Sinorhizobium meliloti pSymB carries genes necessary for arabinose transport and catabolism

Multiple Roles of a Conserved Glutamate Residue for Unique Biophysical Properties in a New Group of Microbial Rhodopsins Homologous to TAT Rhodopsin

TAT rhodopsin, a microbial rhodopsin found in the marine SAR11 bacterium HIMB114, uniquely possesses a Thr-Ala-Thr (TAT) motif in the third transmembrane helix. Because of a low pKa value of the retinal Schiff base (RSB), TAT rhodopsin exhibits both a visible light-absorbing state with the protonated RSB and a UV-absorbing state with the deprotonated RSB at a neutral pH. The UV-absorbing state, in contrast to the visible light-absorbing one, converts to a long-lived photointermediate upon light absorption, implying that TAT rhodopsin functions as a pH-dependent light sensor. Despite detailed biophysical characterization and mechanistic studies on the TAT rhodopsin, it has been unknown whether other proteins with similarly unusual features exist. Here, we identified several new rhodopsin genes homologous to the TAT rhodopsin of HIMB114 (TATHIMB) from metagenomic data. Based on the absorption spectra of expressed proteins from these genes with visible and UV peaks similar to that of TATHIMB, they were classified as Twin-peaked Rhodopsin (TwR) family. TwR genes form a gene cluster with a set of 13 ORFs conserved in subclade IIIa of SAR11 bacteria. A glutamic acid in the second transmembrane helix, Glu54, is conserved in all of the TwRs. We investigated E54Q mutants of two TwRs and revealed that Glu54 plays critical roles in regulating the RSB pKa, oligomer formation, and the efficient photoreaction of the UV-absorbing state. The discovery of novel TwRs enables us to study the universality and individuality of the characteristics revealed so far in the original TATHIMB and contributes to further studies on mechanisms of unique properties of TwRs.

The transport of mannitol in Sinorhizobium meliloti is carried out by a broad-substrate polyol transporter SmoEFGK and is affected by the ability to transport and metabolize fructose

The smo locus (sorbitol mannitol oxidation) is found on the chromosome of S. meliloti's tripartite genome. Mutations at the smo locus reduce or abolish the ability of the bacterium to grow on several carbon sources, including sorbitol, mannitol, galactitol, d-arabitol and maltitol. The contribution of the smo locus to the metabolism of these compounds has not been previously investigated. Genetic complementation of mutant strains revealed that smoS is responsible for growth on sorbitol and galactitol, while mtlK restores growth on mannitol and d-arabitol. Dehydrogenase assays demonstrate that SmoS and MtlK are NAD+-dependent dehydrogenases catalysing the oxidation of their specific substrates. Transport experiments using a radiolabeled substrate indicate that sorbitol, mannitol and d-arabitol are primarily transported into the cell by the ABC transporter encoded by smoEFGK. Additionally, it was found that a mutation in either frcK, which is found in an operon that encodes the fructose ABC transporter, or a mutation in frk, which encodes fructose kinase, leads to the induction of mannitol transport.

Inability to Catabolize Rhamnose by Sinorhizobium meliloti Rm1021Affects Competition for Nodule Occupancy

Rhizobium leguminosarum strains unable to grow on rhamnose as a sole carbon source are less competitive for nodule occupancy. To determine if the ability to use rhamnose as a sole carbon source affects competition for nodule occupancy in Sinorhizobium meliloti, Tn5 mutants unable to use rhamnose as a sole carbon source were isolated. S. meliloti mutations affecting rhamnose utilization were found in two operons syntenous to those of R. leguminosarum. Although the S. meliloti Tn5 mutants were complemented using an R. leguminosarum cosmid that contains the entire wild-type rhamnose catabolic locus, complementation did not occur if the cosmids carried Tn5 insertions within the locus. Through a series of heterologous complementation experiments, enzyme assays, gene fusion, and transport experiments, we show that the S. meliloti regulator, RhaR, is dominant to its R. leguminosarum counterpart. In addition, the data support the hypothesis that the R. leguminosarum kinase is capable of directly phosphorylating rhamnose and rhamnulose, whereas the S. meliloti kinase does not possess rhamnose kinase activity. In nodule competition assays, S. meliloti mutants incapable of rhamnose transport were shown to be less competitive than the wild-type and had a decreased ability to bind plant roots in the presence of rhamnose. The data suggests that rhamnose catabolism is a general determinant in competition for nodule occupancy that spans across rhizobial species.

