Co-transfer of determinants for hydrogenase activity and nodulation ability in Rhizobium leguminosarum

Deciphering the Symbiotic Plant Microbiome: Translating the Most Recent Discoveries on Rhizobia for the Improvement of Agricultural Practices in Metal-Contaminated and High Saline Lands

Rhizosphere and plant-associated microorganisms have been intensely studied for their beneficial effects on plant growth and health. These mainly include nitrogen-fixing bacteria (NFB) and plant-growth promoting rhizobacteria (PGPR). This beneficial fraction is involved in major functions such as plant nutrition and plant resistance to biotic and abiotic stresses, which include water deficiency and heavy-metal contamination. Consequently, crop yield emerges as the net result of the interactions between the plant genome and its associated microbiome. Here, we provide a review covering recent studies on PGP rhizobia as effective inoculants for agricultural practices in harsh soil, and we propose models for inoculant combinations and genomic manipulation strategies to improve crop yield.

Multidisciplinary approaches for studying rhizobium–legume symbioses

The rhizobium–legume symbiosis is a major source of fixed nitrogen (ammonia) in the biosphere. The potential for this process to increase agricultural yield while reducing the reliance on nitrogen-based fertilizers has generated interest in understanding and manipulating this process. For decades, rhizobium research has benefited from the use of leading techniques from a very broad set of fields, including population genetics, molecular genetics, genomics, and systems biology. In this review, we summarize many of the research strategies that have been employed in the study of rhizobia and the unique knowledge gained from these diverse tools, with a focus on genome- and systems-level approaches. We then describe ongoing synthetic biology approaches aimed at improving existing symbioses or engineering completely new symbiotic interactions. The review concludes with our perspective of the future directions and challenges of the field, with an emphasis on how the application of a multidisciplinary approach and the development of new methods will be necessary to ensure successful biotechnological manipulation of the symbiosis.

Apparent incompatibility of plasmid pSfrYC4b of Sinorhizobium fredii with two different plasmids in another strain

Sinorhizobium fredii YC4B is a spontaneous mutant derivative of strain YC4 that is unable to nodulate soybeans. The second-largest plasmid of strain YC4B, termed pSfrYC4b (810 kb), was transferred to S. fredii HN01SR, a strain which contains three large indigenous plasmids (pSfrHN01a, pSfrHN01b and pSfrHN01c). Surprisingly, two stable indigenous plasmids (pSfrHN01a and pSfrHN01b) of strain HN01SR were cured simultaneously by the introduction of pSfrYC4b. Furthermore, a novel, unstable plasmid (pHY4) became visible in agarose gels. The electrophoretic mobility of plasmid pHY4 was slower than that shown by the cured plasmids, indicating that the molecular weight of the former is higher than that of plasmids pSfrYC4b and pSfrHN01b. Replication gene repC-like sequences were detected by polymerase chain reaction (PCR) on pSfrHN01a and pSfrYC4b, but not on pSfrHN01b. Sau3AI and PstI restriction patterns of the PCR-amplified repC-like sequences from HN01SR and YC4B were very similar.

Intergeneric conjugation in Thiobacillus versutus

In plate matings with Escherichia coli HB101/pUW965::Tn5 (KmR) Thiobacillus versutus reacted as an efficient recipient, producing 10(-2) to 10(-3) kanamycin resistant (KmR) T. versutus exconjugants per donor cell. Analysis of agarose gels of plasmid DNA extracted from the exconjugants confirmed that the suicide vector pUW964 did not persist in the recipient, implying that the kanamycin resistance of the exconjugants is based on effective transposition of Tn5 in T. versutus as well as function of the E. coli kanamycin gene. Transfer was equally efficient when a nalidixate-resistant T. versutus mutant was used as recipient. Hybridization evidence for the presence of Tn5 was consistently negative. The significance of this anomalous result is discussed.

