Department of MCD Biology, Sinsheimer Laboratories, University of California, Santa Cruz, CA 95064, USA1
Author for correspondence: Robert A. Ludwig. Tel: +1 831 459 4084. e-mail: ludwig{at}darwin.ucsc.edu
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ABSTRACT |
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Keywords: (methyl)malonate semialdehyde dehydrogenase, microaerobiosis, Rhizobiaceae, microaerophilic bacteria
Abbreviations: DOT, dissolved O2 tension; MSDH, (methyl)malonate semialdehyde dehydrogenase (acylating); PDH, pyruvate dehydrogenase; PHB, poly-ß-hydroxybutyrate
b The GenBank accession number for the sequence determined in this work is AF299324.
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INTRODUCTION |
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Uniquely among rhizobia, Azorhizobium caulinodans fixes N2 both in aerobic culture and in microaerobic symbiosis with the host legume Sesbania rostrata (Dreyfus & Dommergues, 1981 ). Optimal free O2 levels for these processes span some four orders of magnitude. In fully aerobic culture, where dissolved O2 tension (DOT) approximates physiological saturation (
200 µM), A. caulinodans grows vigorously. At 10 µM DOT, A. caulinodans optimally fixes N2. Under true microaerobic conditions (<1 µM DOT), A. caulinodans still grows avidly. At 10 nM DOT in S. rostrata nodules, endosymbiotic A. caulinodans rapidly fixes N2.
For these, various, physiological processes, A. caulinodans, like all microaerophilic rhizobia, prefers as C and energy source the common C4 dicarboxylates succinate and L-malate. Under aerobic conditions, succinate and L-malate are conventionally metabolized via citric acid cycle activity. However, in response to nanomolar DOT, symbiotic nodules require a division of metabolic labour. Infected host plant tissues strictly ferment primary photosynthate (sugars), yielding L-malate; endosymbiotic rhizobia take up and oxidize this L-malate as energy source. Indeed, Rhizobium spp. mutants which lack C4 dicarboxylate active transport capabilities cannot sustain symbiotic N2 fixation (Ronson et al., 1981 ; Finan et al., 1981
, 1983
).
In microaerophilic bacteria, how does metabolic energy transduction adapt when DOT levels change by several orders of magnitude? To help address this question, we sought, identified and studied A. caulinodans mutants defective for microaerobic energy transduction. One such mutant carries a null allele for the E1ß subunit of the pyruvate dehydrogenase (PDH) complex, long considered indispensable to obligate aerobic organisms that lack distinct pyruvate decarboxylase activity. Aerobic growth on sugars and organic acids as C and energy source yields pyruvate as oxidative intermediate. PDH complex activity oxidizes pyruvate using NAD+ as oxidant, yielding acetyl-CoA. Accordingly, PDH complex activity is often referred to as the priming step for citric acid cycle activity, which employs acetyl-CoA as substrate. Surprisingly, in A. caulinodans, PDH activity is indispensable for microaerobic growth but is dispensable for aerobic growth. As corroborated by additional physiological studies, the aerobic and microaerobic energy transduction of A. caulinodans are indeed qualitatively different.
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METHODS |
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Molecular cloning and DNA sequencing.
For DNA sequencing purposes, the wild-type A. caulinodans pdhABC locus on pSKC4 was subcloned and sequenced by standard techniques (Sambrook et al., 1989 ). The multiple short sequences were assembled into a 3134 bp continuous sequence (GenBank accession no. AF299324), which was scanned for ORFs and then translated into presumed protein sequences, which were used to probe the SWISS-PROT database using the BLASTP program at the US National Center for Biotechnology Information; possible homologues identified by these searches were multiply aligned (PIMA; Smith & Smith, 1991
).
2-Oxoacid and pyridine nucleotide pool measurements.