Specialization in a Nitrogen-Fixing Symbiosis: Proteome Differences Between Sinorhizobium medicae Bacteria and Bacteroids

Using tandem mass spectrometry (MS/MS), we analyzed the proteome of Sinorhizobium medicae WSM419 growing as free-living cells and in symbiosis with Medicago truncatula. In all, 3,215 proteins were identified, over half of the open reading frames predicted from the genomic sequence. The abundance of 1,361 proteins displayed strong lifestyle bias. In total, 1,131 proteins had similar levels in bacteroids and free-living cells, and the low levels of 723 proteins prevented statistically significant assignments. Nitrogenase subunits comprised approximately 12% of quantified bacteroid proteins. Other major bacteroid proteins included symbiosis-specific cytochromes and FixABCX, which transfer electrons to nitrogenase. Bacteroids had normal levels of proteins involved in amino acid biosynthesis, glycolysis or gluconeogenesis, and the pentose phosphate pathway; however, several amino acid degradation pathways were repressed. This suggests that bacteroids maintain a relatively independent anabolic metabolism. Tricarboxylic acid cycle proteins were highly expressed in bacteroids and no other catabolic pathway emerged as an obvious candidate to supply energy and reductant to nitrogen fixation. Bacterial stress response proteins were induced in bacteroids. Many WSM419 proteins that are not encoded in S. meliloti Rm1021 were detected, and understanding the functions of these proteins might clarify why S. medicae WSM419 forms a more effective symbiosis with M. truncatula than S. meliloti Rm1021.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Characterization of Mutations That Affect the Nonoxidative Pentose Phosphate Pathway in Sinorhizobium meliloti

Sinorhizobium meliloti is a Gram-negative alphaproteobacterium that can enter into a symbiotic relationship with Medicago sativa and Medicago truncatula Previous work determined that a mutation in the tkt2 gene, which encodes a putative transketolase, could prevent medium acidification associated with a mutant strain unable to metabolize galactose. Since the pentose phosphate pathway in S. meliloti is not well studied, strains carrying mutations in either tkt2 and tal, which encodes a putative transaldolase, were characterized. Carbon metabolism phenotypes revealed that both mutants were impaired in growth on erythritol and ribose. This phenotype was more pronounced for the tkt2 mutant strain, which also displayed auxotrophy for aromatic amino acids. Changes in pentose phosphate pathway metabolite concentrations were also consistent with a mutation in either tkt2 or tal The concentrations of metabolites in central carbon metabolism were also found to shift dramatically in strains carrying a tkt2 mutation. While the concentrations of proteins involved in central carbon metabolism did not change significantly under any conditions, the levels of those associated with iron acquisition increased in the wild-type strain with erythritol induction. These proteins were not detected in either mutant, resulting in less observable rhizobactin production in the tkt2 mutant. While both mutants were impaired in succinoglycan synthesis, only the tkt2 mutant strain was unable to establish symbiosis with alfalfa. These results suggest that tkt2 and tal play central roles in regulating the carbon flow necessary for carbon metabolism and the establishment of symbiosis.IMPORTANCESinorhizobium meliloti is a model organism for the study of plant-microbe interactions and metabolism, especially because it effects nitrogen fixation. The ability to derive the energy necessary for nitrogen fixation is dependent on an organism's ability to metabolize carbon efficiently. The pentose phosphate pathway is central in the interconversion of hexoses and pentoses. This study characterizes the key enzymes of the nonoxidative branch of the pentose phosphate pathway by using defined genetic mutations and shows the effects the mutations have on the metabolite profile and on physiological processes such as the biosynthesis of exopolysaccharide, as well as the ability to regulate iron acquisition.

Metabolic modelling reveals the specialization of secondary replicons for niche adaptation in Sinorhizobium meliloti

The genome of about 10% of bacterial species is divided among two or more large chromosome-sized replicons. The contribution of each replicon to the microbial life cycle (for example, environmental adaptations and/or niche switching) remains unclear. Here we report a genome-scale metabolic model of the legume symbiont Sinorhizobium meliloti that is integrated with carbon utilization data for 1,500 genes with 192 carbon substrates. Growth of S. meliloti is modelled in three ecological niches (bulk soil, rhizosphere and nodule) with a focus on the role of each of its three replicons. We observe clear metabolic differences during growth in the tested ecological niches and an overall reprogramming following niche switching. In silico examination of the inferred fitness of gene deletion mutants suggests that secondary replicons evolved to fulfil a specialized function, particularly host-associated niche adaptation. Thus, genes on secondary replicons might potentially be manipulated to promote or suppress host interactions for biotechnological purposes.

The genome of some bacteria consists of two or more chromosomes or replicons. Here, diCenzo et al. integrate genome-scale metabolic modelling and growth data from a collection of mutants of the plant symbiont Sinorhizobium meliloti to estimate the fitness contribution of each replicon in three environments.

Genomic resources for identification of the minimal N2 -fixing symbiotic genome

The lack of an appropriate genomic platform has precluded the use of gain-of-function approaches to study the rhizobium-legume symbiosis, preventing the establishment of the genes necessary and sufficient for symbiotic nitrogen fixation (SNF) and potentially hindering synthetic biology approaches aimed at engineering this process. Here, we describe the development of an appropriate system by reverse engineering Sinorhizobium meliloti. Using a novel in vivo cloning procedure, the engA-tRNA-rmlC (ETR) region, essential for cell viability and symbiosis, was transferred from Sinorhizobium fredii to the ancestral location on the S. meliloti chromosome, rendering the ETR region on pSymB redundant. A derivative of this strain lacking both the large symbiotic replicons (pSymA and pSymB) was constructed. Transfer of pSymA and pSymB back into this strain restored symbiotic capabilities with alfalfa. To delineate the location of the single-copy genes essential for SNF on these replicons, we screened a S. meliloti deletion library, representing > 95% of the 2900 genes of the symbiotic replicons, for their phenotypes with alfalfa. Only four loci, accounting for < 12% of pSymA and pSymB, were essential for SNF. These regions will serve as our preliminary target of the minimal set of horizontally acquired genes necessary and sufficient for SNF.