Metabolic pathways in Paracoccus denitrificans and closely related bacteria in relation to the phylogeny of prokaryotes

Denitrification and methylotrophy in Paracoccus denitrificans are discussed. The properties of the enzymes of denitrification: the nitrate-nitrite antiporter, nitrate reductase, nitrite reductase, nitric oxide reductase and nitrous oxide reductase are described. The genes for none of these proteins have yet been cloned and sequenced from P. denitrificans. A number of sequences are available for enzymes from Escherichia coli, Pseudomonas stutzeri and Pseudomonas aeruginosa. It is concluded that pathway specific c-type cytochromes are involved in denitrification. At least 40 genes are involved in denitrification. In methanol oxidation at least 20 genes are involved. In this case too pathway specific c-type cytochromes are involved. The sequence homology between the quinoproteins methanol dehydrogenase, alcoholde-hydrogenase and glucose dehydrogenase is discussed. This superfamily of proteins is believed to be derived from a common ancestor. The moxFJGI operon determines the structural components of methanol dehydrogenase and the associated c-type cytochrome. Upstream of this operon 3 regulatory proteins were found. The moxY protein shows the general features of a sensor protein and the moxX protein those of a regulatory protein. Thus a two component regulatory system is involved in both denitrification and methylotrophy. The phylogeny of prokaryotes based on 16S rRNA sequence is discussed. It is remarkable that the 16S rRNA of Thiosphaera pantotropha is identical to that of P. denitrificans. Still these bacteria show a number of differences. T. pantotropha is able to denitrify under aerobic circumstances and it shows heterotrophic nitrification. Nitrification and heterotrophic nitrification are found in species belonging to the beta-and gamma-subdivisions of purple non-sulfur bacteria. Thus the occurrence of heterotrophic nitrification in T. pantotropha, which belongs to the alpha-subdivision of purple non-sulfur bacteria is a remarkable property. Furthermore T. pantotropha contains two nitrate reductases of which the periplasmic one is supposed to be involved in aerobic denitrification. The nitrite reductase is of the Cu-type and not of the cytochrome cd1 type as in P. denitrificans. Also the cytochrome b of the Qbc complex of T. pantotropha is highly similar to its counterpart in P. denitrificans. It is hypothesized that the differences between these two organisms which both contain large megaplasmids is due to a combination of loss of genetic information and plasmid-coded properties. The distribution of a number of complex metabolic systems in eubacteria and in a number of species belonging to the alpha-group of purple non sulphur bacteria is reviewed.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential expression of hydrogen uptake (hup) genes in vegetative and symbiotic cells of Rhizobium leguminosarum

The genetic determinants responsible for H2-uptake (hup genes) in Rhizobium leguminosarum are organized in six transcriptional units, designated regions hupI to hupVI, with region hupI coding for the hydrogenase structural genes (Leyva et al. 1990). Regulation of the expression of hup genes from R. leguminosarum was examined by using hup-lacZ fusions and mRNA dot-blot analysis. None of the six hup regions is transcribed in vegetative cells grown under normal aerobic conditions, whereas all six regions are transcribed in pea bacteroids. Additionally, exposure of cell cultures to low oxygen tensions specifically induces the expression of regions hupV and hupVI. By studying the expression of hupV- and hupVI-lacZ fusions in R. meliloti mutants it was determined that the microaerobic induction of these two regions is dependent on the regulatory fixLJ system, and that this control is exerted through fixK. Such expression was also shown to be nifA and ntrA independent. The functions of the hupV and hupVI gene products are unknown. The possibility that they play a regulatory role in hup gene expression is unlikely, since pea bacteroids from R. leguminosarum Hup- mutants carrying Tn5 insertions in regions hupV and hupVI contained normal levels of mRNA transcripts corresponding to the remaining hup regions.