For 2-oxoacid quantitation, 2,4-dinitrophenylhydrazone derivatives from cleared, cell-free supernatants were prepared and separated by liquid chromatography (Supelco Gel C-610H) using 0·1% phosphoric acid as mobile phase; the flow rate was 0·5 ml min-1 at 30 °C, the spectrophotometric absorbance wavelength was 360 nm. Pyridine nucleotides were measured by modified cycling assays (Bernovsky & Swan, 1973 ). Culture samples were obtained at the indicated times, vigorously mixed with 0·5 vol. of either 7% perchloric acid (for total NAD) or 1 M NaOH (for NADH), heated for 10 min at 60 °C, and neutralized. For measurments of NADH, reactions included 40 mM Bicine pH 7·8, 0·5 mM EDTA, 15 mM phenazine methosulfate, 4 mM MTT [3-(4,5-dimethythiazol-2-yl)-2,5-diphenyltetrazolium bromide], 0·5 M ethanol and neutralized, cell-free alkali extract; reactions were conducted in the dark at 30 °C. For measurements of NAD+ by cycling, reactions contained neutralized, cell-free acid extract and were supplemented and initiated with 0·5 mg (200 U) purified Saccharomyces cerevisiae alcohol dehydrogenase. All reactions were monitored by spectrophotometric absorbance at 570 nm.
Pyruvate dehydrogenase assay.
Mid-exponential-phase cultures in ORS-MM were isolated and cell-free extracts were prepared by ultrasonication. Extracts were then cleared by centrifugation (1 h at 100000 g); extract protein concentrations were measured by standard Bradford assays. PDH activity at 30 °C was inferred as pyruvate-dependent reduction of NAD+ measured as spectrophotometric absorbance at 339 nm. Reactions comprised 0·1 M Tris/HCl pH 7·5, 10 mM MgCl2, 0·12 mM coenzyme A, 6 mM dithiothreitol, 1·2 mM NAD+, supernatant fractions of cell-free extracts (500 µg total protein); they were initiated with 10 mM pyruvate.
Cytochrome-dependent pyruvate oxidase assay.
Pyruvate-dependent cytochrome reduction was assayed by ferricyanide reduction. Batch cultures were grown on ORS-MM supplemented with 0·2% potassium succinate, 0·1% monosodium glutamate and 100 µM potassium nicotinate; early-exponential-phase cultures were harvested and washed; cell pellets were resuspended and lysed by ultrasonication. Crude cell-free extracts were then used to measure pyruvate-dependent reduction of ferricyanide at 30 °C measured as spectrophotometric absorbance at 420 nm. Reactions contained 0·1 M Tris/HCl pH 7·5, 10 mM MgCl2, 0·07 mM potassium ferricyanide and added crude cell-free extract (100 µg total protein), and were initiated with 20 mM pyruvate.
(Methyl)malonate semialdehyde dehydrogenase (acylating) assay.
Strains were cultured at 30 °C in ORS-MM supplemented with 0·4% L-valine as sole C and N source; mid-exponential-phase cultures were isolated and cell-free extracts were prepared by ultrasonication. Extracts were then cleared by centrifugation (1 h at 100000 g); extract protein concentrations were measured by standard Bradford assays. MSDH activity at 30 and 42 °C was inferred as (methyl)malonate semialdehyde-dependent reduction of NAD+. Reactions contained: 0·1 M Tris/HCl pH 8·5, 10 mM MgCl2, 0·20 mM coenzyme A, 1 mM dithiothreitol, cell-free extract (500 µg total protein), and either 10 mM malonate semialdehyde or 10 mM methylmalonate semialdehyde. These substrates were freshly prepared by acid hydrolysis of 3,3-diethoxypropionate or 3,3-diethoxyisobutyrate, respectively, as described by Hayaishi et al. (1961) . Reactions were initiated with addition of 1·5 mM NAD+ and monitored by spectrophotometric absorbance at 340 nm. Both activities were absolutely dependent upon added coenzyme A.
Dinitrogenase (N2ase) assay.