L-Hydroxyproline and d-Proline Catabolism in Sinorhizobium meliloti

Sinorhizobium meliloti forms N2-fixing root nodules on alfalfa, and as a free-living bacterium, it can grow on a very broad range of substrates, including l-proline and several related compounds, such as proline betaine, trans-4-hydroxy-l-proline (trans-4-l-Hyp), and cis-4-hydroxy-d-proline (cis-4-d-Hyp). Fourteen hyp genes are induced upon growth of S. meliloti on trans-4-l-Hyp, and of those, hypMNPQ encodes an ABC-type trans-4-l-Hyp transporter and hypRE encodes an epimerase that converts trans-4-l-Hyp to cis-4-d-Hyp in the bacterial cytoplasm. Here, we present evidence that the HypO, HypD, and HypH proteins catalyze the remaining steps in which cis-4-d-Hyp is converted to α-ketoglutarate. The HypO protein functions as a d-amino acid dehydrogenase, converting cis-4-d-Hyp to Δ(1)-pyrroline-4-hydroxy-2-carboxylate, which is deaminated by HypD to α-ketoglutarate semialdehyde and then converted to α-ketoglutarate by HypH. The crystal structure of HypD revealed it to be a member of the N-acetylneuraminate lyase subfamily of the (α/β)8 protein family and is consistent with the known enzymatic mechanism for other members of the group. It was also shown that S. meliloti can catabolize d-proline as both a carbon and a nitrogen source, that d-proline can complement l-proline auxotrophy, and that the catabolism of d-proline is dependent on the hyp cluster. Transport of d-proline involves the HypMNPQ transporter, following which d-proline is converted to Δ(1)-pyrroline-2-carboxylate (P2C) largely via HypO. The P2C is converted to l-proline through the NADPH-dependent reduction of P2C by the previously uncharacterized HypS protein. Thus, overall, we have now completed detailed genetic and/or biochemical characterization of 9 of the 14 hyp genes.

IMPORTANCE

Hydroxyproline is abundant in proteins in animal and plant tissues and serves as a carbon and a nitrogen source for bacteria in diverse environments, including the rhizosphere, compost, and the mammalian gut. While the main biochemical features of bacterial hydroxyproline catabolism were elucidated in the 1960s, the genetic and molecular details have only recently been determined. Elucidating the genetics of hydroxyproline catabolism will aid in the annotation of these genes in other genomes and metagenomic libraries. This will facilitate an improved understanding of the importance of this pathway and may assist in determining the prevalence of hydroxyproline in a particular environment.

Arabinose and protocatechuate catabolism genes are important for growth of Rhizobium leguminosarum biovar viciae in the pea rhizosphere

BACKGROUND AND AIMS

To form nitrogen-fixing nodules on pea roots, Rhizobium leguminosarum biovar viciae must be competitive in the rhizosphere. Our aim was to identify genes important for rhizosphere fitness.

METHODS

Signature-tagged mutants were screened using microarrays to identify mutants reduced for growth in pea rhizospheres. Candidate mutants were assessed relative to controls for growth in minimal medium, growth in pea rhizospheres and for infection of peas in mixed inoculants. Mutated genes were identified by DNA sequencing and confirmed by transduction.

RESULTS

Of 5508 signature-tagged mutants, microarrays implicated 50 as having decreased rhizosphere fitness. Growth tests identified six mutants with rhizosphere-specific phenotypes. The mutation in one of the genes (araE) was in an arabinose catabolism operon and blocked growth on arabinose. The mutation in another gene (pcaM), encoding a predicted solute binding protein for protocatechuate and hydroxybenzoate uptake, decreased growth on protocatechuate. Both mutants were decreased for nodule infection competitiveness with mixed inoculants, but nodulated peas normally when inoculated alone. Other mutants with similar phenotypes had mutations predicted to affect secondary metabolism.

CONCLUSIONS

Catabolism of arabinose and protocatechuate in the pea rhizosphere is important for competitiveness of R.l. viciae. Other genes predicted to be involved in secondary metabolism are also important.

