Nodulation of specific legumes is controlled by several distinct loci in Rhizobium trifolii

The Absence of the N-acyl-homoserine-lactone Autoinducer Synthase Genes traI and ngrI Increases the Copy Number of the Symbiotic Plasmid in Sinorhizobium fredii NGR234

Plant-released flavonoids induce the transcription of symbiotic genes in rhizobia and one of the first bacterial responses is the synthesis of so called Nod factors. They are responsible for the initial root hair curling during onset of root nodule development. This signal exchange is believed to be essential for initiating the plant symbiosis with rhizobia affiliated with the Alphaproteobacteria. Here, we provide evidence that in the broad host range strain Sinorhizobium fredii NGR234 the complete lack of quorum sensing molecules results in an elevated copy number of its symbiotic plasmid (pNGR234a). This in turn triggers the expression of symbiotic genes and the production of Nod factors in the absence of plant signals. Therefore, increasing the copy number of specific plasmids could be a widespread mechanism of specialized bacterial populations to bridge gaps in signaling cascades.

Comparison of characteristics of the nodX genes from various Rhizobium leguminosarum strains

We have analyzed the nucleotide sequences of the nodX genes from two strains of Rhizobium leguminosarum bv. viciae able to nodulate Afghan peas (strains A1 and Himalaya) and from two strains of R. leguminosarum bv. trifolii (ANU843 and CSF). The nodX genes of strains A1 and ANU843 were shown to be functional for the induction of nodules on Afghan peas. To analyze the cause of phenotypic differences of strain A1 and strain TOM we have studied the composition of the lipochitin-oligosaccharides (LCOs) produced by strain A1 after induction by the flavonoid naringenin or various pea root exudates. The structural analysis of the LCOs by mass spectrometry revealed that strain A1 synthesizes a family of at least 23 different LCOs. The use of exudates instead of naringenin resulted only in quantitative differences in the ratios of various LCOs produced.

Structural identification of metabolites produced by the NodB and NodC proteins of Rhizobium leguminosarum

The Rhizobium nodulation genes nodABC are involved in the synthesis of lipo-chitin oligosaccharides. We have analysed the metabolites which are produced in vivo and in vitro by Rhizobium strains which express the single nodA, nodB and nodC genes or combinations of the three. In vivo radioactive labelling experiments, in which D-[1-14C]-glucosamine was used as a precursor, followed by mass spectrometric analysis of the purified radiolabelled metabolic products, showed that Rhizobium strains that only express the combination of the nodB and nodC genes do not produce lipo-chitin oligosaccharides but instead produce chitin oligomers (mainly pentamers) which are devoid of the N-acetyl group on the non-reducing terminal sugar residue (designated NodBC metabolites). Using the same procedure we have shown that when the nodL gene is expressed in addition to the nodBC genes the majority of metabolites contain an additional O-acetyl substituent on the non-reducing terminal sugar residue (designated NodBCL metabolites). The NodBC and NodBCL metabolites purified after in vivo labelling were compared with the radiolabelled metabolites produced in vitro by Rhizobium bacterial cell lysates to which UDP-N-acetyl-D-[U-14C]-glucosamine was added using thin-layer chromatography. The results show that the lysates of strains which expressed the nodBC or nodBCL genes can also produce NodBC and NodBCL metabolites. The same results were obtained when the NodB and NodC proteins were produced separately in two different strains. On the basis of these and other recent results, we propose that NodB is a chitin oligosaccharide deacetylase, NodC an N-acetylglucosaminyltransferase and, by default, NodA is involved in lipid attachment.

Regulation and function of rhizobial nodulation genes

This review focuses on the functions of nodulation (nod) genes in the interaction between rhizobia and legumes. The nod genes are the key bacterial determinants of the signal exchange between the two symbiotic partners. The product of the nodD gene is a transcriptional activator protein that functions as receptor for a flavonoid plant compound. This signaling induces the expression of a set of nod genes that produces several related Nod factors, substituted lipooligosaccharides. The Nod factors are then excreted and serve as signals sent from the bacterium to the plant. The plant responds with the development of a root nodule. The plant-derived flavonoid, as well as the rhizobial signal, must have distinct chemical structures which guarantee that only matching partners are brought together.

