Genetic and physical mapping and expression in Pseudomonas aeruginosa of the chromosomally encoded ribulose bisphosphate carboxylase genes of Alcaligenes eutrophus

Phylogeny and functional expression of ribulose 1,5-bisphosphate carboxylase/oxygenase from the autotrophic ammonia-oxidizing bacterium Nitrosospira sp. isolate 40KI

The autotrophic ammonia-oxidizing bacteria (AOB), which play an important role in the global nitrogen cycle, assimilate CO(2) by using ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO). Here we describe the first detailed study of RubisCO (cbb) genes and proteins from the AOB. The cbbLS genes from Nitrosospira sp. isolate 40KI were cloned and sequenced. Partial sequences of the RubisCO large subunit (CbbL) from 13 other AOB belonging to the beta and gamma subgroups of the class Proteobacteria are also presented. All except one of the beta-subgroup AOB possessed a red-like type I RubisCO with high sequence similarity to the Ralstonia eutropha enzyme. All of these new red-like RubisCOs had a unique six-amino-acid insert in CbbL. Two of the AOB, Nitrosococcus halophilus Nc4 and Nitrosomonas europaea Nm50, had a green-like RubisCO. With one exception, the phylogeny of the AOB CbbL was very similar to that of the 16S rRNA gene. The presence of a green-like RubisCO in N. europaea was surprising, as all of the other beta-subgroup AOB had red-like RubisCOs. The green-like enzyme of N. europaea Nm50 was probably acquired by horizontal gene transfer. Functional expression of Nitrosospira sp. isolate 40KI RubisCO in the chemoautotrophic host R. eutropha was demonstrated. Use of an expression vector harboring the R. eutropha cbb control region allowed regulated expression of Nitrosospira sp. isolate 40KI RubisCO in an R. eutropha cbb deletion strain. The Nitrosospira RubisCO supported autotrophic growth of R. eutropha with a doubling time of 4.6 h. This expression system may allow further functional analysis of AOB cbb genes.

Something from almost nothing: carbon dioxide fixation in chemoautotrophs

The last decade has seen significant advances in our understanding of the physiology, ecology, and molecular biology of chemoautotrophic bacteria. Many ecosystems are dependent on CO2 fixation by either free-living or symbiotic chemoautotrophs. CO2 fixation in the chemoautotroph occurs via the Calvin-Benson-Bassham cycle. The cycle is characterized by three unique enzymatic activities: ribulose bisphosphate carboxylase/oxygenase, phosphoribulokinase, and sedoheptulose bisphosphatase. Ribulose bisphosphate carboxylase/oxygenase is commonly found in the cytoplasm, but a number of bacteria package much of the enzyme into polyhedral organelles, the carboxysomes. The carboxysome genes are located adjacent to cbb genes, which are often, but not always, clustered in large operons. The availability of carbon and reduced substrates control the expression of cbb genes in concert with the LysR-type transcriptional regulator, CbbR. Additional regulatory proteins may also be involved. All of these, as well as related topics, are discussed in detail in this review.

The Calvin cycle enzyme phosphoglycerate kinase of Xanthobacter flavus required for autotrophic CO2 fixation is not encoded by the cbb operon

During autotrophic growth of Xanthobacter flavus, energy derived from the oxidation of hydrogen methanol or formate is used to drive the assimilation of CO2 via the Calvin cycle. The genes encoding the Calvin cycle enzymes are organized in the cbb operon, which is expressed only during autotrophic growth. Although it has been established that the transcriptional activator CbbR is required for the expression of the cbb operon, it is unclear whether CbbR is the only factor contributing to the regulation of the cbb operon. This paper describes the isolation of X. flavus mutants which were affected in the regulation of the cbb operon. One of the mutant strains was subject to an enhanced repression of the cbb operon promoter by the gluconeogenic substrate succinate and in addition failed to grow autotrophically. The rate of growth of the X. flavus mutant on succinate-containing medium was lower than that of the wild-type strain, but rates of growth on medium supplemented with gluconate were identical. A genomic library of X. flavus was constructed and was used to complement the mutant strain. The nucleotide sequence of the DNA fragment required to restore autotrophic growth of the X. flavus mutant was determined. One open reading frame that displayed extensive similarities to phosphoglycerate kinase-encoding genes (pgk) was identified. The X. flavus mutant lacked phosphoglycerate kinase activity following growth on gluconate or succinate. Introduction of the pgk gene into the X. flavus mutant partially restored the activity of phosphoglycerate kinase. Induction of the cbb operon of the X. flavus wild-type strain resulted in a simultaneous and parallel increase in the activities of ribulose-1,5-biphosphate carboxylase and phosphoglycerate kinase, whereas the latter activity remained absent in the X. flavus pgk mutant. It is concluded that X. flavus employees a single phosphoglycerate kinase enzyme and this is not encoded within the cbb operon.