Immunochemical analysis of lipopolysaccharides from free-living and endosymbiotic forms of Rhizobium leguminosarum

Rhizobium leguminosarum B556 and 8002 differ only with respect to carrying symbiotic plasmids with specificity for Pisum or Phaseolus hosts, respectively. Protease-treated samples derived from free-living cultures of both strains revealed a ladder of lipopolysaccharide (LPS-1) bands after periodate-silver staining of sodium dodecyl sulfate-polyacrylamide gels. These bands were arranged as doublets. After Western (immuno-) blotting, all LPS-1 bands reacted with monoclonal antibody JIM 21, whereas monoclonal antibody MAC 57 reacted only with the upper (slower-migrating) band and monoclonal antibody MAC 114 reacted only with the lower band of each doublet pair. Preparations obtained from bacteroids of Pisum or Phaseolus nodules showed significant differences in the size distribution and antigenicity of LPS. In bacteroids from Phaseolus sp., JIM 21 and MAC 57 each stained a ladder of LPS-1 bands on sodium dodecyl sulfate-polyacrylamide gels which corresponded in mobility to the upper band of each doublet pair seen in free-living cultures. MAC 114 did not react with the LPS from Phaseolus sp.-derived bacteroids. In bacteroids from Pisum sp., only fast-migrating (lower-molecular-weight) forms of LPS-1 could be visualized on gels, but both upper and lower bands of each doublet were still present and could be stained by the appropriate monoclonal antibody, MAC 57 or MAC 114, respectively. Similarly, bacteroids from R. leguminosarum 3841, which nodulates Pisum species, differed with respect to the structure and antigenicity of their LPS-1 from bacteroids of a related strain, B625, which nodulates Phaseolus species. Physiological factors were investigated that could account for these differences between the structures of LPS-1 from free-living cultures of B556 and 8002 and that from bacteroids. The following modifications in growth conditions each tended to reduce the expression of MAC 114 antigen and enhance the expression of MAC 57 antigen: succinate rather than glucose as the carbon source; microaerobic (2.5%, vol/vol) oxygen concentrations; and acidic (pH 5 to 6) culture medium. When all three of these conditions were combined, the LPS-1 that resulted was very similar to that in bacteroids from Pisum nodules. However, it was not possible to reproduce the LPS-1 pattern observed for bacteroids from Phaseolus nodules, which maintained a ladder of LPS bands reacting with MAC 57 antibody.

Genetic organization of the hydrogen uptake (hup) cluster from Rhizobium leguminosarum

In symbiosis with peas, Rhizobium leguminosarum UPM791 induces the synthesis of a hydrogen uptake (Hup) system that recycles hydrogen generated in nodules by nitrogenase. A cosmid (pAL618) containing hup genes from this strain on a 20-kilobase-pair (kb) DNA insert has previously been isolated in our laboratory (A. Leyva, J. M. Palacios, T. Mozo, and T.Ruiz-Argüeso, J. Bacteriol. 169:4929-4934, 1987). Here we show that cosmid pAL618 contains all of the genetic information required to confer high levels of hydrogenase activity on the naturally Hup- strains R. leguminosarum UML2 and Rhizobium phaseoli CFN42, and we also describe in detail the organization of hup genes on pAL618. To study hup gene organization, site-directed transposon mutagenesis and complementation analysis were carried out. According to the Hup phenotype associated with the transposon insertions, hup genes were found to span a 15-kilobase-pair region within pAL618 insert DNA. Complementation analysis revealed that Hup- mutants fell into six distinct complementation groups that define six transcriptional units, designated regions hupI to hupVI. Region hupI was subcloned and expressed in Escherichia coli cells under the control of a bacteriophage T7 promoter. A polypeptide of ca. 65 kilodaltons that was cross-reactive with antiserum against the large subunit of Bradyrhizobium japonicum hydrogenase was detected both in E. coli cells carrying the cloned hupI region and in pea bacteroids from strain UPM791, indicating that region hupI codes for structural genes of R. leguminosarum hydrogenase.

Characterization and nucleotide sequence of a novel gene fixW upstream of the fixABC operon in Rhizobium leguminosarum

On the Rhizobium leguminosarum PRE sym plasmid, fixABC and a novel gene fixW were identified upstream of the regulatory gene nifA. The molecular masses of FixABC, 29, 44 and 50 kDa respectively, were estimated by polyacrylamide gel electrophoresis (PAGE) and of FixW, 25 kDa, by PAGE and nucleotide sequencing. Hybridization studies using bacteroid mRNA as a probe showed that fixABC is one operon which can be transcribed independently of fixW. Nucleotide sequencing revealed that both fixW and fixA are preceded by a nif consensus promoter. The fixA promoter partly overlaps the 3'-terminal coding region of fixW, indicating that readthrough from fixW into fixA is possible. Two open reading frames, ORF71 and ORF79, precede fixW and form one operon with fixW. ORF71 contains sequences homologous to the fixA promoter and 5'-terminal coding region. One more duplication of fixA sequences was detected, also located within the sym plasmid nif/fix clusters. One duplication of fixW sequences was found. No fixW homologue could be found in other nitrogen fixing organisms except in a number of R. leguminosarum strains.