A cultivar-specific interaction between Rhizobium leguminosarum bv. trifolii and subterranean clover is controlled by nodM, other bacterial cultivar specificity genes, and a single recessive host gene

Insertion mutagenesis identified two negatively acting gene loci which restrict the ability of Rhizobium leguminosarum bv. trifolii TA1 to infect the homologous host Trifolium subterraneum cv. Woogenellup. One locus was confirmed by DNA sequence analysis as the nodM gene, while the other locus, designated csn-1 (cultivar-specific nodulation), is not located on the symbiosis plasmid. The presence of these cultivar specificity loci could be suppressed by the introduction of the nodT gene from ANU843, a related R. leguminosarum bv. trifolii strain. Other nod genes, present in R. leguminosarum bv. viciae (including nodX) and R. meliloti, were capable of complementing R. leguminosarum bv. trifolii TA1 for nodulation on cultivar Woogenellup. Nodulation studies conducted with F2 seedlings from a cross between cultivar Geraldton and cultivar Woogenellup indicated that a single recessive gene, designated rwt1, is responsible for the Nod- association between strain TA1 and cultivar Woogenellup. Parallels can be drawn between this association and gene-for-gene systems common in interactions between plants and biotrophic pathogens.

nodT, a positively-acting cultivar specificity determinant controlling nodulation of Trifolium subterraneum by Rhizobium leguminosarum biovar trifolii

Rhizobium leguminosarum biovar trifolii strain TA1 nodulates a range of Trifolium plants including red, white and subterranean clovers. Nitrogen-fixing nodules are promptly initiated on the tap roots of these plants at the site of inoculation. In contrast to these associations, strain TA1 has a 'Nod-' phenotype on a particular cultivar of subterranean clover called Woogenellup (A.H. Gibson, Aust J Agric Sci 19: (1968) 907-918) where it induces rare, poorly developed, slow-to-appear and ineffective lateral root nodules. By comparing the nodulation gene region of strain TA1 with that of another R. leguminosarum bv. trifolii strain ANU843, which is capable of efficiently nodulating cv. Woogenellup, we have shown that the nodT gene (B.P. Surin et al., Mol Microbiol 4: (1990) 245-252) is essential for nodulation on cv. Woogenellup. The nodT gene is naturally absent in strain TA1. A cosmid clone spanning the entire nodulation gene region of strain TA1 was capable of conferring nodulation ability to R.l. bv. trifolii strains deleted for nodulation genes, but only on cultivars of subterranean clovers nodulated by strain TA1. This shows that cultivar recognition events are, in part, determined by genes in the nodulation region of strain TA1. Complementation studies also indicated that strain TA1 contains negatively-acting genes located on the Sym plasmid and elsewhere, which specifically block nodulation of cv. Woogenellup.

Subcellular localization of the Rhizobium leguminosarum nodI gene product

By the use of antibodies raised against a fusion protein of lacZ'-nodI (produced in Escherichia coli) which specifically react with NodI protein, it was shown that in wild-type Rhizobium leguminosarum biovar viciae NodI protein (i) is recovered with the cytoplasmic membrane fraction and (ii) is translated as part of the nodABCIJ operon. In addition, it was found that the bacterial chromosomal background strongly influences the expression of several nod genes.

New Field Isolates of Rhizobium leguminosarum Biovar Viciae That Nodulate the Primitive Pea Cultivar Afghanistan in Addition to Modern Cultivars

A collection of 13 field isolates of Rhizobium leguminosarum bv. viciae that have the ability to nodulate the roots of current North American cultivars of peas as well as a “primitive” cultivar, Afghanistan, was examined. These isolates originated in diverse geographical regions of the world, which indicates that this phenotype is not restricted to isolates from any one region. When subclones of the nodulation region from one plasmid were used to examine Eco RI-fragment-length polymorphisms in this collection of strains as well as in a collection comprising strains that do not nodulate the primitive cultivar, polymorphism was found in both collections. With one exception, RisφA6, all strains that nodulated cv. Afghanistan pea contained a region called nodX as an extension to the nodA BCIJ operon that has been observed in all R. leguminosarum bv. viciae strains, including those that do not nodulate cv. Afghanistan pea. RisφA6 was also the only strain in which nodulating ability could not be associated with a conjugative plasmid.