A. caulinodans N2 fixation was measured in culture by acetylene reduction and N2-dependent growth. Cells were grown to late-exponential phase in ORS-MM supplemented with either 0·4% succinate or 0·4% acetate as C source, 0·1% L-glutamate as N source, and 100 µM nicotinate, harvested, washed twice, resuspended (4x108 cells ml-1) in NIF medium, placed in stoppered serum vials and sparged with 3% O2/1% CO2/96% argon for 8 h at 30 °C. Vials were then injected with 0·2 atm acetylene (substrate), freshly generated by hydration of calcium carbide, and assayed for ethylene production versus time using gas chromatography (Donald et al., 1985 ).
Sesbania rostrata nodulation tests.
S. rostrata seedlings were germinated aseptically and grown on sterile, defined medium under N limitation (Kwon & Beevers, 1992 ). Three-week-old plants, about 50 cm in height, were inoculated with the desired A. caulinodans strain between the first and second stem internodes, for which region stem-nodule development is synchronized (Donald et al., 1986
). Starting 6 d post-inculation, maturing stem-nodules were excised and tested for N2 fixation activity by acetylene reduction. Thereafter, nodules were recovered, crushed and homogenized (Polytron), and cell-free supernatants were analysed for leghaemoglobin, measured as spectrophotometric absorbance at 540 nm. All developmental nodulation tests were done in triplicate.
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RESULTS |
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For genetic complementation tests, pSUPC4x-series plasmids were transmitted to A. caulinodans 62004 by intergeneric conjugation using, as agent, E. coli DH5/pRK2073 (Table 1
). Resulting A. caulinodans 62004/pSUPC4x partial diploids were tested for normal aerobic growth (large, opaque colonies) on defined (ORS-MM) salts plus 0·4% succinate as sole C and energy source, 15 mM
as N source, and 16 µM nicotinate. In such complementation tests, several 62004/pSUPC4x partial diploid strains grew as wild-type. From physical analysis, all complementing, recombinant plasmids carried the same 3·1 kbp SalI DNA fragment derived from
C4. Accordingly, one representative plasmid, pSUPC4, was chosen for further study.
Structure of the A. caulinodans pdhABC locus
The complete sequence of the 3·1 kbp insert DNA of pSUPC4 was determined (see Methods). Four long ORFs were identified. Three of the ORFs shared substantial homology with pyruvate dehydrogenase subunits E1 and E1ß, as well as dihydrolipoamide S-acetyltransferase E2, all components of the PDH complex; ORF4 remained unidentified. The inferred A. caulinodans genes, respectively labelled pdhA, pdhB and pdhC, were tightly organized, presumably as a single operon (Fig. 1
). The DNA sequence of recombinant pVi62004 (Table 1
) was similarly determined. Co-alignment of the wild-type genome and recombinant pVi62004 DNA sequences placed the insertion mutation in strain 62004 at nucleotide 925 of the pdhB gene (pdhB4 allele; Fig. 1
).
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A. caulinodans E1ß was likewise analysed. As with E1, A. caulinodans E1ß was most similar to the Z. mobilis E1ß orthologue, sharing 62% identity and 75% similarity. The alignment is shown in a supplementary figure with the online version of the paper at http://mic.sgmjournals.org. Interestingly, A. caulinodans E1ß residues 487 define a lipoyl-binding domain of the type usually found with the eukaryotic-type E2, dihydrolipoamide S-acetyltransferase. Likewise, Z. mobilis E1ß also shows this domain organization. A. caulinodans E1ß also has a highly conserved E2 binding interface, spanning residues 163197, again conserved in Z. mobilis E1ß. Distal to the A. caulinodans pdhB gene is a third ORF, the pdhC gene, whose inferred product carries a similar lipoyl-binding domain and which very likely encodes E2, dihydrolipoamide S-acetyltransferase, as is the case for Z. mobilis. The pdhC lipoyl-binding domain exhibits a high degree of homology (84% identity, 95% similarity) to amino acids 487 of E1ß as well as to the N-terminus of Z. mobilis E2. As evidenced by a high degree of sequence conservation, the A. caulinodans pdhABC locus, like that of Sinorhizobium meliloti (Cabanes et al., 2000
) very likely encodes the first two catalytic activities of the PDH complex.