Expression and regulation of Bradyrhizobium japonicum and Xanthobacter flavus CO2 fixation genes in a photosynthetic bacterial host

Calvin cycle carbon dioxide fixation genes encoded on DNA fragments from two nonphotosynthetic, chemolithoautotrophic bacteria, Bradyrhizobium japonicum and Xanthobacter flavus, were found to complement and support photosynthetic growth of a ribulose 1,5-bisphosphate carboxylase-oxygenase (RubisCO) deletion mutant of the purple nonsulfur bacterium Rhodobacter sphaeroides. The regulation of RubisCO expression was analyzed in the complemented R. sphaeroides RubisCO deletion mutant. Distinct differences in the regulation of RubisCO synthesis were revealed when the complemented R. sphaeroides strains were cultured under photolithoautotrophic and photoheterotrophic growth conditions, e.g., a reversal in the normal pattern of RubisCO gene expression. These studies suggest that sequences and molecular signals which regulate the expression of diverse RubisCO genes may be probed by using the R. sphaeroides complementation system.

Reversible inactivation and characterization of purified inactivated form I ribulose 1,5-bisphosphate carboxylase/oxygenase of Rhodobacter sphaeroides

Form I ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) from Rhodobacter sphaeroides is inactivated upon the addition of organic acids to photolithoautotrophically grown cultures. Activity recovers after the dissipation of the organic acid from the culture. The inactivation process depends on both the concentration of the organic compound and the nitrogen status of the cells. The inactivated RubisCO has been purified and was shown to exhibit mobility on both nondenaturing and sodium dodecyl sulfate gels different from that of the active enzyme prepared from cells not treated with organic acids. However, the Michaelis constants for ribulose 1,5-bisphosphate and CO2 or O2 were not dramatically altered. Purified inactivated RubisCO could be activated in vitro by increasing the temperature or the levels of Mg(II), and this activation was accompanied by changes in the electrophoretic mobility of the protein. When foreign bacterial RubisCO genes were expressed in an R. sphaeroides host strain lacking the ability to synthesize endogenous RubisCO, only slight inactivation of RubisCO activity was attained.

Eukaryote-eukaryote endosymbioses: insights from studies of a cryptomonad alga

It has been proposed that those plants which contain photosynthetic plastids surrounded by more than two membranes have arisen through secondary endosymbiotic events. Molecular evidence confirms this proposal, but the nature of the endosymbiont(s) and the number of endosymbioses remain unresolved. Whether plastids arose from one type of prokaryotic ancestor or multiple types is the subject of some controversy. In order to try to resolve this question, the plastid gene content and arrangement has been studied from a cryptomonad alga. Most of the gene clusters common to photosynthetic prokaryotes and plastids are preserved and seventeen genes which are not found on the plastid genomes of land plants have been found. Together with previously published phylogenetic analyses of plastid genes, the present data support the notion that the type of prokaryote involved in the initial endosymbiosis was from within the cyanobacterial assemblage and that an early divergence giving rise to the green plant lineage and the rhodophyte lineage resulted in the differences in plastid gene content and sequence between these two groups. Multiple secondary endosymbiotic events involving a eukaryotic (probably rhodophytic alga) and different hosts are hypothesized to have occurred subsequently, giving rise to the chromophyte, cryptophyte and euglenophyte lineages.

