partial neutralization of the highly negatively charged MIIA domain. Mass spectrometry and isothermal titration calorimetry showed two Ca2+-binding sites which promote structure formation with a total binding enthalpy of -110 kJ mol-1 at a low micromolar Kd. Putative binding motifs were identified by sequence similarity to EF-hand domains and their structure analyzed by molecular dynamics simulations. The stoichiometric Ca2+-dependent induction of structure correlated with catalytic activity and may provide a "host-sensing" mechanism that is shared among pathogens that use a T3SS for efficient secretion of disordered proteins.

Expression of the Bradyrhizobium japonicum type III secretion system in legume nodules and analysis of the associated tts box promoter.

In Bradyrhizobium japonicum, as in some other rhizobia, symbiotic efficiency is influenced by a type III secretion system (T3SS). Most genes encoding the transport machinery and secreted proteins are preceded by a conserved 30-bp motif, the type-three secretion (tts) box. In this study, we found that regions downstream of 34 tts boxes are transcribed. For nopB, nopL, and gunA2, the transcriptional start sites were found to be 12, 11, and 10 bp downstream of their tts boxes, respectively. The deletion of this motif or modification of two or more conserved residues strongly reduced expression of nopB. This indicates that the tts box is an essential promoter element. Data obtained with lacZ reporter gene fusions of five genes preceded by a tts box (gunA2, nopB, rhcV, nopL, and blr1806) revealed that they are expressed in 4-week-old nodules of Macroptilium atropurpureum. These data suggest that the T3SS is active in mature nitrogen-fixing nodules. The two-component response regulator TtsI is required for the expression of rhcV, nopL, and blr1806 in bacteroids. Staining of inoculated roots showed that nopB is also expressed in early infection stages.

Characterization of a novel MIIA domain-containing protein (MdcE) in Bradyrhizobium spp.

Several genes coding for proteins with metal ion-inducible autocleavage (MIIA) domains were identified in type III secretion system tts gene clusters from draft genomes of recently isolated Bradyrhizobium spp. MIIA domains have been first described in the effectors NopE1 and NopE2 of Bradyrhizobium diazoefficiens USDA 110. All identified genes are preceded by tts box promoter motifs. The identified proteins contain one or two MIIA domains. A phylogenetic analysis of 35 MIIA domain sequences from 16 Bradyrhizobium strains revealed four groups. The protein from Bradyrhizobium sp. LmjC strain contains a single MIIA domain and was designated MdcE (MdcELmjC). It was expressed as a fusion to maltose-binding protein (MalE) in Escherichia coli and subsequently purified by affinity chromatography. Recombinant MalE-MdcELmjC-Strep protein exhibited autocleavage in the presence of Ca2+, Cu2+, Cd2+ and Mn2+, but not in the presence of Mg2+, Ni2+ or Co2+. Site-directed mutagenesis at the predicted cleavage site abolished autocleavage activity of MdcELmjC. An LmjC mdcE- mutant was impaired in the ability to nodulate Lupinus angustifolius and Macroptilium atropurpureum.

A regioselective tryptophan 5-halogenase is involved in pyrroindomycin biosynthesis in Streptomyces rugosporus LL-42D005.

The antibiotic compound pyrroindomycin B contains an indole ring chlorinated in the 5 position. The indole ring is probably derived from tryptophan, and thus primers derived from conserved regions of tryptophan halogenases were used to amplify and clone a DNA fragment that was then used to isolate a tryptophan 5-halogenase gene (pyrH) from a cosmid library of the pyrroindomycin producer Streptomyces rugosporus LL-42D005. A gene disruption mutant in the tryptophan 5-halogenase gene no longer produced pyrroindomycin B, but still produced pyrroindomycin A, the nonhalogenated derivative. The halogenase gene could be overexpressed in Pseudomonas fluorescens BL915 DeltaORF1 and was purified to homogeneity by immobilized metal chelate ion affinity chromatography. Chlorinating and brominating activities with tryptophan as a substrate were detected in cell-free extracts and for the purified enzyme.

Identification of genistein-inducible and type III-secreted proteins of Bradyrhizobium japonicum.

Flagellin is the bulk protein secreted by Bradyrhizobium japonicum. For easier identification of minor protein fractions, the flagellin genes bll6865 and bll6866 were deleted. Extracellular proteins of the corresponding mutant were purified and separated by 2D gel electrophoresis. Several of the protein spots were detectable only after addition of genistein to the growth medium-genistein is an isoflavone secreted by soybean that activates the expression of genes encoding a type III secretion system. These secreted proteins were not present in supernatants of mutants in which conserved genes of the type III secretion system or the regulatory gene ttsI, which is essential for activation of the type III secretion system, are deleted. Out of 22 genistein-inducible protein spots 8 different proteins could be identified by mass spectrometry. One of the proteins, Blr1752, has similarity to NopP of Rhizobium sp. strain NGR234 that is known to be secreted. Another protein is Blr1656 (GunA2) that was shown previously to have endoglucanase activity. Three proteins have similarity to subunits of the flagellar apparatus. Some proteins appeared in several separate spots indicating posttranslational modification. A conserved tts box motif was found in the putative promoter region of six genes encoding secreted proteins.

