PubMed Health⌕ Search

Biomedical subjects

F Debellé

Publications and source records attributed to F Debellé.

18 recordsLinked to original sources
PubMed2001

Unusual methyl-branched alpha,beta-unsaturated acyl chain substitutions in the Nod Factors of an arctic rhizobium, Mesorhizobium sp. strain N33 (Oxytropis arctobia).

Mesorhizobium sp. strain N33 (Oxytropis arctobia), a rhizobial strain isolated in arctic Canada, is able to fix nitrogen at very low temperatures in association with a few arctic legume species belonging to the genera Astragalus, Onobrychis, and Oxytropis. Using mass spectrometry and nuclear magnetic resonance spectroscopy, we have determined the structure of N33 Nod factors, which are major determinants of nodulation. They are pentameric lipochito-oligosaccharides 6-O sulfated at the reducing end and exhibit other original substitutions: 6-O acetylation of the glucosamine residue next to the nonreducing terminal glucosamine and N acylation of the nonreducing terminal glucosamine by methyl-branched acyl chains of the iso series, some of which are alpha,beta unsaturated. These unusual substitutions may contribute to the peculiar host range of N33. Analysis of N33 whole-cell fatty acids indicated that synthesis of the methyl-branched fatty acids depended on the induction of bacteria by plant flavonoids, suggesting a specific role for these fatty acids in the signaling process between the plant and the bacteria. Synthesis of the methyl-branched alpha,beta-unsaturated fatty acids required a functional nodE gene.

Acyltransferases↗
PubMed2001

Nod genes and Nod signals and the evolution of the Rhizobium legume symbiosis.

The establishment of the nitrogen-fixing symbiosis between rhizobia and legumes requires an exchange of signals between the two partners. In response to flavonoids excreted by the host plant, rhizobia synthesize Nod factors (NFs) which elicit, at very low concentrations and in a specific manner, various symbiotic responses on the roots of the legume hosts. NFs from several rhizobial species have been characterized. They all are lipo-chitooligosaccharides, consisting of a backbone of generally four or five glucosamine residues N-acylated at the non-reducing end, and carrying various O-substituents. The N-acyl chain and the other substituents are important determinants of the rhizobial host specificity. A number of nodulation genes which specify the synthesis of NFs have been identified. All rhizobia, in spite of their diversity, possess conserved nodABC genes responsible for the synthesis of the N-acylated oligosaccharide core of NFs, which suggests that these genes are of a monophyletic origin. Other genes, the host specific nod genes, specify the substitutions of NFs. The central role of NFs and nod genes in the Rhizobium-legume symbiosis suggests that these factors could be used as molecular markers to study the evolution of this symbiosis. We have studied a number of NFs which are N-acylated by alpha,beta-unsaturated fatty acids. We found that the ability to synthesize such NFs does not correlate with taxonomic position of the rhizobia. However, all rhizobia that produce NFs such nodulate plants belonging to related tribes of legumes, the Trifolieae, Vicieae, and Galegeae, all of them being members of the so-called galegoid group. This suggests that the ability to recognize the NFs with alpha-beta-unsaturated fatty acids is limited to this group of legumes, and thus might have appeared only once in the course of legume evolution, in the galegoid phylum.

Biological Evolution↗
PubMed2000

The common nodulation genes of Astragalus sinicus rhizobia are conserved despite chromosomal diversity.

The nodulation genes of Mesorhizobium sp. (Astragalus sinicus) strain 7653R were cloned by functional complementation of Sinorhizobium meliloti nod mutants. The common nod genes, nodD, nodA, and nodBC, were identified by heterologous hybridization and sequence analysis. The nodA gene was found to be separated from nodBC by approximately 22 kb and was divergently transcribed. The 2. 0-kb nodDBC region was amplified by PCR from 24 rhizobial strains nodulating A. sinicus, which represented different chromosomal genotypes and geographic origins. No polymorphism was found in the size of PCR products, suggesting that the separation of nodA from nodBC is a common feature of A. sinicus rhizobia. Sequence analysis of the PCR-amplified nodA gene indicated that seven strains representing different 16S and 23S ribosomal DNA genotypes had identical nodA sequences. These data indicate that, whereas microsymbionts of A. sinicus exhibit chromosomal diversity, their nodulation genes are conserved, supporting the hypothesis of horizontal transfer of nod genes among diverse recipient bacteria.