Catalytic properties of recombinant octameric, hexadecameric, and heterologous cyanobacterial/bacterial ribulose- 1,5-bisphosphate carboxylase/oxygenase

The recent isolation of a catalytically competent recombinant octameric core of the hexadecameric ribulose-1,5-bisphosphate carboxylase/oxygenase from the cyanobacterium Anacystis nidulans (Synechococcus) (B. Lee and F. R. Tabita, 1990, Biochemistry 29, 9352-9357) has provided a useful system for examining the properties of this enzyme in the absence of small subunits. Unlike most sources of hexadecameric ribulose bisphosphate carboxylase, the nonactivated Anacystis holoenzyme was not inhibited markedly by preincubation with ribulose 1,5-bisphosphate. This was also true for the Anacystis octameric core and a heterologous recombinant enzyme that comprised large subunits from Anacystis and small subunits from the bacterium Alcaligenes eutrophus, suggesting that substrate-mediated inactivation is not influenced by small subunits. In addition, the CO2/O2 specificity factor was not affected by the source of the small subunits incorporated into the structure of the hexadecameric protein, in agreement with previous in vitro heterologous reconstitution studies. The activated octameric Anacystis enzyme, however, was significantly more sensitive to inhibition by the phosphorylated effector 6-phosphogluconate than were the hexadecameric Alcaligenes and Anacystis enzymes and the heterologous Anacystis-Alcaligenes hybrid.

Evidence for two sets of structural genes coding for ribulose bisphosphate carboxylase in Thiobacillus ferrooxidans

Previously, we reported the cloning of the ribulose-1,5-bisphosphate carboxylase genes (rbcL1-rbcS1) of Thiobacillus ferrooxidans Fe1 (T. Kusano, K. Sugawara, C. Inoue, and N. Suzuki, Curr. Microbiol. 22:35-41, 1991). With these genes as probes, a second set of ribulose-1,5-bisphosphate carboxylase genes (rbcL2-rbcS2) was identified in the same strain and cloned. rbcL1 and rbcL2 encode the large subunits, and rbcS1 and rbcS2 encode the small subunits. Similar restriction patterns between these gene sets suggested a high level of sequence homology. In fact, sequence analysis showed that a 2.2-kb region, including the entire large and small subunit structural genes, was totally conserved in rbcL1-rbcS1 and rbcL2-rbcS2. The rbcL1 (rbcL2) and rbcS1 (rbcS2) genes were 1,422 and 333 bp in length and encoded 473- and 110-amino-acid proteins, respectively. The genes were separated by a 90-bp spacer sequence and were preceded by possible ribosome-binding sites. The N-terminal amino acid sequences of the subunit proteins, synthesized in Escherichia coli, were determined by Edman degradation and found to agree with the deduced amino acid sequences, except for the N-terminal methionine residue. The transcriptional start site of the rbc genes was determined by primer extension, and the size of the rbc transcript was estimated to be about 2.1 kb, suggestive of the cotranscription of rbcL1-rbcS1 and/or rbcL2-rbcS2 mRNAs. Comparisons of amino acid sequences of both subunits with those of other organisms revealed that the ribulose-1,5-bisphosphate carboxylase of T. ferrooxidans, a chemoautotrophic bacterium, is phylogenetically closer to the photosynthetic bacterium Chromatium vinosum than to another chemoautotrophic bacterium, Alcaligenes eutrophus.

Expression of endogenous and foreign ribulose 1,5-bisphosphate carboxylase-oxygenase (RubisCO) genes in a RubisCO deletion mutant of Rhodobacter sphaeroides

A Rhodobacter sphaeroides ribulose 1,5-bisphosphate carboxylase-oxygenase (RubisCO) deletion strain was constructed that was complemented by plasmids containing either the form I or form II CO2 fixation gene cluster. This strain was also complemented by genes encoding foreign RubisCO enzymes expressed from a Rhodospirillum rubrum RubisCO promoter. In R. sphaeroides, the R. rubrum promoter was regulated, resulting in variable levels of disparate RubisCO molecules under different growth conditions. Photosynthetic growth of the R. sphaeroides deletion strain complemented with cyanobacterial RubisCO revealed physiological properties reflective of the unique cellular environment of the cyanobacterial enzyme. The R. sphaeroides RubisCO deletion strain and R. rubrum promoter system may be used to assess the properties of mutagenized proteins in vivo, as well as provide a potential means to select for altered RubisCO molecules after random mutagenesis of entire genes or gene regions encoding RubisCO enzymes.