Genetic complementation and polarity
Transposon insertions frequently yield polar mutations that confound genetic complementation tests. As inferred from DNA sequence analysis, pSUPC4x plasmids carry an incomplete pdh operon yet complement strain 62004. So, either the genomic pdh locus carries internal promoter(s) distal to the pdhB4 insertion, or else VP2021 (8·7 kb) itself acts as mobile promoter. [Note that genomic VP2021 insertions carry direct, terminal IS50R element repeats (Donald et al., 1986 ).] To help distinguish these possibilities, strain 62004 derivatives in which the complex VP2021 insertion was resolved as a simple IS50R insertion were identified (Loroch et al., 1995
). Such strains presumably arise by homologous IS50R excisive recombination. One resulting strain, 62004R (Table 1
), now carrying the pdhB4R allele, was similarly tested for genetic complementation. The aerobic growth phenotype of strain 62004R/pSUPC4x partial diploids was intermediate between the 62004 parent and wild-type; some aerobic growth enhancement was observed when cells were tested with succinate as sole C and energy source. Therefore, internal VP2021 sequences might contain mobile promoter(s) when inserted and specifically oriented toward distal genes.
The A. caulinodans pdhB4R mutant completely lacks pyruvate dehydrogenase activity
For obligate aerobes such as A. caulinodans, PDH activity is considered indispensable for growth on both sugars and organic acids not directly yielding acetyl-CoA. Yet, for the pdhB4R mutant, the IS50R insertion presumably yielded a null allele. Accordingly, PDH catalytic activities for both wild-type and the pdhB4R mutant were measured and compared (see Methods). For these enzyme assays, strains were cultured aerobically on minimal, defined medium with succinate as sole C and energy source (which presumably requires PDH activity for synthesis of acetyl-CoA as citric acid cycle substrate). In cell-free extracts, pyruvate-dependent initial velocities of 0·29 µmol NADH min-1 (mg protein)-1 for wild-type and -0·03 µmol NADH min-1 (mg protein)-1 for the pdhB4R mutant were obtained. In this, as in all other similar assays, the pdhB4R mutant lacked detectable PDH activity. In accord with both biochemical and genetic evidence, the pdhB4R allele is, therefore, null.
The A. caulinodans pdhB4R mutant shows no cytochrome-dependent pyruvate oxidation
To help reconcile both the viability of the pdhB4R mutant and its lack of PDH activity, A. caulinodans was tested for cytochrome-dependent pyruvate oxidase activity. The archetypal E. coli POXB pyruvate oxidase activity is membrane associated (but not integral), yields CO2 and free acetate, and transfers reducing equivalents to bacterial respiration (Koland et al., 1984 ). Accordingly, crude cell extracts of both A. caulinodans wild-type and the pdhB4R mutant were tested for pyruvate oxidase activity, measured as pyruvate-dependent reduction of ferricyanide (Methods). Wild-type extracts showed pyruvate dependent ferricyanide reduction at high initial rates [0·35 µmol ferrocyanide min-1 (mg protein)-1], whereas the pdhB4R mutant showed no detectable pyruvate dependent ferricyanide reduction [<0·01 µmol ferrocyanide min-1 (mg protein)-1]. Thus, the pdhB4R mutant completely lacks discrete pyruvate oxidase activity. In wild-type crude cell extracts, the observed rates for pyruvate oxidase activity may be attributable to coupled PDH and NADH-CoQ oxidoreductase activities. In summary, the pdhB4R mutant completely lacks any catalytic activity mediating pyruvate oxidation. Therefore, A. caulinodans PDH activity is dispensable for aerobic growth.