Acyltransferases↗
PubMed1999

Structure of the Mesorhizobium huakuii and Rhizobium galegae Nod factors: a cluster of phylogenetically related legumes are nodulated by rhizobia producing Nod factors with alpha,beta-unsaturated N-acyl substitutions.

Rhizobia are symbiotic bacteria that synthesize lipochitooligosaccharide Nod factors (NFs), which act as signal molecules in the nodulation of specific legume hosts. Based on the structure of their N-acyl chain, NFs can be classified into two categories: (i) those that are acylated with fatty acids from the general lipid metabolism; and (ii) those (= alphaU-NFs) that are acylated by specific alpha,beta-unsaturated fatty acids (containing carbonyl-conjugated unsaturation(s)). Previous work has described how rhizobia that nodulate legumes of the Trifolieae and Vicieae tribes produce alphaU-NFs. Here, we have studied the structure of NFs from two rhizobial species that nodulate important genera of the Galegeae tribe, related to Trifolieae and Vicieae. Three strains of Mesorhizobium huakuii, symbionts of Astragalus sinicus, produced as major NFs, pentameric lipochitooligosaccharides O-sulphated and partially N-glycolylated at the reducing end and N-acylated, at the non-reducing end, by a C18:4 fatty acid. Two strains of Rhizobium galegae, symbionts of Galega sp., produced as major NFs, tetrameric O-carbamoylated NFs that could be O-acetylated on the glucosamine residue next to the non-reducing terminal glucosamine and were N-acylated by C18 and C20 alpha,beta-unsaturated fatty acids. These results suggest that legumes nodulated by rhizobia synthesizing alphaU-NFs constitute a phylogenetic cluster in the Galegoid phylum.

Acylation↗
PubMed1996

The common nodABC genes of Rhizobium meliloti are host-range determinants.

Symbiotic bacteria of the genus Rhizobium synthesize lipo-chitooligosaccharides, called Nod factors (NFs), which act as morphogenic signal molecules on legume hosts. The common nodABC genes, present in all Rhizobium species, are required for the synthesis of the core structure of NFs. NodC is an N-acetylglucosaminyltransferase, and NodB is a chitooligosaccharide deacetylase; NodA is involved in N-acylation of the aminosugar backbone. Specific nod genes are involved in diverse NF substitutions that confer plant specificity. We transferred to R. tropici, a broad host-range tropical symbiont, the ability to nodulate alfalfa, by introducing nod genes of R. meliloti. In addition to the specific nodL and nodFE genes, the common nodABC genes of R. meliloti were required for infection and nodulation of alfalfa. Purified NFs of the R. tropici hybrid strain, which contained chitin tetramers and were partly N-acylated with unsaturated C16 fatty acids, were able to elicit nodule formation on alfalfa. Inactivation of the R. meliloti nodABC genes suppressed the ability of the NFs to nodulate alfalfa. Studies of NFs from nodA, nodB, nodC, and nodI mutants indicate that (i) NodA of R. meliloti, in contrast to NodA of R. tropici, is able to transfer unsaturated C16 fatty acids onto the chitin backbone and (ii) NodC of R. meliloti specifies the synthesis of chitin tetramers. These results show that allelic variation of the common nodABC genes is a genetic mechanism that plays an important role in signaling variation and in the control of host range.

Acylation↗
PubMed1996

The NodA proteins of Rhizobium meliloti and Rhizobium tropici specify the N-acylation of Nod factors by different fatty acids.