Exopolysaccharide production in response to medium acidification is correlated with an increase in competition for nodule occupancy

Sinorhizobium meliloti strains unable to utilize galactose as a sole carbon source, due to mutations in the De-Ley Doudoroff pathway (dgoK), were previously shown to be more competitive for nodule occupancy. In this work, we show that strains carrying this mutation have galactose-dependent exopolysaccharide (EPS) phenotypes that were manifested as aberrant Calcofluor staining as well as decreased mucoidy when in an expR(+) genetic background. The aberrant Calcofluor staining was correlated with changes in the pH of the growth medium. Strains carrying dgoK mutations were subsequently demonstrated to show earlier acidification of their growth medium that was correlated with an increase expression of genes associated with succinoglycan biosynthesis as well as increased accumulation of high and low molecular weight EPS in the medium. In addition, it was shown that the acidification of the medium was dependent on the inability of S. meliloti strains to initiate the catabolism of galactose. To more fully understand why strains carrying the dgoK allele were more competitive for nodule occupancy, early nodulation phenotypes were investigated. It was found that strains carrying the dgoK allele had a faster rate of nodulation. In addition, nodule competition experiments using genetic backgrounds unable to synthesize either succinoglycan or EPSII were consistent with the hypothesis that the increased competition phenotype was dependent upon the synthesis of succinoglycan. Fluorescent microscopy experiments on infected root-hair cells, using the acidotropic dye Lysotracker Red DND-99, provide evidence that the colonized curled root hair is an acidic compartment.

Analysis of a taurine-dependent promoter in Sinorhizobium meliloti that offers tight modulation of gene expression

Background

Genetic models have been developed in divergent branches of the class Alphaproteobacteria to help answer a wide spectrum of questions regarding bacterial physiology. For example, Sinorhizobium meliloti serves as a useful representative for investigating rhizobia-plant symbiosis and nitrogen fixation, Caulobacter crescentus for studying cell cycle regulation and organelle biogenesis, and Zymomonas mobilis for assessing the potentials of metabolic engineering and biofuel production. A tightly regulated promoter that enables titratable expression of a cloned gene in these different models is highly desirable, as it can facilitate observation of phenotypes that would otherwise be obfuscated by leaky expression.

Results

We compared the functionality of four promoter regions in S. meliloti (P_araA, P_tauA, P_rhaR, and P_melA) by constructing strains carrying fusions to the uidA reporter in their genomes and measuring beta-glucuronidase activities when they were induced by arabinose, taurine, rhamnose, or melibiose. P_tauA was chosen for further study because it, and, to a lesser extent, P_melA, exhibited characteristics suitable for efficient modulation of gene expression. The levels of expression from P_tauA depended on the concentrations of taurine, in both complex and defined media, in S. meliloti as well as C. crescentus and Z. mobilis. Moreover, our analysis indicated that TauR, TauC, and TauY are each necessary for taurine catabolism and substantiated their designated roles as a transcriptional activator, the permease component of an ABC transporter, and a major subunit of the taurine dehydrogenase, respectively. Finally, we demonstrated that P_tauA can be used to deplete essential cellular factors in S. meliloti, such as the PleC histidine kinase and TatB, a component of the twin-arginine transport machinery.

Conclusions

The P_tauA promoter of S. meliloti can control gene expression with a relatively inexpensive and permeable inducer, taurine, in diverse alpha-proteobacteria. Regulated expression of the same gene in different hosts can be achieved by placing both tauR and P_tauA on appropriate vectors, thus facilitating inspection of conservation of gene function across species.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-014-0295-2) contains supplementary material, which is available to authorized users.

Examination of prokaryotic multipartite genome evolution through experimental genome reduction

Many bacteria carry two or more chromosome-like replicons. This occurs in pathogens such as Vibrio cholerea and Brucella abortis as well as in many N₂-fixing plant symbionts including all isolates of the alfalfa root-nodule bacteria Sinorhizobium meliloti. Understanding the evolution and role of this multipartite genome organization will provide significant insight into these important organisms; yet this knowledge remains incomplete, in part, because technical challenges of large-scale genome manipulations have limited experimental analyses. The distinct evolutionary histories and characteristics of the three replicons that constitute the S. meliloti genome (the chromosome (3.65 Mb), pSymA megaplasmid (1.35 Mb), and pSymB chromid (1.68 Mb)) makes this a good model to examine this topic. We transferred essential genes from pSymB into the chromosome, and constructed strains that lack pSymB as well as both pSymA and pSymB. This is the largest reduction (45.4%, 3.04 megabases, 2866 genes) of a prokaryotic genome to date and the first removal of an essential chromid. Strikingly, strains lacking pSymA and pSymB (ΔpSymAB) lost the ability to utilize 55 of 74 carbon sources and various sources of nitrogen, phosphorous and sulfur, yet the ΔpSymAB strain grew well in minimal salts media and in sterile soil. This suggests that the core chromosome is sufficient for growth in a bulk soil environment and that the pSymA and pSymB replicons carry genes with more specialized functions such as growth in the rhizosphere and interaction with the plant. These experimental data support a generalized evolutionary model, in which non-chromosomal replicons primarily carry genes with more specialized functions. These large secondary replicons increase the organism's niche range, which offsets their metabolic burden on the cell (e.g. pSymA). Subsequent co-evolution with the chromosome then leads to the formation of a chromid through the acquisition of functions core to all niches (e.g. pSymB).