The alternative process for conversion of succinate to acetyl-CoA requires malonate semialdehyde dehydrogenase (acylating) activity
The pdhB4R mutant remains able to grow aerobically with C4 dicarboxylates (succinate, L-malate) as sole C and energy source, although growth is poor. In the absence of PDH activity, how is succinate converted to acetyl-CoA as substrate for citric acid cycle activity? Accordingly, conditional-lethal derivatives of strain 62004R carrying spontaneous mutants were isolated by penicillin-type enrichment. Strain 62004R was repeatedly subcultured on defined medium with, as sole C and N source, L-valine, on which A. caulinodans wild-type grows reasonably well, and in the presence of Timentin (see Methods) as selective agent. In these enrichment cultures, 30 °C was used as permissive and 42 °C as non-permissive temperature (see Methods). From 10 parallel culture-series, 51 temperature-sensitive (ts) spontaneous mutants that completely failed to grow on L-valine at 42 °C, but grew normally at 30 °C, were identified.
Upon replica-plate testing of these 51 candidates, 12 ts strains also failed to grow on defined ORS-MM medium with succinate and ammonium at 42 °C. To each of these 12 conditional-lethal mutants, plasmid pSUPC4 was introduced by intergeneric conjugation, as previously described. Resulting pdh partial-diploids were retested for growth on defined medium supplemented with succinate and ammonium, or alternatively with L-valine as sole C and N source, both at 42 °C. Two ts candidates, strains 62042 and 62047 (Table 1), recovered some growth on succinate and ammonium when carrying pSUPC4 (recall that pSUPC4 only weakly complements strain 62004R), but remained completely unable to grow with L-valine as sole C and N source at 42 °C; the other ts mutants remained conditional-lethal for all growth at 42 °C when carrying pSUPC4. Together with wild-type, strains 62042 and 62047 were tested for MSDH activity at both 30 and 42 °C (see Methods). For both ts mutants, MSDH activity was present at wild-type levels at 30 °C but was completely lost at 42 °C (Table 2
). In subsequent assays, malonate semialdehyde was substituted for methylmalonate semialdehyde as substrate. Similar activities were recorded; again, both ts mutants specifically lacked malonate semialdehyde-dependent activity at 42 °C (Table 2
). With both malonate semialdehyde and methylmalonate semialdehyde as substrates, observed activities were completely dependent on added coenzyme A; hence oxidative MSDH activity is acylating, yielding acetyl-CoA. In A. caulinodans wild-type, the relatively low MSDH activities measured (in comparison to PDH activities) may be responsible for the poor growth of the pdhB4 mutant on C4 dicarboxylates as sole C and energy source. Thus, MSDH activity also facilitates conversion of succinate to acetyl-CoA. Specifically, L-malate, as produced from succinate, might be diverted according to the following pathway (see Fig. 2
):
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Relevant activities include: NAD-malate dehydrogenase, aspartate aminotransferase, aspartate -decarboxylase, ß-alanine aminotransferase (Hayaishi et al., 1961
) and MSDH (acylating).
The pdhB4R mutant displays growth properties consistent with physiological acetyl-CoA limitation
General patterns of C source utilization for both A. caulinodans wild-type and the pdhB4R mutant were compared. To mitigate NAD+ limitation with pdhB4R mutant cultures (see below), nicotinate levels of liquid batch cultures in defined media were increased to 100 µM (see Methods). Because growth yields for both wild-type and the pdhB4R mutant cultured without additional N source(s) were very low, 100 µM nicotinate as N source was still insufficient to sustain normal culture growth. In subsequent experiments, 15 mM ammonium as N source was included, and various C sources were tested. Both wild-type and the pdhB4R mutant were first cultured to mid-exponential phase in rich (GYPC) medium; cells were harvested by centrifugation, washed twice with phosphate buffer, and resuspended in defined (ORS-MM) medium containing various C sources and at cell densities of 5x107 ml-1. Subcultures were then vigorously aerated at 30 °C. Growth was monitored both spectrophotometrically by light scattering and by viable cell counts (see Methods). When wild-type was shifted to succinate, a preferred C and energy source, exponential growth resumed quickly; whereas with acetate as C source, resumption of growth was delayed some 16 h. By contrast, when the pdhB4R mutant was shifted to succinate, growth was delayed some 24 h and then very slow, whereas when the pdhB4R mutant was shifted to acetate, growth was decidedly faster. Because it adapts much more quickly than wild-type when shifted from rich medium to defined medium with acetate as sole C source, the pdhB4R mutant seems acetyl-CoA limited on rich medium. In contrast to the pdhB4R mutant, the ability of A. caulinodans wild-type to use acetate and other secondary C sources which yield acetyl-CoA independent of pyruvate seems inducible.