Rhizobia synthesize mono-N-acylated chitooligosaccharide signals, called Nod factors, that are required for the specific infection and nodulation of their legume hosts. The biosynthesis of Nod factors is under the control of nodulation (nod) genes, including the nodABC genes present in all rhizobial species. The N-acyl substitution can vary between species and can play a role in host specificity. In Rhizobium meliloti, an alfalfa symbiont, the acyl chain is a C16 unsaturated or a (omega-1) hydroxylated fatty acid, whereas in Rhizobium tropici, a bean symbiont, it is vaccenic acid (C18:1). We constructed R. meliloti derivatives having a non-polar deletion of nodA, and carrying a plasmid with either the R. meliloti or the R. tropici nodA gene. The strain with the R. tropici nodA gene produced Nod factors acylated by vaccenic acid, instead of the C16 unsaturated or hydroxylated fatty acids characteristic of R. meliloti Nod factors, and infected and nodulated alfalfa with a significant delay. These results show that NodA proteins of R. meliloti and R. tropici specify the N-acylation of Nod factors by different fatty acids, and that allelic variation of the common nodA gene can contribute to the determination of host range.

Acylation↗
PubMed1996

Rhizobium lipo-chitooligosaccharide nodulation factors: signaling molecules mediating recognition and morphogenesis.

Rhizobia elicit on their specific leguminous hosts the formation of new organs, called nodules, in which they fix nitrogen. The rhizobial nodulation genes specify the synthesis of lipo-chitooligosaccharide signals, the Nod factors (NFs). Each rhizobial species has a characteristic set of nodulation genes that specifies the length of the chitooligosaccharide backbone and the type of substitutions at both ends of the molecule, thus making the NFs specific for a given plant host. At extremely low concentrations, purified NFs are capable of eliciting on homologous legume hosts many of the plant developmental responses characteristic of the bacteria themselves, including cell divisions, and the triggering of a plant organogenic program. This review summarizes our current knowledge on the biosynthesis, structure, and function of this new class of signaling molecules. Finally we discuss the possibility that these signals could be part of a new family of plant lipo-chitooligosaccharide growth regulators.

Carbohydrate Sequence↗
PubMed1994

Rhizobium meliloti lipooligosaccharide nodulation factors: different structural requirements for bacterial entry into target root hair cells and induction of plant symbiotic developmental responses.

Rhizobium meliloti produces lipochitooligosaccharide nodulation NodRm factors that are required for nodulation of legume hosts. NodRm factors are O-acetylated and N-acylated by specific C16-unsaturated fatty acids. nodL mutants produce non-O-acetylated factors, and nodFE mutants produce factors with modified acyl substituents. Both mutants exhibited a significantly reduced capacity to elicit infection thread (IT) formation in alfalfa. However, once initiated, ITs developed and allowed the formation of nitrogen-fixing nodules. In contrast, double nodF/nodL mutants were unable to penetrate into legume hosts and to form ITs. Nevertheless, these mutants induced widespread cell wall tip growth in trichoblasts and other epidermal cells and were also able to elicit cortical cell activation at a distance. NodRm factor structural requirements are thus clearly more stringent for bacterial entry than for the elicitation of developmental plant responses.

Biological Assay↗
PubMed1993

Role of the Rhizobium meliloti nodF and nodE genes in the biosynthesis of lipo-oligosaccharidic nodulation factors.

Rhizobia nodulation (nod) genes are involved in the synthesis of symbiotic signals, the Nod factors, which are mono-N-acylated chito-oligosaccharides. Nod factors elicit, in a specific manner, various plant responses on legume roots. In this report we address the question of the role of nodFEG genes in the synthesis of the acyl moiety of Rhizobium meliloti Nod factors. In a Nod factor-overproducing strain with the wild-type nod region, in addition to the delta 2,9-C16:2 and delta 2, 4,9-C16:3 acyl groups already described, delta 9-C16:1 was also found, together with a series of C18 to C26 (omega-1)-hydroxylated fatty acids. A deletion of nodE resulted in the absence of C16:2 and C16:3 fatty acids, which were replaced by vaccenic acid (delta 11-C18:1), but did not change the proportion of (omega-1)-hydroxylated fatty acids. A nodF deletion, non-polar with respect to nodE, resulted in the same alterations in the Nod factor N-acyl composition, showing that both nodF and nodE are required for the synthesis of the C16 polyunsaturated chains. In contrast, nodG mutations did not result in a detectable change in the Nod factor N-acyl moiety. When a plasmid carrying the nodFE genes of Rhizobium leguminosarum bv. viciae was introduced into R. meliloti nodFE- and nodFEG-deleted strains, Nod factors with polyunsaturated C18 fatty acids (C18:2, C18:3, and C18:4) could be detected. These results provide evidence that the molecular basis of allelic variation between the R. meliloti and R. leguminosarum bv. viciae host range nodFE genes lies in the fact that the two nodFE alleles specify the synthesis of unsaturated fatty acid substituents with a different carbon length.