Either of two nod gene loci can complement the nodulation defect of a nod deletion mutant of Rhizobium leguminosarum bv viciae

A deletion mutant of Rhizobium leguminosarum biovar viciae lacking the host-specific nodulation (nod) gene region (nodFEL nodMNT and nodO) but retaining the other nod genes (nodD nodABCIJ) was unable to nodulate peas or Vicia hirsuta, although it did induce root hair deformation. The mutant appeared to be blocked in its ability to induce infection threads and could be rescued for nodulation of V. hirsuta in mixed inoculation experiments with an exopolysaccharide deficient mutant (which is also Nod-). The nodulation deficiency of the deletion mutant strain could be partially restored by plasmids carrying the nodFE, nodFEL or nodFELMNT genes but not by nodLMN. Surprisingly, the mutant strain could also be complemented with a plasmid that did not carry any of the nodFELMNT genes but which did carry the nodO gene on a 30 kb cloned region of DNA. Using appropriate mutations it was established that nodO is essential for nodulation in the absence of nodFE. Thus, either of two independent nod gene regions can complement the deletion mutant for nodulation of V. hirsuta. Similar observations were made for pea nodulation except that nodL was required in addition to nodO for nodulation in the absence of the nodFE genes. These observations show that nodulation can occur via either of two pathways encoded by non-homologous genes.

Molecular characterization of the nodulation gene, nodT, from two biovars of Rhizobium leguminosarum

DNA sequencing of the nodIJ region from Rhizobium leguminosarum biovar trifolii revealed the nodT gene immediately downstream of nodJ. DNA hybridizations using a nodT-specific probe showed that nodT is present in several R. leguminosarum strains. Interestingly, a flavonoid-inducible nodT gene homologue in R. leguminosarum bv. viciae is not in the nodABCIJ operon but is located downstream of nodMN. The sequence of the nodT gene from bv. viciae was determined and a comparison of the predicted amino-acid sequences of the two nodT genes shows them to be conserved; the predicted protein sequences appear to have a potential transit sequence typical of outer-membrane proteins. Mutations affecting nodT in either biovar had no observed effect on nodulation of the legumes tested.

Expression of Rhizobium leguminosarum CFN42 genes for lipopolysaccharide in strains derived from different R. leguminosarum soil isolates

Two mutant derivatives of Rhizobium leguminosarum ANU843 defective in lipopolysaccharide (LPS) were isolated. The LPS of both mutants lacked O antigen and some sugar residues of the LPS core oligosaccharides. Genetic regions previously cloned from another Rhizobium leguminosarum wild-type isolate, strain CFN42, were used to complement these mutants. One mutant was complemented to give LPS that was apparently identical to the LPS of strain ANU843 in antigenicity, electrophoretic mobility, and sugar composition. The other mutant was complemented by a second CFN42 lps genetic region. In this case the resulting LPS contained O-antigen sugars characteristic of donor strain CFN42 and reacted weakly with antiserum against CFN42 cells, but did not react detectably with antiserum against ANU843 cells. Therefore, one of the CFN42 lps genetic regions specifies a function that is conserved between the two R. leguminosarum wild-type isolates, whereas the other region, at least in part, specifies a strain-specific LPS structure. Transfer of these two genetic regions into wild-type strains derived from R. leguminosarum ANU843 and 128C53 gave results consistent with this conclusion. The mutants derived from strain ANU843 elicited incompletely developed clover nodules that exhibited low bacterial populations and very low nitrogenase activity. Both mutants elicited normally developed, nitrogen-fixing clover nodules when they carried CFN42 lps DNA that permitted synthesis of O-antigen-containing LPS, regardless of whether the O antigen was the one originally made by strain ANU843.