Homology and distribution of CO dehydrogenase structural genes in carboxydotrophic bacteria

The 17 (S), 30 (M) and 87 kDa (L) subunits of CO dehydrogenases from the CO-oxidizing bacteria Pseudomonas carboxydoflava, Pseudomonas carboxydohydrogena and Pseudomonas carboxydovorans OM5 were isolated and purified. The N-terminal sequences of same subunits from different bacteria showed distinct homologies. Dot blot hybridization employing oligonucleotide probes derived from the sequences of the S-subunit of P. carboxydovorans OM5 and the M-subunit of P. carboxydohydrogena and DNA of the plasmid-containing CO-oxidizing bacteria Alcaligenes carboxydus, Azomonas B1, P. carboxydoflava, P. carboxydovorans OM2, OM4 and OM5 indicated that all genes encoding these subunits reside on plasmids. That in P. carboxydovorans OM5 CO dehydrogenase structural genes are located entirely on plasmid pHCG3 was evident from the absence of hybridization employing DNA from the cured mutant strain OM5-12. CO dehydrogenase structural genes could be identified on the chromosome of the plasmid-free bacteria Arthrobacter 11/x, Bacillus schlegelii, P. carboxydohydrogena and P. carboxydovorans OM3. There was no example of a plasmid-harboring carboxydotrophic bacterium that did not carry CO dehydrogenase structural genes on the plasmid. The N-terminal sequences of CO dehydrogenase structural genes were found to be conserved among carboxydotrophic bacteria of distinct taxonomic position, independent of the presence of plasmids. It is discussed whether this might be the consequence of horizontal gene transfer.

Diversity within Serogroups of Rhizobium leguminosarum biovar viceae in the Palouse Region of Eastern Washington as Indicated by Plasmid Profiles, Intrinsic Antibiotic Resistance, and Topography

Serology, plasmid profiles, and intrinsic antibiotic resistance (IAR) were determined for 192 isolates of Rhizobium leguminosarum biovar viceae from nodules of peas ( Pisum sativum L.) grown on the south slope and bottomland topographic positions in eastern Washington State. A total of 3 serogroups and 18 plasmid profile groups were identified. Nearly all isolates within each plasmid profile group were specific for one of the three serogroups. Cluster analysis of IAR data showed that individual clusters were dominated by one serogroup and by one or two plasmid profile groups. Plasmid profile analysis and IAR analysis grouped 72% of the isolates similarly. Most plasmid profile groups and several IAR clusters favored either the south slope or the bottomland topographic position. These findings show that certain intraserogroup strains possess a greater competitiveness for nodulation and/or possess a greater ability to survive in adjacent soil environments.

Cloning and characterization of hydrogen uptake genes from Rhizobium leguminosarum

A gene library of genomic DNA from the hydrogen uptake (Hup)-positive strain 128C53 of Rhizobium leguminosarum was constructed by using the broad-host-range mobilizable cosmid vector pLAFR1. The resulting recombinant cosmids contained insert DNA averaging 21 kilobase pairs (kb) in length. Two clones from the above gene library were identified by colony hybridization with DNA sequences from plasmid pHU1 containing hup genes of Bradyhizobium japonicum. The corresponding recombinant cosmids, pAL618 and pAL704, were isolated, and a region of about 28 kb containing the sequences homologous to B. japonicum hup-specific DNA was physically mapped. Further hybridization analysis with three fragments from pHU1 (5.9-kb HindIII, 2.9-kb EcoRI, and 5.0-kb EcoRI) showed that the overall arrangement of the R. leguminosarum hup-specific region closely parallels that of B. japonicum. The presence of functional hup genes within the isolated cosmid DNA was demonstrated by site-directed Tn5 mutagenesis of the 128C53 genome and analysis of the Hup phenotype of the Tn5 insertion strains in symbiosis with peas. Transposon Tn5 insertions at six different sites spanning 11 kb of pAL618 completely suppressed the hydrogenase activity of the pea bacteroids.