Acyltransferases↗
PubMed1992

Signaling and host range variation in nodulation.

Rhizobium, Bradyrhizobium, and Azorhizobium strains, collectively referred to as rhizobia, elicit on their leguminous hosts, in a specific manner, the formation of nodules in which they fix nitrogen. Rhizobial nod genes, which determine host specificity, infection, and nodulation, are involved in the exchange of low molecular weight signal molecules between the plant and the bacteria as follows. Transcription of the nod operons is under the control of NodD regulatory proteins, which are specifically activated by plant flavonoid signals. The common and species-specific structural nod genes are involved in turn in the synthesis of specific lipo-oligosaccharides that signal back to the plant to elicit root-hair deformations, cortical-cell divisions, and nodule-meristem formation.

Fabaceae↗
PubMed1991

Molecular basis of symbiotic host specificity in Rhizobium meliloti: nodH and nodPQ genes encode the sulfation of lipo-oligosaccharide signals.

The symbiosis between Rhizobium and legumes is highly specific. For example, R. meliloti elicits the formation of root nodules on alfalfa and not on vetch. We recently reported that R. meliloti nodulation (nod) genes determine the production of acylated and sulfated glucosamine oligosaccharide signals. We now show that the biochemical function of the major host-range genes, nodH and nodPQ, is to specify the 6-O-sulfation of the reducing terminal glucosamine. Purified Nod factors (sulfated or not) from nodH+ or nodH- strains exhibited the same plant specificity in a variety of bioassays (root hair deformations, nodulation, changes in root morphology) as the bacterial cells from which they were purified. These results provide strong evidence that the molecular mechanism by which the nodH and nodPQ genes mediate host specificity is by determining the sulfation of the extracellular Nod signals.

Amino Acid Sequence↗
PubMed1990

Role of the nodD and syrM genes in the activation of the regulatory gene nodD3, and of the common and host-specific nod genes of Rhizobium meliloti.

To analyse the regulation of the nodulation (nod) genes of Rhizobium meliloti RCR2011 we have isolated lacZ gene fusions to a number of common, host-range and regulatory nod genes, using the mini-Mu-lac bacteriophage transposon MudII1734. Common (nodA, nodC, nod region IIa) and host-range (nodE, nodG, nodH) genes were found to be regulated similarly. They were activated (i) by the regulatory nodD1 gene in the presence of flavones such as chrysoeriol, luteolin and 7,3',4'-trihydroxyflavone, (ii) by nodD2 in the presence of alfalfa root exudate but not with the NodD1-activating flavones, and (iii) by the regulatory genes syrM-nodD3 even in the absence of plant inducers. Thus common and host-range nod genes belong to the same regulon. In contrast to the nodD1 gene, the regulatory nodD3 gene was not expressed constitutively and exhibited a complex regulation. It required syrM for expression, was activated by nodD1 in the presence of luteolin and was positively autoregulated.

DNA, Bacterial↗
PubMed1988

Interference between Rhizobium meliloti and Rhizobium trifolii nodulation genes: genetic basis of R. meliloti dominance.

Transfer of an IncP plasmid carrying the Rhizobium meliloti nodFE, nodG, and nodH genes to Rhizobium trifolii enabled R. trifolii to nodulate alfalfa (Medicago sativa), the normal host of R. meliloti. Using transposon Tn5-linked mutations and in vitro-constructed deletions of the R. meliloti nodFE, nodG, and nodH genes, we showed that R. meliloti nodH was required for R. trifolii to elicit both root hair curling and nodule initiation on alfalfa and that nodH, nodFE, and nodG were required for R. trifolii to elicit infection threads in alfalfa root hairs. Interestingly, the transfer of the R. meliloti nodFE, nodG, and nodH genes to R. trifolii prevented R. trifolii from infecting and nodulating its normal host, white clover (Trifolium repens). Experiments with the mutated R. meliloti nodH, nodF, nodE, and nodG genes demonstrated that nodH, nodF, nodE, and possibly nodG have an additive effect in blocking infection and nodulation of clover.