Single step purification of recombinant proteins using the metal ion-inducible autocleavage (MIIA) domain as linker for tag removal.

For fast and easy purification, proteins are typically fused with an affinity tag, which often needs to be removed after purification. Here, we present a method for the removal of the affinity tag from the target protein in a single step protocol. The protein VIC_001052 of the coral pathogen Vibrio coralliilyticus ATCC BAA-450 contains a metal ion-inducible autocatalytic cleavage (MIIA) domain. Its coding sequence was inserted into an expression vector for the production of recombinant fusion proteins. Following, the target proteins MalE and mCherry were produced as MIIA-Strep fusion proteins in Escherichia coli. The target proteins could be separated from the MIIA-Strep part simply by the addition of calcium or manganese(II) ions within minutes. The cleavage is not affected in the pH range from 5.0 to 9.0 or at low temperatures (6°C). Autocleavage was also observed with immobilized protein on an affinity column. The protein yield was similar to that achieved with a conventional purification protocol.

The domain of unknown function DUF1521 exhibits metal ion-inducible autocleavage activity - a novel example from a putative effector protein of Vibrio coralliilyticus ATCC BAA-450.

Vibrio coralliilyticus ATCC BAA-450 is a pathogen causing coral bleaching at elevated seawater temperatures. Based on the available genome sequence, the strain has a type III secretion system. Within the corresponding gene cluster, VIC_001052 is encoded, which contains a conserved domain of unknown function DUF1521. In this study, we show that the purified domain exhibits autocleavage activity in the presence of several divalent metal ions, for example, calcium and manganese but not with magnesium or zinc. Autocleavage is not affected by temperatures between 0 and 30 °C, indicating that seawater temperature is not a critical factor for this activity. The DUF1521 domain and the cleavage site are conserved in several proteins from proteobacteria, suggesting a similar cleavage activity for these proteins.

Analysis of the secretome of the soybean symbiont Bradyrhizobium japonicum.

Proteins from the supernatant of Bradyrhizobium japonicum were separated by two-dimensional gel electrophoresis and stained with Coomassie. This revealed more than 100 protein spots. Sixty-eight proteins were identified by mass spectrometry. Thirty-five are predicted to contain an N-terminal signal peptide characteristic for proteins transported by the general secretory pathway. Most of these appear to be substrate-binding proteins of the ABC transporter family. Ten proteins were categorized as unclassified conserved or hypothetical. None of the proteins has similarity to proteins transported by a type I secretion system or to autotransporters. Three of the proteins might be located in the outer membrane. The addition of genistein led to changes in the spot pattern of three flagellar proteins and resulted in the identification of the nodulation outer protein Pgl. Moreover, the application of shot-gun mass spectrometry resulted in the first-time identification of NopB, NopH and NopT, which were present only after genistein induction. Replacing genistein with daidzein or coumestrol reduced the amount of the type III-secreted protein GunA2.

Genetic organization and functional analysis of the type III secretion system of Bradyrhizobium elkanii.

Cloning and sequencing of a 47.1-kb chromosomal DNA region revealed the presence of a type III secretion system (T3SS) in Bradyrhizobium elkanii USDA61. The identified genes are likely to encode the transcriptional activator TtsI, core components of the secretion apparatus and secreted proteins. Several ORFs within the cluster are not conserved in other rhizobia. Nine tts box motifs, a promoter element of TtsI-regulated genes, were found; six of them upstream of annotated genes. For functional analyses, the rhcC2 and rhcJ genes were disrupted. These mutations had a cultivar-specific effect on nodulation. Vigna radiata cv. KPS1 developed nodules if infected with the mutant strains but not with the wild type. In contrast, V. radiata cv. CN36 was nodulated by all strains. Nodulation of rj(1) soybean depended on the T3SS. A comparison of the protein patterns from supernatants of the wild type and rhcJ mutant by two-dimensional gel electrophoresis revealed proteins that are secreted only in the wild-type background. These results show that B. elkanii encodes a functional T3SS that is involved in the interaction with host legumes.

Characterization of two sets of subpolar flagella in Bradyrhizobium japonicum.

Bradyrhizobium japonicum is one of the soil bacteria that form nodules on soybean roots. The cell has two sets of flagellar systems, one thick flagellum and a few thin flagella, uniquely growing at subpolar positions. The thick flagellum appears to be semicoiled in morphology, and the thin flagella were in a tight-curly form as observed by dark-field microscopy. Flagellin genes were identified from the amino acid sequence of each flagellin. Flagellar genes for the thick flagellum are scattered into several clusters on the genome, while those genes for the thin flagellum are compactly organized in one cluster. Both types of flagella are powered by proton-driven motors. The swimming propulsion is supplied mainly by the thick flagellum. B. japonicum flagellar systems resemble the polar-lateral flagellar systems of Vibrio species but differ in several aspects.