Rhizobia are free-living bacteria of agricultural and environmental importance that form root-nodules on leguminous plants and provide these plants with fixed nitrogen. Many of the rhizobia have a multipartite genome, as do several plant and animal pathogens. All isolates of the alfalfa symbiont, Sinorhizobium meliloti, carry three large replicons, the chromosome (∼3.7 Mb), pSymA megaplasmid (∼1.4 Mb), and pSymB chromid (∼1.7 Mb). To gain insight into the role and evolutionary history of these replicons, we have ‘reversed evolution’ by constructing a S. meliloti strain consisting solely of the chromosome and lacking the pSymB chromid and pSymA megaplasmid. As the resulting strain was viable, we could perform a detailed phenotypic analysis and these data provided significant insight into the biology and metabolism of S. meliloti. The data lend direct experimental evidence in understanding the evolution and role of the multipartite genome. Specifically the large secondary replicons increase the organism's niche range, and this advantage offsets the metabolic burden of these replicons on the cell. Additionally, the single-chromosome strain offers a useful platform to facilitate future forward genetic approaches to understanding and manipulating the symbiosis and plant-microbe interactions.

Physiology, genetics, and biochemistry of carbon metabolism in the alphaproteobacterium Sinorhizobium meliloti

A large proportion of genes within a genome encode proteins that play a role in metabolism. The Alphaproteobacteria are a ubiquitous group of bacteria that play a major role in a number of environments. For well over 50 years, carbon metabolism in Rhizobium has been studied at biochemical and genetic levels. Here, we review the pre- and post-genomics literature of the metabolism of the alphaproteobacterium Sinorhizobium meliloti. This review provides an overview of carbon metabolism that is useful to readers interested in this organism and to those working on other organisms that do not follow other model system paradigms.

A link between arabinose utilization and oxalotrophy in Bradyrhizobium japonicum

Rhizobia have a versatile catabolism that allows them to compete successfully with other microorganisms for nutrients in the soil and in the rhizosphere of their respective host plants. In this study, Bradyrhizobium japonicum USDA 110 was found to be able to utilize oxalate as the sole carbon source. A proteome analysis of cells grown in minimal medium containing arabinose suggested that oxalate oxidation extends the arabinose degradation branch via glycolaldehyde. A mutant of the key pathway genes oxc (for oxalyl-coenzyme A decarboxylase) and frc (for formyl-coenzyme A transferase) was constructed and shown to be (i) impaired in growth on arabinose and (ii) unable to grow on oxalate. Oxalate was detected in roots and, at elevated levels, in root nodules of four different B. japonicum host plants. Mixed-inoculation experiments with wild-type and oxc-frc mutant cells revealed that oxalotrophy might be a beneficial trait of B. japonicum at some stage during legume root nodule colonization.

Ecological interactions among phlebotomines (Diptera: Psychodidae) in an agroforestry environment of northeast Brazil

Phlebotomine vectors transmit parasites and can cause visceral leishmaniasis (VL) or cutaneous leishmaniasis (TL). Phlebotomine females are hematophagous but need to ingest carbohydrates, possibly promoting the development of protozoan parasites in their digestive tract. The present study evaluated the species composition and abundance across several habitats in a metropolitan landscape, as well as associations among phlebotomines, plants, and local climatic parameters. Three consecutive monthly collections were carried out in an Atlantic Forest fragment, using CDC light traps in peridomestic areas and cashew, coconut, and mango tree. plantations. Eight species of phlebotomine were captured: Evandromyia evandroi, Lutzomyia longipalpis, Psathyromyia shannoni, Sciopemyia sordellii, Evandromyia walkeri, Psychodopygus wellcomei, Nyssomyia whitmani, and Nyssomyia intermedia, primarily from the forest environment. L. longipalpis was confirmed as a species adapted to anthropic environments, while P. wellcomei was shown to be predominately forest-dwelling. Phlebotomines exhibited diversified food consumption patterns in relation to carbohydrate sources. They fed on both native and exotic species of arboreal and shrubby vegetables and gramineous plants.

Carbohydrate kinase (RhaK)-dependent ABC transport of rhamnose in Rhizobium leguminosarum demonstrates genetic separation of kinase and transport activities

In Rhizobium leguminosarum the ABC transporter responsible for rhamnose transport is dependent on RhaK, a sugar kinase that is necessary for the catabolism of rhamnose. This has led to a working hypothesis that RhaK has two biochemical functions: phosphorylation of its substrate and affecting the activity of the rhamnose ABC transporter. To address this hypothesis, a linker-scanning random mutagenesis of rhaK was carried out. Thirty-nine linker-scanning mutations were generated and mapped. Alleles were then systematically tested for their ability to physiologically complement kinase and transport activity in a strain carrying an rhaK mutation. The rhaK alleles generated could be divided into three classes: mutations that did not affect either kinase or transport activity, mutations that eliminated both transport and kinase activity, and mutations that affected transport activity but not kinase activity. Two genes of the last class (rhaK72 and rhaK73) were found to have similar biochemical phenotypes but manifested different physiological phenotypes. Whereas rhaK72 conferred a slow-growth phenotype when used to complement rhaK mutants, the rhaK73 allele did not complement the inability to use rhamnose as a sole carbon source. To provide insight to how these insertional variants might be affecting rhamnose transport and catabolism, structural models of RhaK were generated based on the crystal structure of related sugar kinases. Structural modeling suggests that both rhaK72 and rhaK73 affect surface-exposed residues in two distinct regions that are found on one face of the protein, suggesting that this protein's face may play a role in protein-protein interaction that affects rhamnose transport.