Conserved Plasmid Hydrogen-Uptake ( hup )-Specific Sequences within Hup + Rhizobium leguminosarum Strains

Thirteen Rhizobium leguminosarum strains previously reported as H 2 -uptake hydrogenase positive (Hup + ) or negative (Hup − ) were analyzed for the presence and conservation of DNA sequences homologous to cloned Bradyrhizobium japonicum hup -specific DNA from cosmid pHU1 (M. A. Cantrell, R. A. Haugland, and H. J. Evans, Proc. Natl. Acad. Sci. USA 80:181-185, 1983). The Hup phenotype of these strains was reexamined by determining hydrogenase activity induced in bacteroids from pea nodules. Five strains, including H 2 oxidation-ATP synthesis-coupled and -uncoupled strains, induced significant rates of H 2 -uptake hydrogenase activity and contained DNA sequences homologous to three probe DNA fragments (5.9-kilobase [kb] Hin dIII, 2.9-kb Eco RI, and 5.0-kb Eco RI) from pHU1. The pattern of genomic DNA Hin dIII and Eco RI fragments with significant homology to each of the three probes was identical in all five strains regardless of the H 2 -dependent ATP generation trait. The restriction fragments containing the homology totalled about 22 kb of DNA common to the five strains. In all instances the putative hup sequences were located on a plasmid that also contained nif genes. The molecular sizes of the identified hup-sym plasmids ranged between 184 and 212 megadaltons. No common DNA sequences homologous to B. japonicum hup DNA were found in genomic DNA from any of the eight remaining strains showing no significant hydrogenase activity in pea bacteroids. These results suggest that the identified DNA region contains genes essential for hydrogenase activity in R. leguminosarum and that its organization is highly conserved within Hup + strains in this symbiotic species.

Conservation of IS66 homologue of octopine Ti plasmid DNA in Rhizobium fredii plasmid DNA

DNA sequences homologous to the T-DNA region of the octopine Ti plasmid from Agrobacterium tumefaciens are found in various fast-growing Rhizobium fredii strains. The largest fragment (BamHI fragment 2) at the right-boundary region of the 'core' T-DNA hybridizes to more than one plasmid present in R. fredii. However, one smaller fragment (EcoRI fragment 19a) adjacent to the 'core' T-DNA shows homology only with the plasmid carrying the symbiotic nitrogen-fixation genes (pSym). Hybridization data obtained with digested R. fredii USDA193 pSym DNA suggests that the homology is mainly with two HindIII fragments, 1.7 kb and 8.8 kb in size, of the plasmid. The 1.7 kb HindIII fragment also hybridizes to two regions of the virulence plasmid of A. tumefaciens, pAL1819, a deletion plasmid derived from the octopine Ti plasmid, pTiAch5. Hybridization studies with an insertion element IS66 from A. tumefaciens indicate that the 1.7 kb HindIII fragment of R. fredii plasmid, homologous to the T-DNA and the virulence region of Ti plasmid, is itself an IS66 homologue.

Cloning of DNA fragments carrying hydrogenase genes of Rhodopseudomonas capsulata