When cell doubling times were measured in mid-exponential phase after shift from rich to defined, minimal medium, growth responses were cleanly divided between preferred energy substrates (metabolically) upstream of PDH activity (succinate, L-malate, L-lactate) and those downstream (acetate, nicotinate and ß-D-hydroxybutyrate). The former were rapidly used by wild-type but slowly utilized by the pdhB4 mutant (Table 3).
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A. caulinodans microaerobic growth requires PDH activity
To study C source utilization under true microaerobic conditions, A. caulinodans wild-type and the pdhB4R mutant were aerobically cultured to early exponential phase (1x108 ml-1) in defined medium with 15 mM ammonium as N source and shifted to continuous sparge with a prepared gas mixture of 0·1% O2/1% CO2/balance (99%) argon. After such shifts, DOT values for all cultures dropped to submicromolar levels within 2 h (see Methods). Upon shift from aerobic to microaerobic conditions, the wild-type continued to grow normally with succinate as sole C and energy source. However, with acetate as sole C source, further growth ceased immediately after shift (Fig. 4a
). Upon shift from aerobic to microaerobic conditions, the pdhB4R mutant failed to grow whether provided with either succinate or acetate as sole C and energy source (Fig. 4b
). Microaerobic growth was also assessed as colony-forming ability on solid, defined medium (Methods). A. caulinodans cytochrome cbb3 oxidase mutant 64611, which cannot grow in microaerobic culture (Kaminski et al., 1996
) was included as negative control in these experiments. The wild-type was able to grow with either succinate, L-malate, L-lactate, 2-oxoglutarate or L-valine, but not with acetate, ß-D-hydroxybutyrate or nicotinate as sole C and energy source (Table 4
). By contrast, the pdhB4R mutant did not grow under microaerobic conditions with any combination of these, various C and energy sources (Table 4
). As measured by both physiological shift experiments and colony-forming ability, the pdhB4R mutant failed to show microaerobic growth. Therefore, PDH activity is indispensable under microaerobic conditions.
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In S. rostrata nodules, symbiotic N2 fixation requires PDH activity
In culture under 10 µM DOT, the pdhB4R mutant retains the ability to fix N2. In nodule symbiosis, N2 fixation operates in the presence of excess leghaemoglobin and, hence, is a microaerobic process. As inferred from reconstruction experiments, DOT in active nodule tissues decreases to nanomolar levels in response to leghaemoglobin induction (Bergersen et al., 1986 ; Bergersen & Turner, 1990
). To assess the role of PDH in symbiotic N2 fixation, a (temporal) developmental study was conducted with, as host plant, S. rostrata, which yields determinate (developmentally synchronous) nodules on both stems and roots. Three-week-old S. rostrata plants were inoculated at stem internodes with fresh A. caulinodans cultures (see Methods). In incited S. rostrata stem nodules, leghaemoglobin synthesis dramatically increased, beginning 11 d after inoculation, and became fully active 14 d after inoculation. Starting 6 d and continuing to 24 d post-inoculation, individual stem nodules were excised and tested for N2 fixation activity by acetylene reduction (Fig. 5
). For wild-type stem nodules, some N2 fixation activity was evident prior to leghaemoglobin induction (presumably at >1 µM DOT). Nevertheless, N2 fixation (acetylene reduction) activity increased dramatically in parallel with leghaemoglobin (spectrophotometric absorbance). The pdhB4R mutant also showed incremental N2 fixation activity early in stem nodule development. However, N2 fixation responded adversely to leghaemoglobin induction, which was both delayed and limited. In mature stem nodules elicited by the pdhB4R mutant, N2 fixation not only failed to induce, it decayed (Fig. 5
). Therefore, the A. caulinodans pdhB4R mutant is symbiotically incompetent. In summary, A. caulinodans N2 fixation in S. rostrata nodules requires its own PDH activity.