Chromosome Deletion↗
PubMed1986

Nucleotide sequence of Rhizobium meliloti RCR2011 genes involved in host specificity of nodulation.

A 6 kb DNA segment of the R. meliloti 2011 pSym megaplasmid, which contains genes controlling host specificity of root hair infection and of nodulation, was cloned and sequenced. The DNA sequence analysis, in conjunction with previous genetic data, allowed identification of four nod genes designated as E, F, G and H. nodH is divergently transcribed with respect to nodFE and nodG. A conserved nucleotide sequence was found around 200 bp upstream of the translation start of nodF, nodH and nodA. This sequence is also present upstream of common nodA and species specific nodF genes of other Rhizobium species. The predicted protein products of nodF and nodG show homology with acyl carrier protein and ribitol dehydrogenase, respectively. The nodH product contains a rare sequence of four contiguous proline residues. Comparison with the nod gene products of R. leguminosarum shows that species specific nodFE products are as well conserved as those of common nodABC and nodD genes.

Amino Acid Sequence↗
PubMed1986

Assignment of symbiotic developmental phenotypes to common and specific nodulation (nod) genetic loci of Rhizobium meliloti.

Rhizobium meliloti nodulation (nod) genes required for specific infection and nodulation of alfalfa have been cloned. Transposon Tn5 mutagenesis defined three nod regions spanning 16 kilobases of the pSym megaplasmid. Genetic and cytological studies of 62 nodulation-defective mutants allowed the assignment of symbiotic developmental phenotypes to common and specific nod loci. Root hair curling was determined by both common (region I) and specific (region III) nod transcription units; locus IIIb (nodH gene) positively controlled curling on the homologous host alfalfa, whereas loci IIIa (nodFE) and IIIb (nodH) negatively controlled curling on heterologous hosts. Region I (nodABC) was required for bacterial penetration and infection thread initiation in shepherd's crooks, and the nodFE transcription unit controlled infection thread development within the alfalfa root hair. In contrast, induction of nodule organogenesis, which can be triggered from a distance, seemed to be controlled by common nodABC genes and not to require specific nod genes nodFE and nodH. Region II affected the efficiency of hair curling and infection thread formation.

Cloning, Molecular↗
PubMed1985

Identification of a Rhizobium meliloti pSym2011 region controlling the host specificity of root hair curling and nodulation.

In Rhizobium meliloti 2011 nodulation genes (nod) required to nodulate specifically alfalfa are located on a pSym megaplasmid. Nod- derivatives carrying large pSym deletions were isolated. By complementation of these strains with in vivo- and in vitro-constructed episomes containing pSym of sequences and introduction of these episomes into Agrobacterium tumefaciens, we show (i) that from a region of pSym of about 360 kilobases, genes required for specific alfalfa nodulation are clustered in a DNA fragment of less than 30 kilobases and (ii) that a nod region located between nifHDK and the common nod genes is absolutely required for alfalfa nodulation and controls the specificity of root hair curling and nodule organogenesis initiation.

Chromosome Deletion↗
PubMedDate not supplied

The Rhizobium, Bradyrhizobium, and Azorhizobium NodC proteins are homologous to yeast chitin synthases.

The nodABC genes of rhizobia are essential for the synthesis of lipo-oligosaccharidic (N-acylated chitin oligomers) nodulation signals. nodC gene products from Rhizobium, Bradyrhizobium, and Azorhizobium exhibit extensive homology with chitin synthases, suggesting that the NodC proteins are involved in the synthesis of the chitin oligomer backbone by catalyzing the beta-1,4-linkage between N-acetyl-D-glucosamine residues.

Bacterial Proteins↗