Characterization of Rhizobium phaseoli Sym plasmid regions involved in nodule morphogenesis and host-range specificity

Two nodulation regions from the symbiotic plasmid (pSym) of Rhizobium phaseoli CE-3 were identified. The two regions were contained in overlapping cosmids pSM927 and pSM991. These cosmids, in a R. phaseoli pSym-cured strain background, induced ineffective nodules on Phaseolus vulgaris roots. Transconjugants of Rhizobium meliloti harbouring pSM991 induced nodule-like structures on bean roots, suggesting that this cosmid contains host-range determinants. Analysis of deletions and insertional mutations in the sequences of pSM991 indicated that the genes responsible for the induction and development of nodules in P. vulgaris are organized in two regions 20 kb apart. One region, located in a 6.8 kb EcoRI fragment, includes the common nodABC genes. The other region, located in a 3.5 kb EcoRI fragment, contains information required for host-range determination.

Localization of functional regions of the Rhizobium nodD product using hybrid nodD genes

The flavonoid-inducible nod promoters of Rhizobium are positively regulated by the nodD gene which is highly conserved in various Rhizobium species. The nodD gene are functionally different in (i) their response to various exogenously added flavonoid inducers, (ii) the extent to which they mediate the activation of the flavonoid-inducible promoters, and (iii) the extent to which they repress their own constitutive transcription. In order to localize the regions of the nodD product which determine these differences, two series of nodD hybrid genes have been constructed. In one series the 5' moiety is derived from the R. meliloti nodD1 gene and the 3' moiety from the R. trifolii nodD gene. In the other series, the origins of the nodD moieties are reversed. Two regions of the nodD product appeared to be involved in autoregulation and it was also shown that the nodD promoters differ in their susceptibility to autoregulation. Many regions, dispersed over the entire nodD product, are involved in the specificity of activation by flavonoids. Several hybrid nodD genes were characterized which activate transcription with novel inducers. Furthermore, two classes of hybrid nodD genes were found from which the activation characteristics differ completely from those of the parental nodD genes. The first class activates the nodABCIJ promoter to the maximum level in the absence of flavonoid inducer. This level can no longer be influenced, positively or negatively, by the presence of (iso-)flavonoids. With the second class of hybrids, activation of the nodABCIJ promoter, even in the presence of flavonoid inducers, is no longer possible.

Rhizobium leguminosarum genes required for expression and transfer of host specific nodulation

The contributions of various nod genes from Rhizobium leguminosarum biovar viceae to host-specific nodulation have been assessed by transferring specific genes and groups of genes to R. leguminosarum bv. trifolii and testing the levels of nodulation on Pisum sativum (peas) and Vicia hirsuta. Many of the nod genes are important in determination of host-specificity; the nodE gene plays a key (but not essential) role and the efficiency of transfer of host specific nodulation increased with additional genes such that nodFE < nodFEL < nodFELMN. In addition the nodD gene was shown to play an important role in host-specific nodulation of peas and Vicia whilst other genes in the nodABCIJ gene region also appeared to be important. In a reciprocal series of experiments involving nod genes cloned from R. leguminosarum bv. trifolii it was found that the nodD gene enabled bv. viciae to nodulate Trifolium pratense (red clover) but the nodFEL gene region did not. The bv. trifolii nodD or nodFEL genes did significantly increase nodulation of Trifolium subterraneum (sub-clover) by R. leguminosarum bv. viciae. It is concluded that host specificity determinants are encoded by several different nod genes.