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DISCUSSION |
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From genetic experiments, MSDH activity was identified as alternative source of acetyl-CoA in aerobic culture. Notably, in wild-type cell extracts, valine-induced MSDH activities were quite low in comparison to succinate-induced PDH activities, which may help explain why MSDH-dependent acetyl-CoA production is growth-limiting under aerobic conditions with C4 dicarboxylates as C and energy source. Under microaerobic conditions, the MSDH dependent pathway does not function. By inference, oxaloacetate, and thus NAD+-dependent malate dehydrogenase (MDH) activity, is then required for acetyl-CoA synthesis. Conceivably, MDH activity is sufficiently inhibited by the high microaerobic NADH:NAD+ ratio so as to limit malonate semialdehyde production. Inhibition of MSDH-dependent acetyl-CoA synthesis would then render PDH activity indispensable. While L-valine strongly induces MSDH activity in aerobic cultures and sustains microaerobic growth of wild-type, it does not allow microaerobic growth of the pdhB4R mutant. Accordingly, this growth defect is not attributable to lack of MSDH activity.
The ts alleles carried by strains 62042 and 62047 lack MSDH activity at the restrictive temperature. For both ts mutants, activity with both malonate semialdehyde and methylmalonate semialdehyde as substrate is then completely lost. Most likely, these ts mutants involve a single, catalytic protein. In that case, either a single subunit is used to make two, different enzymes or else a single enzyme activity uses both substrates. In mammals, MSDH accepts as substrate both methylmalonate semialdehyde, yielding propanoyl-CoA, and malonate semialdehyde, yielding acetyl-CoA (Yamada & Jakoby, 1960 ). Hypothetically, A. caulinodans might produce malonate semialdehyde from L-aspartate via ß-alanine. However, such N-containing compounds accumulate in rhizobia (Reid et al., 1996
) and thus are poorly oxidizable energy substrates. Further complicating this analysis, both L-aspartate and ß-alanine, when catabolized at high rates, also liberate (excess) ammonium. Therefore, confirmation of this pathway is problematical and requires experiments other than simple physiological growth tests.
In culture, the A. caulinodans pdhB4R mutant remains able to fix N2 at 10 µM DOT, a partially aerobic condition. However, during symbiosis, the pdhB4R mutant displays an absolute defect in N2 fixation that correlates with leghaemoglobin induction. Conceivably, dihydrolipoamide dehydrogenase, the E3 flavoprotein of the PDH complex, might serve as metabolic electron source for chemical reduction of N2 under microaerobiosis. However, changes in energy metabolism under microaerobic conditions characteristic of symbiotic N2 fixation in planta almost certainly contribute to this phenotype. In symbiotic nodules, bacteroids accumulate large amounts (3060% dried cell mass) of PHB (Bergersen & Turner, 1990 ), as do microaerophilic bacterial cultures generally (Jackson & Dawes, 1976
; Senior et al., 1972
; Steinbüchel & Schlegel, 1991
). Indeed, A. caulinodans PHB synthase mutants are symbiotically defective (Mandon et al., 1998
).
For obligate aerobic organisms, physiological O2 limitation frequently leads to steady-state excess of acetyl-CoA, the fuel of oxidative energy transduction (citric acid cycle; glyoxylate cycle). Formally, this condition obtains when acetyl-CoA production rates exceed oxidation rates; the metabolic steady-state is then constrained by available oxidant. To compensate, some acetyl-CoA may then itself serve as endogenous oxidant, in which case partially reduced compounds, among them ß-hydroxybutyrate and other ketone bodies, accumulate. In mammals, to help mitigate adverse effects of such ketosis, ß-hydroxybutyrate equivalents are then excreted. In microaerophilic bacteria, ß-hydroxybutyrate equivalents are instead polymerized in situ, forming poly-hydroxyalkanoate (such as PHB) granules. Both bacterial and mammalian processes presumably help reoxidize pyridine nucleotide (cofactor) pools and thus revitalize stalled acetyl-CoA oxidation via concerted glyoxylate and citric acid cycle activities.