Root exudation of phytochemicals in Arabidopsis follows specific patterns that are developmentally programmed and correlate with soil microbial functions

Plant roots constantly secrete compounds into the soil to interact with neighboring organisms presumably to gain certain functional advantages at different stages of development. Accordingly, it has been hypothesized that the phytochemical composition present in the root exudates changes over the course of the lifespan of a plant. Here, root exudates of in vitro grown Arabidopsis plants were collected at different developmental stages and analyzed using GC-MS. Principle component analysis revealed that the composition of root exudates varied at each developmental stage. Cumulative secretion levels of sugars and sugar alcohols were higher in early time points and decreased through development. In contrast, the cumulative secretion levels of amino acids and phenolics increased over time. The expression in roots of genes involved in biosynthesis and transportation of compounds represented in the root exudates were consistent with patterns of root exudation. Correlation analyses were performed of the in vitro root exudation patterns with the functional capacity of the rhizosphere microbiome to metabolize these compounds at different developmental stages of Arabidopsis grown in natural soils. Pyrosequencing of rhizosphere mRNA revealed strong correlations (p<0.05) between microbial functional genes involved in the metabolism of carbohydrates, amino acids and secondary metabolites with the corresponding compounds released by the roots at particular stages of plant development. In summary, our results suggest that the root exudation process of phytochemicals follows a developmental pattern that is genetically programmed.

The tRNAarg gene and engA are essential genes on the 1.7-Mb pSymB megaplasmid of Sinorhizobium meliloti and were translocated together from the chromosome in an ancestral strain

Bacterial genomes with two (or more) chromosome-like replicons are known, and these appear to be particularly frequent in alphaproteobacteria. The genome of the N(2)-fixing alfalfa symbiont Sinorhizobium meliloti 1021 contains a 3.7-Mb chromosome and 1.4-Mb (pSymA) and 1.7-Mb (pSymB) megaplasmids. In this study, the tRNA(arg) and engA genes, located on the pSymB megaplasmid, are shown to be essential for growth. These genes could be deleted from pSymB when copies were previously integrated into the chromosome. However, in the closely related strain Sinorhizobium fredii NGR234, the tRNA(arg) and engA genes are located on the chromosome, in a 69-kb region designated the engA-tRNA(arg)-rmlC region. This region includes bacA, a gene that is important for intracellular survival during host-bacterium interactions for S. meliloti and the related alphaproteobacterium Brucella abortus. The engA-tRNA(arg)-rmlC region lies between the kdgK and dppF2 (NGR_c24410) genes on the S. fredii chromosome. Synteny analysis showed that kdgK and dppF2 orthologues are adjacent to each other on the chromosomes of 15 sequenced strains of S. meliloti and Sinorhizobium medicae, whereas the 69-kb engA-tRNA(arg)-rmlC region is present on the pSymB-equivalent megaplasmids. This and other evidence strongly suggests that the engA-tRNA(arg)-rmlC region translocated from the chromosome to the progenitor of pSymB in an ancestor common to S. meliloti and S. medicae. To our knowledge, this work represents one of the first experimental demonstrations that essential genes are present on a megaplasmid.

Rhizobial plasmids — replication, structure and biological role

Soil bacteria, collectively named rhizobia, can establish mutualistic relationships with legume plants. Rhizobia often have multipartite genome architecture with a chromosome and several extrachromosomal replicons making these bacteria a perfect candidate for plasmid biology studies. Rhizobial plasmids are maintained in the cells using a tightly controlled and uniquely organized replication system. Completion of several rhizobial genome-sequencing projects has changed the view that their genomes are simply composed of the chromosome and cryptic plasmids. The genetic content of plasmids and the presence of some important (or even essential) genes contribute to the capability of environmental adaptation and competitiveness with other bacteria. On the other hand, their mosaic structure results in the plasticity of the genome and demonstrates a complex evolutionary history of plasmids. In this review, a genomic perspective was employed for discussion of several aspects regarding rhizobial plasmids comprising structure, replication, genetic content, and biological role. A special emphasis was placed on current post-genomic knowledge concerning plasmids, which has enriched the view of the entire bacterial genome organization by the discovery of plasmids with a potential chromosome-like role.

Characterization of the twin-arginine transport secretome in Sinorhizobium meliloti and evidence for host-dependent phenotypes

The twin-arginine transport (Tat) system is essential for cell viability in Sinorhizobium meliloti and may play a role during the development of root nodules. Utilizing an in vivo recombination strategy, we have constructed 28 strains that contain deletions in predicted Tat substrates. Testing of these mutations for symbiotic proficiency on the plant hosts alfalfa and sweet clover shows that some of these mutations affect associations with these hosts differentially.