Expression of Nodulation Genes in Rhizobium leguminosarum biovar trifolii Is Affected by Low pH and by Ca and Al Ions

Early stages in the infection of leguminous plants by Rhizobium spp. are restricted at low pH and are further influenced by the presence of Ca and Al ions. In the experiments reported here, transcriptional and translational fusions of the Escherichia coli lacZ gene to Rhizobium leguminosarum biovar trifolii nodulation ( nod ) genes were used to investigate the effects of pH and of Ca and Al ions on nod gene expression. The regulatory nodD gene in R. leguminosarum biovar trifolii was constitutively expressed at a range of pH levels between 4.8 and 6.5, and expression was not affected by the addition of 22.5 μM Al or 1,250 μM Ca. Induction of expression of nodA, nodF, and region II nodulation genes in the presence of 5 × 10 −7 M 7,4′-dihydroxyflavone was restricted at a pH of <5.7 and was extremely poor at pH 4.8. Induction of nodA expression was further restricted by 22.5 μM Al over a range of pH levels but was increased in the presence of Ca. The addition of Ca, however, only slightly alleviated the Al-mediated inhibition of nodA induction. Induction of expression of nodA was equally sensitive to low pH in three strains of R. leguminosarum biovar trifolii (ANU845, ANU815, and ANU1184), which exhibited contrasting growth abilities in solution culture at a pH of <5.0. Aluminum, however, differentially affected the induction of nodA in these three strains, with the most Al-tolerant strain for growth being the most Al-sensitive strain for nod gene induction. Poor induction of expression of nodulation genes in R. leguminosarum biovar trifolii was considered to be an important factor contributing to the acid-sensitive step of legume root infection.

Induction of nitrogen-fixing nodules on clover requires only 32 kilobase pairs of DNA from the Rhizobium trifolii symbiosis plasmid

Overlapping subclones from the Rhizobium trifolii symbiosis plasmid pRt843a were generated by using in vivo and in vitro methods. Subclones were assayed for symbiotic phenotype by introducing them into a derivative of R. trifolii ANU843 cured of its symbiosis plasmid and testing the transconjugant strains for the ability to induce nitrogen-fixing nodules on clover. One subclone spanning 32 kilobase pairs (kb) of DNA from pRt843a was found to restore nitrogen fixation ability. This subclone included all known nodulation genes of R. trifolii ANU843 and the nitrogenase structural genes nifHDK. In addition, regions homologous to fixABC, nifA, nifB, nifE, and nifN genes of other nitrogen-fixing bacteria were identified in this 32-kb subclone by DNA-DNA hybridization. Transposon mutagenesis of this subclone confirmed that regions containing these nif and fix genes were required for induction of nitrogen-fixing nodules on clover. In addition, a region located 5 kb downstream of the nifK gene was found to be required for induction of nitrogen-fixing nodules. No homology to known nif and fix genes could be detected in this latter region.

Characterization of the Rhizobium leguminosarum genes nodLMN involved in efficient host-specific nodulation

Three nodulation genes, nodL, nodM and nodN, were isolated from Rhizobium leguminosarum and their DNA sequences were determined. The three genes are in the same orientation as the previously described nodFE genes and the predicted molecular weights of their products are 20,105 (nodL), 65,795 (nodM) and 18,031 (nodN). Analysis of gene regulation using operon fusions showed that nodL, nodM and nodN are induced in response to flavanone molecules and that this induction is nodD-dependent. In addition, it was shown that the nodM and nodN genes are in one operon which is preceded by a conserved 'nod-box' sequence, whereas the nodL gene is in the same operon as the nodFE genes. DNA hybridizations using specific gene probes showed that strongly homologous genes are present in Rhizobium trifolii but not Rhizobium meliloti or Bradyrhizobium japonicum. A mutation within nodL strongly reduced nodulation of peas, Lens and Lathyrus but had little effect on nodulation of Vicia species. A slight reduction in nodulation of Vicia hirsuta was observed with strains carrying mutations in nodM or nodN.