Pushing this limit further, microaerophilic bacteria remain capable of oxidative energy transduction under nanomolar DOT, which represents extreme O2 limitation. In principle, this capability may be owed to either peripheral factors and/or fundamentally different core processes. As an example of the former, in response to extreme O2 limitation, A. caulinodans produces ultrahigh-affinity terminal oxidases, such as cytochrome cbb3 and cytochrome bd oxidases, which possess apparent Km(O2) values sufficient to allow growth at submicromolar DOT (Kaminski et al., 1996 ). However, even such ultrahigh-affinity terminal oxidases may not be fully O2-saturated at nanomolar DOT, so overall metabolic rates may then be limited by terminal oxidase rates.
However, rates are not the whole story. Consider the overall equation for respiration:
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In microaerophilic bacteria such as A. caulinodans, oxidative phosphorylation continues unabated and at high efficiency during microaerobic growth (Senior et al., 1972 ). To maintain a similarly high
P:O ratio at nanomolar DOT (Bergersen et al., 1986
), the steady-state NADH:NAD+ ratio needs to increase significantly. Correspondingly, A. caulinodans cultures show a dramatic increase in NADH:NAD+ upon microaerobic shift. In principle, this high NADH:NAD+ ratio might lead to qualitative metabolic changes, such as increased diversion of L-malate to pyruvate in lieu of oxaloacetate. In contrast to the reaction catalysed by conservative NAD-malate dehydrogenase (yielding oxaloacetate), a highly endergonic process, that catalysed by decarboxylative malate dehydrogenase (also known as malic enzyme), yielding pyruvate is exergonic under standard conditions. Indeed, in E. coli, NAD-dependent malic enzyme is the only such activity among this group of L-malate-dependent enzymes not allosterically inhibited by NADH (Sanwal, 1970
). In Sinorhizobium meliloti, NAD-dependent malic enzyme activity is indeed required for microaerobic energy transduction in Medicago sativa nodules (Driscoll & Finan, 1993
).
For aerobic energy transduction via the citric acid cycle, oxaloacetate and acetyl-CoA undergo bimolecular condensation to yield citrate, regenerating coenzyme A. In Saccharomyces cerevisiae, NAD-malate dehydrogenase and citrate synthase activities are metabolically coupled (Vélot & Srere, 2000 ). In E. coli, both citrate synthase and NAD-malate dehydrogenase activities are powerfully allosterically inhibited by NADH (Sanwal, 1969
, 1970
). Accordingly, upon shift to microaerobic conditions, an increased NADH:NAD+ ratio may render acetyl-CoA oxidation rates growth-limiting. Consequently, citric acid cycle and glyoxylate cycle activities might then be restricted. Indeed, both Bradyrhizobium japonicum aconitase mutants (Thöny-Meyer & Künzler, 1996
) and 2-oxoglutarate dehydrogenase mutants (Green et al., 2000
) remain competent for symbiotic N2 fixation with host soybean (Glycine max) plants and, by inference, remain capable of microaerobic energy transduction.
Thus, excess acetyl-CoA serves as endogenous oxidant, being converted to PHB as metabolic end product. Nevertheless, energy transduction with C4 dicarboxylate (succinate, L-malate) as energy substrate must remain oxidative and NADH as substrate for oxidative phosphorylation must be produced (Fig. 6). However, the (linear) conversion of L-malate to PHB is only partially (<50%) oxidative, yielding four CO2 per two substrate L-malate molecules converted to ß-D-hydroxybutyryl-CoA for polymerization. Beneficially, a less oxidative energy transduction helps microaerophilic bacteria maintain high substrate fluxes, and requisite high enzyme activities, whether microaerobic or aerobic. In turn, such oxidative metabolic gearing facilitates rapid adaptation in response to dramatic changes in environmental DOT.
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ACKNOWLEDGEMENTS |
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REFERENCES |
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Received 24 January 2001;
revised 28 March 2001;
accepted 2 April 2001.