Inability to catabolize galactose leads to increased ability to compete for nodule occupancy in Sinorhizobium meliloti

A mutant unable to utilize galactose was isolated in Sinorhizobium meliloti strain Rm1021. The mutation was found to be in a gene annotated dgoK1, a putative 2-keto-3-deoxygalactonokinase. The genetic region was isolated on a complementing cosmid and subsequently characterized. Based on genetic and bioinformatic evidence, the locus encodes all five enzymes (galD, dgoK, dgoA, SMc00883, and ilvD1) involved in the De Ley-Doudoroff pathway for galactose catabolism. Although all five genes are present, genetic analysis suggests that the galactonase (SMc00883) and the dehydratase (ilvD1) are dispensable with respect to the ability to catabolize galactose. In addition, we show that the transport of galactose is partially facilitated by the arabinose transporter (AraABC) and that both glucose and galactose compete with arabinose for transport. Quantitative reverse transcription-PCR (qRT-PCR) data show that in a dgoK background, the galactose locus is constitutively expressed, and the induction of the ara locus seems to be enhanced. Assays of competition for nodule occupancy show that the inability to catabolize galactose is correlated with an increased ability to compete for nodule occupancy.

Genetic characterization of a complex locus necessary for the transport and catabolism of erythritol, adonitol and L-arabitol in Sinorhizobium meliloti

The Sinorhizobium meliloti locus necessary for the utilization of erythritol as a sole carbon source, contains 17 genes, including genes that encode an ABC transporter necessary for the transport of erythritol, as well as the genes encoding EryA, EryB, EryC, TpiB and the regulators EryD and EryR (SMc01615). Construction of defined deletions and complementation experiments show that the other genes at this locus encode products that are necessary for the catabolism of adonitol (ribitol) and l-arabitol, but not d-arabitol. These analyses show that aside from one gene that is specific for the catabolism of l-arabitol (SMc01619, lalA), the rest of the catabolic genes are necessary for both polyols (SMc01617, rbtC; SMc01618, rbtB; SMc01622, rbtA). Genetic and biochemical data show that in addition to utilizing erythritol as a substrate, EryA is also capable of utilizing adonitol and l-arabitol. Similarly, transport experiments using labelled erythritol show that adonitol, l-arabitol and erythritol share a common transporter (MptABCDE). Quantitative RT-PCR experiments show that transcripts containing genes necessary for adonitol and l-arabitol utilization are induced by these sugars in an eryA-dependent manner.

Characterization of the mmsAB-araD1 (gguABC) genes of Agrobacterium tumefaciens

The chvE-gguABC operon plays a critical role in both virulence and sugar utilization through the activities of the periplasmic ChvE protein, which binds to a variety of sugars. The roles of the GguA, GguB, and GguC are not known. While GguA and GguB are homologous to bacterial ABC transporters, earlier genetic analysis indicated that they were not necessary for utilization of sugars as the sole carbon source. To further examine this issue, in-frame deletions were constructed separately for each of the three genes. Our growth analysis clearly indicated that GguA and GguB play a role in sugar utilization and strongly suggests that GguAB constitute an ABC transporter with a wide range of substrates, including L-arabinose, D-fucose, D-galactose, D-glucose, and D-xylose. Site-directed mutagenesis showed that a Walker A motif was vital to the function of GguA. We therefore propose renaming gguAB as mmsAB, for multiple monosaccharide transport. A gguC deletion affected growth only on L-arabinose medium, suggesting that gguC encodes an enzyme specific to L-arabinose metabolism, and this gene was renamed araD1. Results from bioinformatics and experimental analyses indicate that Agrobacterium tumefaciens uses a pathway involving nonphosphorylated intermediates to catabolize L-arabinose via an L-arabinose dehydrogenase, AraA(At), encoded at the Atu1113 locus.

A locus necessary for the transport and catabolism of erythritol in Sinorhizobium meliloti

In this work we have genetically defined an erythritol utilization locus in Sinorhizobium meliloti. A cosmid containing the locus was isolated by complementation of a transposon mutant and was subsequently mutagenized using Tn5 : : B20. The locus was found to consist of five transcriptional units, each of which was necessary for the utilization of erythritol. Genetic complementation experiments using genes putatively annotated as erythritol catabolic genes clearly showed that, of the 17 genes at this locus, six genes are not necessary for the utilization of erythritol as a sole carbon source. The remaining genes encode EryA, EryB, EryC and TpiB as well as an uncharacterized ABC-type transporter. Transport experiments using labelled erythritol showed that components of the ABC transporter are necessary for the uptake of erythritol. The locus also contains two regulators: EryD, a SorC class regulator, and SMc01615, a DeoR class regulator. Quantitative RT-PCR experiments showed that each of these regulators negatively regulates its own transcription. In addition, induction of the erythritol locus was dependent upon EryD and a product of erythritol catabolism. Further characterization of polar mutations revealed that in addition to erythritol, the locus contains determinants for adonitol and l-arabitol utilization. The context of the mutations suggests that the locus is important for both the transport and catabolism of adonitol and l-arabitol.