Rhizobium japonicum USDA 191 has two nodD genes that differ in primary structure and function

Several Rhizobium genes (designated nod genes) are involved in early steps in nodule formation. Here we present the results of DNA sequence and functional analysis of two nodD genes from the symbiotic plasmid of USDA 191, a fast-growing strain that forms nitrogen-fixing nodules on soybeans. Both genes encoded full-length nodD-related polypeptides, which were 69% homologous to each other. One of these genes, nodD1, complemented a Rhizobium trifolii nodD::Tn5 mutant for clover nodulation; the other gene, nodD2, did not. The nodD1 coding region was preceded by a conserved DNA sequence previously noted in other rhizobia, but no such sequence was found in front of nodD2. Plants inoculated with a nodD1 insertion mutant appeared to be nitrogen starved and had a greatly reduced nodule number. Plants inoculated with a nodD2 mutant had a partially nitrogen-starved appearance and normal nodule number, were slightly delayed in nodule formation, and formed nodules that contained reduced levels of nodulin-35 and had fewer bacteroids per infected plant cell. Thus, both of these genes are involved in symbiosis. USDA 191 carrying extra copies of nodD2 on a plasmid vector had an altered colony morphology that suggested inhibition of exopolysaccharide synthesis. The predicted gene products of nodD1 and nodD2 both showed homology to LysR, an E. coli regulatory protein. We conclude that nodD1 probably has the same function as nodD in temperate rhizobia, namely, activation of nodABC transcription in the presence of plant signals. nodD2 may be involved in regulation of exopolysaccharide synthetic genes.

Multiple host-specificity loci of the broad host-range Rhizobium sp. NGR234 selected using the widely compatible legume Vigna unguiculata

Specificity in legume-Rhizobium symbiosis depends on plant and rhizobial genes. As our objective was to study broad host-range determinants of rhizobia, we sought a legume and a Rhizobium with the lowest possible specificity. By inoculating 12 different legumes with a heterogenous collection of 35 fast-growing rhizobia, we found Rhizobium sp. NGR234 to be the Rhizobium and Vigna unguiculata to be the plant with the lowest specificities. Transfer of cloned fragments of the Sym-plasmid pNGR234a into heterologous rhizobia, screening for extension of host-range of the transconjugants to include V. unguiculata, and restriction mapping of the Hsn- and overlapping clones, proved that there were at least three distinct Hsn-regions (HsnI, II, and III) on pNGR234a. HsnI is located next to nodD, HsnII is linked to nifKDH and HsnIII to nodC. In addition to nodulation of Vigna, HsnI conferred upon the transconjugants the ability to nodulate Glycine max, Macroptilium atropurpureum and Psophocarpus tetragonolobus. All three Hsn-regions, when transferred to the appropriate recipients, induced root-hair-curling on M. atropurpureum. Hsn-region III was able to complement a mutation in the host-range gene nodH of R. meliloti strain 2011. Homology to "nod-box"-sequences could be shown only for the sub-clones containing HsnII and HsnIII, thus suggesting different regulation mechanisms for HsnI and HsnII/III.

Flavonoids induce Rhizobium leguminosarum to produce nodDABC gene-related factors that cause thick, short roots and root hair responses on common vetch

Rhizobium leguminosarum produced a factor(s) that caused thick, short roots (Tsr phenotype) as well as root hair induction (Hai phenotype) and deformation (Had phenotype) in Vicia sativa plants upon incubation with root exudate or with one of the nod gene inducers naringenin or apigenin; this was a nodDABC gene-dependent process. Detection of the Hai and Had phenotypes was much more sensitive than that of the Tsr phenotype.

Split-Root Assays Using Trifolium subterraneum Show that Rhizobium Infection Induces a Systemic Response That Can Inhibit Nodulation of Another Invasive Rhizobium Strain