A cosmid library of Rhodopseudomonas capsulata DNA was constructed in Escherichia coli HB101 using the broad-host-range cosmid vector pLAFR1. More than ninety per cent of the clones in the bank contained cosmids with DNA inserts averaging 20 kilobase pairs in length. Mutants deficient in uptake hydrogenase (Hup-) were obtained from R. capsulata strain B10 by ethylmethylsulfonate (EMS) mutagenesis. The content of hydrogenase protein in Hup- mutant cells was tested by rocket immunoelectrophoresis. Hup- mutants (Rifr) were complemented with the clone bank by conjugation and, from the transconjugants selected by rifampicin and tetracycline resistance, Hup+ transconjugants were screened for the ability to grow photoautotrophically and to reduce methylene blue in a colony assay. The recombinant plasmid pAC57 restored hydrogenase activity in the Hup- mutants RCC8, RCC10, RCC12 and ST410 whereas pAG202 restored that of IR4. The cloned R. capsulata DNA insert of pAC57 gave 5 restriction fragments by cleavage with EcoRI endonuclease. Fragment 1 (7 kb) restored hydrogenase activity in Hup- mutant strains RCC12 and ST410 and fragment 5 (1.3 kb) in strains RCC8 and RCC10. Since the 2 cosmids pAC57 and pAG202 are different cosmids, as indicated by restriction analyses and absence of cross hybridization, it is concluded that at least two hup genes are required for the expression of hydrogenase activity in R. capsulata.

Genetics of hydrogenase from aerobic lithoautotrophic bacteria

Aerobic facultatively autotrophic hydrogen bacteria are distinguished on the basis of their hydrogen-oxidizing enzyme system (Hox). The major group, represented by Paracoccus denitrificans and Pseudomonas facilis, contains a membrane-bound, electron transport-coupled protein. Species of Nocardia are characterized by the possession of a cytoplasmic NAD-dependent hydrogenase. Both enzymes are present in strains of Alcaligenes. All hydrogenases from lithoautotrophs are H2-consuming nickel-iron-sulfur proteins. Despite these common characteristics, hydrogenases differ in catalytic and molecular properties, in particular in the regulation of enzyme synthesis. Hydrogenase formation is either inducible by H2 (e.g. P. denitrificans strain F1, Alcaligenes hydrogenophilus) or subject to derepression in response to the supply of reductant, temperature, and oxygen (e.g. Alcaligenes eutrophus). The only plasmid-encoded Hox function has been conclusively identified in species of Alcaligenes. Structural and regulatory hox genes reside on megaplasmids, ranging in size between 400 and 500 kilobase pairs (kb). Most of the plasmids are self-transmissible by conjugation. Hox genes of A. eutrophus H16 have been localized by plasmid curing, genetic transfer, molecular cloning and analysis of plasmid deletions and insertions. They seem to be clustered in a DNA sequence of approximately 50 kb, representing several transcriptional units. In addition, a chromosomally encoded regulatory function is required for the expression of plasmid-linked hox genes. Plasmid pHGl of A. eutrophus H16 has been transferred to the non-lithoautotrophic soil bacterium JMP222. Both hydrogenases are expressed in the new host. The current state of hydrogenase genetics in Alcaligenes is discussed in reference to hydrogenase systems of other lithoautotrophic bacteria.

Organization of genes necessary for growth of the hydrogen-methanol autotroph Xanthobacter sp. strain H4-14 on hydrogen and carbon dioxide

Mutants unable to grow on H2 and CO2 were isolated in the hydrogen-methanol autotroph Xanthobacter sp. strain H4-14 and complemented with a clone bank constructed in a broad-host-range cosmid vector. The mutants fell into two classes. Class I mutants (Cfx-) cannot grow on hydrogen or methanol and are deficient in one or more of the key enzymes of the Calvin Cycle. Class II mutants (Hox-) can grow on methanol but not on hydrogen and lack hydrogenase activity. Restriction maps of the complementing clones show that each class is not linked to the other. Subcloning and Tn5 mutagenesis have localized the regions of DNA complementing these mutants. The region complementing a class I mutant which is deficient in ribulosebisphosphate carboxylase activity is approximately 3.2 kilobase pairs in size. Expression of this enzyme activity from cloned DNA gave evidence for the orientation of an operon containing the structural genes for this enzyme. The region complementing most of the class II mutants is 3 to 4.5 kilobase pairs in size.