Deletion of citrate synthase restores growth of Sinorhizobium meliloti 1021 aconitase mutants

The symbiotic nitrogen-fixing bacterium Sinorhizobium meliloti 1021 encodes only one predicted aconitase (AcnA) in its genome. AcnA has a significant degree of similarity with other bacterial aconitases that behave as dual proteins: enzymes and posttranscriptional regulators of gene expression. Similar to the case with these bacterial aconitases, AcnA activity was reversibly labile and was regained upon reconstitution with reduced iron. The aconitase promoter was active in root nodules. acnA mutants grew very poorly, had secondary mutations, and were quickly outgrown by pseudorevertants. The acnA gene was stably interrupted in a citrate synthase (gltA) null background, indicating that the intracellular accumulation of citrate may be deleterious for survival of strain 1021. No aconitase activity was detected in this mutant, suggesting that the acnA gene encodes the only functional aconitase of strain 1021. To uncover a function of AcnA beyond its catalytic role in the tricarboxylic acid cycle pathway, the gltA acnA double mutant was compared with the gltA single mutant for differences in motility, resistance to oxidative stress, nodulation, and growth on different substrates. However, no differences in any of these characteristics were found.

Control of gluconate utilization in Sinorhizobium meliloti

The Sinorhizobium meliloti megaplasmid pSymA has previously been implicated in gluconate utilization. We report a locus on pSymA encoding a putative tripartite ATP-independent periplasmic (TRAP) transporter that is required for gluconate utilization. The expression of this locus is negatively regulated by a GntR family regulator encoded adjacent to the transporter operon.

Formate-dependent autotrophic growth in Sinorhizobium meliloti

It was found that S. meliloti strain SmA818, which is cured of pSymA, could not grow on defined medium containing only formate and bicarbonate as carbon sources. Growth experiments showed that Rm1021 was capable of formate/bicarbonate-dependent growth, suggesting that it was capable of autotrophic-type growth. The annotated genome of S. meliloti Rm1021 contains three formate dehydrogenase genes. A systematic disruption of each of the three formate dehydrogenase genes, as well as the genes encoding determinants of the Calvin-Benson-Bassham, cycle was carried out to determine which of these determinants played a role in growth on this defined medium. The results showed that S. meliloti is capable of formate-dependent autotrophic growth. Formate-dependent autotrophic growth is dependent on the presence of the chromosomally located fdsABCDG operon, as well as the cbb operon carried by pSymB. Growth was also dependent on the presence of either of the two triose-phosphate isomerase genes (tpiA or tpiB) that are found in the genome. In addition, it was found that fdoGHI carried by pSymA encodes a formate dehydrogenase that allows Rm1021 to carry out formate-dependent respiration. Taken together, the data allow us to present a model of how S. meliloti can grow on defined medium containing only formate and bicarbonate as carbon sources.

Eukaryotic and bacterial gene clusters related to an alternative pathway of nonphosphorylated L-rhamnose metabolism

The Entner-Doudoroff (ED) pathway is a classic central pathway of d-glucose metabolism in all three phylogenetic domains. On the other hand, Archaea and/or bacteria possess several modified versions of the ED pathway, in which nonphosphorylated intermediates are involved. Several fungi, including Pichia stipitis and Debaryomyces hansenii, possess an alternative pathway of L-rhamnose metabolism, which is different from the known bacterial pathway. Gene cluster related to this hypothetical pathway was identified by bioinformatic analysis using the metabolic enzymes involved in analogous sugar pathways to the ED pathway. Furthermore, the homologous gene cluster was found not only in many other fungi but also several bacteria, including Azotobacter vinelandii. Four putative metabolic genes, LRA1-4, were cloned, overexpressed in Escherichia coli, and purified. Substrate specificity and kinetic analysis revealed that nonphosphorylated intermediates related to L-rhamnose are significant active substrates for the purified LRA1-4 proteins. Furthermore, L-2-keto-3-deoxyrhamnonate was structurally identified as both reaction products of dehydration by LRA3 and aldol condensation by LRA4. These results suggested that the LRA1-4 genes encode L-rhamnose 1-dehydrogenase, L-rhamnono-gamma-lactonase, L-rhamnonate dehydratase, and L-KDR aldolase, respectively, by which L-rhamnose is converted into pyruvate and L-lactaldehyde through analogous reaction steps to the ED pathway. There was no evolutionary relationship between L-KDR aldolases from fungi and bacteria.

Characterization of Sinorhizobium meliloti triose phosphate isomerase genes

A Tn5 mutant strain of Sinorhizobium meliloti with an insertion in tpiA (systematic identifier SMc01023), a putative triose phosphate isomerase (TPI)-encoding gene, was isolated. The tpiA mutant grew more slowly than the wild type on rhamnose and did not grow with glycerol as a sole carbon source. The genome of S. meliloti wild-type Rm1021 contains a second predicted TPI-encoding gene, tpiB (SMc01614). We have constructed mutations and confirmed that both genes encode functional TPI enzymes. tpiA appears to be constitutively expressed and provides the primary TPI activity for central metabolism. tpiB has been shown to be required for growth with erythritol. TpiB activity is induced by growth with erythritol; however, basal levels of TpiB activity present in tpiA mutants allow for growth with gluconeogenic carbon sources. Although tpiA mutants can be complemented by tpiB, tpiA cannot substitute for mutations in tpiB with respect to erythritol catabolism. Mutations in tpiA or tpiB alone do not cause symbiotic defects; however, mutations in both tpiA and tpiB caused reduced symbiotic nitrogen fixation.