Subterranean clover plants possessing two equally infectible and robust lateral root systems (“split roots”) were used in conjunction with several specific mutant strains (derived from Rhizobium trifolii ANU843) to investigate a systemic plant response induced by infective Rhizobium strains. This plant response controls and inhibits subsequent nodulation on the plant. When strain ANU843 was inoculated onto both root systems simultaneously or 24, 48, 72, or 96 h apart, an inhibitory response occurred which retarded nodulation on the root exposed to the delayed inoculum but only when the delay period between inocula was greater than 24 h. Equal numbers of nodules were generated on both roots when ANU843 was inoculated simultaneously or 24 h apart. The ability to infect subterranean clover plants was required to initiate the plant inhibitory response since preexposure of one root system to non-nodulating strains did not retard the ability of the wild-type strain to nodulate the opposing root system (even when the delay period was 96 h). Moreover, the use of specific Tn 5 -induced mutants subtly impaired in their ability to nodulate demonstrated that the plant could effectively and rapidly discriminate between infections initiated by either the parent or the mutant strains. When inoculated alone onto clover plants, these mutant strains were able to infect the most susceptible plant cells at the time of inoculation and induce nitrogen-fixing nodules. However, the separate but simultaneous inoculation on opposing root systems of the parent and the mutant strains resulted in the almost complete inhibition of the nodulation ability of the mutant strains. We concluded that the mutants were affected in their competitive ability, and this finding was reflected by poor nodule occupancy when the mutants were coinoculated with the parent strain onto a single root system. Thus the split-root system may form the basis of a simple screening method for the ranking of competitiveness of various rhizobia on small seeded legumes.

Rhizobium nod genes are involved in the induction of two early nodulin genes in Vicia sativa root nodules

Nodulin gene expresison was studied in Vicia sativa (common vetch) root nodules induced by several Rhizobium and Agrobacterium strains. An Agrobacterium transconjugant containing a R. leguminosarum symplasmid instead of its Ti-plasmid, that was previously shown to form "empty" nodules on pea, induced nodules on Vicia roots in which nodule cells were infected with bacteria. In the Vicia nodules induced by this transconjugant, two so-called early nodulin genes were found to be expressed, whereas in the nodules formed on pea the expression of only one early nodulin gene was detected. In both cases the majority of the nodulin genes was not expressed.Apparently, an intracellular location of the bacteria is not sufficient for the induction of the majority of the nodulin genes. All nodulin genes were expressed in nodules induced by cured Rhizobium strains containing cosmid clones that have a 10 kb nod region of the sym-plasmid in common. Since in tumours no nodulin gene expression was found at all, the Agrobacterium chromosome does not contribute to the induction of nodulin genes. Therefore it is concluded that the signal for the induction of the expression of the two Vicia early nodulin genes is encoded by the nod-region, and the signal involved in the induction of all other nodulin genes has to be located outside the sym-plasmid, on the Rhizobium chromosome. The apparent difference in early nodulin gene expression between pea and Vicia is discussed in the light of the usefulness of Agrobacterium transconjugants in the study of nodulin gene expression.

Promoters in the nodulation region of the Rhizobium leguminosarum Sym plasmid pRL1JI

A region of 16.8 kb of the Sym(biosis) plasmid pRL1JI of Rhizobium leguminosarum, consisting of the established 9.7 kb nodulation region which confers nodulation ability on Vicia hirsuta and a region of 7.1 kb which appeared to be necessary for nodulation on V. sativa and Trifolium subterraneum, was subcloned as fragments of maximally 2.5 kb in a newly developed IncQ transcriptional fusion vector. The expression of these fragments was studied in Rhizobium. One constitutive promoter, pr.nodD, and three plant-exudate inducible promoters were found, namely the known pr.nodA and pr.nodF as well as a new promoter designated pr.nodM. The latter promoters were localized within 114 bp, 330 bp and 630 bp respectively and they regulate the transcription of the operons nodA, B, C, I, J, nodF, E and of an operon of at least 2.5 kb located in the 7.1 kb region. Induction of the three inducible operons required plant exudate and a functional nodD product. The flavanone naringenin could replace plant exudate. Each of the three inducible promoters contained a nod-box. A consensus for the nod-box sequence, based on known sequences, is proposed. The 114 bp fragment which contains pr.nodA activity was used to localize pr.nodA by means of deletion mapping. The fragment which appeared necessary for complete pr.nodA activity is 72 bp in size, contains the complete nod-box and in addition a region of 21 bp downstream of the nod-box, in which the loosely conserved sequence AT(T)AG appears to be important for promoter activity.