Transposon Tn5 specifies streptomycin resistance in Rhizobium spp

Transposon Tn5 conferred streptomycin resistance on different strains of Rhizobium meliloti, Rhizobium leguminosarum, and Rhizobium trifolii but not on Escherichia coli. A gene (str) specifying this phenotype has been identified and localized on the physical and genetic map of Tn5. It is transcribed from the promoter of neo, the gene that encodes neomycin phosphotransferase. The str gene is downstream from neo in a single transcriptional unit, as revealed by molecular cloning of different segments of Tn5 and by cloning of the neo-str region of Tn5 downstream from a lac promoter. Fusion of the SalI-generated rightward segment of Tn5 (devoid of neo) to a part of a tetracycline resistance gene, tet, in a plasmid or downstream from a lac promoter in a plasmid resulted in significant levels of streptomycin resistance in an R. meliloti host, suggesting that the str gene product can function independent of neomycin phosphotransferase. A natural isolate of R. meliloti that does not express Tn5-associated streptomycin resistance has been identified. We have used the str of Tn5 as a genetic marker in Rhizobium spp.

Effect of Plasmid pIJ1008 from Rhizobium leguminosarum on Symbiotic Function of Rhizobium meliloti

Plasmid pIJ1008, which carries determinants for uptake hydrogenase (Hup) activity, was transferred from Rhizobium leguminosarum to Rhizobium meliloti without impairing the capacity of the latter species to form root nodules on alfalfa. The plasmid was still present in rhizobia reisolated from the root nodules of 12 different alfalfa cultivars, but only low levels of Hup activity were detected in alfalfa.

Alcaligenes eutrophus hydrogenase genes (Hox)

Mutants of Alcaligenes eutrophus H16 lacking catalytically active soluble hydrogenase (Hos-) grew very slowly lithoautotrophically with hydrogen. Mutants devoid of particulate hydrogenase activity (Hop-) were not affected in growth with hydrogen. The use of Hos- and Hop- mutants as donors of hydrogen-oxidizing ability in crosses with plasmid-free recipients impaired in both hydrogenases (Hox-) resulted in transconjugants which had inherited the plasmid and the phenotype of the donor. This indicates that the structural genes which code for the hydrogenases reside on plasmid pHG1. The Hox function of one class of Hox- mutants could not be restored by conjugation. These mutants exhibited a pleiotropic phenotype since they were unable to grow with hydrogen and also failed to grow heterotrophically with nitrate (Hox- Nit-). Nitrate was scarcely utilized as electron acceptor or as nitrogen source. Hox- Nit- mutants did not act as recipients but could act as donors of the Hox character. Transconjugants derived from those crosses were Hox+ Nit+, indicating that the mutation which leads to the Hox- Nit- phenotype maps on the chromosome. Apparently, the product of a chromosomal gene is involved in the expression of plasmid-encoded Hox genes. We observed that the elimination of plasmid pHG1 coincided with the occurrence of multiple resistances to various antibiotics. Since Hox+ transconjugate retained the antibiotic-resistant phenotype, we conclude that this property is not directly plasmid associated.

Suicide plasmid vehicles for insertion mutagenesis in Rhizobium meliloti and related bacteria

We describe the construction and use of a set of plasmid vectors of the transposons Tn1, Tn5, and Tn9 that are suicidal in Rhizobium species and therefore suitable for mutagenesis with these three transposons. The vectors are composed of the p15A replicon which functions in Escherichia coli but not in Rhizobium species and a region encoding the N type of bacterial conjugation system which is very efficient in matings between E. coli and Rhizobium species. The usefulness of the vectors has been most extensively assessed in Rhizobium meliloti. It is likely that they will be useful for mutagenesis and genome manipulation in other bacteria as well.

Growth regulators, Rhizobium and nodulation in peas : Indole-3-acetic acid from the culture medium of nodulating and non-nodulating strains of R. leguminosarum

Indole-3-acetic acid (IAA) has been identified in the culture medium of nodulating and non-nodulating strains of Rhizobium lebuminosarum by gas chromatography-mass spectrometry. The levels of IAA produced by the different strains have been quantified using multiple ion monitoring and a deuterated internal standard. Indole-3-acetic acid is produced in the absence of exogenous tryptophan in all strains but its level is greatly stimulated by applied tryptophan. No correlation has been established between the ability to nodulate peas and the ability to produce IAA.