Gene for the ribulose-1,5-bisphosphate carboxylase small subunit protein of the marine chromophyte Olisthodiscus luteus is similar to that of a chemoautotrophic bacterium

The photosynthetic enzyme ribulose-1,5-bisphosphate carboxylase [3-phospho-D-glycerate carboxylase (dimerizing), EC 4.1.1.39] small subunit protein is encoded by the gene rbcS in the chloroplast genome of the unicellular alga Olisthodiscus luteus. This observation contrasts sharply with that seen in terrestrial plants and green algae, where rbcS is nuclear-localized. In this study, the O. luteus rbcS gene has been sequenced. The predicted primary structure of the protein sequence is 139 amino acids in length and lacks an N-terminal signal sequence. Unexpectedly, the O. luteus rbcS amino acid sequence shows the greatest similarity (56% identity) to that of the chemolithotrophic bacterium Alcaligenes eutrophus. A comparison of the N-terminal amino acid rbcS sequence of A. eutrophus to those of O. luteus and brown alga Fucus species shows extensive sequence similarity (68.3% identity). This observation suggests that the rbcS genes of these organisms are evolutionary homologues and may provide useful information in the study of small-subunit function.

Expressed genes for plant-type ribulose 1,5-bisphosphate carboxylase/oxygenase in the photosynthetic bacterium Chromatium vinosum, which possesses two complete sets of the genes

Two sets of genes for the large and small subunits of ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) were detected in the photosynthetic purple sulfur bacterium Chromatium vinosum by hybridization analysis with RuBisCO gene probes, cloned by using the lambda Fix vector, and designated rbcL-rbcS and rbcA-rbcB. rbcL and rbcA encode the large subunits, and rbcS and rbcB encode the small subunits. rbcL-rbcS was the same as that reported previously (A. M. Viale, H. Kobayashi, T. Takabe, and T. Akazawa, FEBS Lett. 192:283-288, 1985). A DNA fragment bearing rbcA-rbcB was subcloned in plasmid vectors and sequenced. We found that rbcB was located 177 base pairs downstream of the rbcA coding region, and both genes were preceded by plausible procaryotic ribosome-binding sites. rbcA and rbcD encoded polypeptides of 472 and 118 amino acids, respectively. Edman degradation analysis of the subunits of RuBisCO isolated from C. vinosum showed that rbcA-rbcB encoded the enzyme present in this bacterium. The large- and small-subunit polypeptides were posttranslationally processed to remove 2 and 1 amino acid residues from their N-termini, respectively. Among hetero-oligomeric RuBisCOs, the C. vinosum large subunit exhibited higher homology to that from cyanobacteria, eucaryotic algae, and higher plants (71.6 to 74.2%) than to that from the chemolithotrophic bacterium Alcaligenes eutrophus (56.6%). A similar situation has been observed for the C. vinosum small subunit, although the homology among small subunits from different organisms was lower than that among the large subunits.

D-ribulose-1,5-bisphosphate carboxylase/oxygenase: function-dependent structural changes

The key carboxylating enzyme of the reductive pentose phosphate cycle, D-ribulose-1,5-bisphosphate carboxylase/oxygenase [RuBisCO] isolated from the chemolithoautotrophic, H2-oxidizing bacterium Alcaligenes eutrophus H16 has been analyzed by several different techniques that allow conclusions about structure and function-dependent structural changes. The techniques include a novel approach in which the enzyme was induced to form 2D-crystals suitable for electron microscopy in each of its three stable functional states: as active enzyme [Ea] (in the presence of Mg2+ and HCO3-); as inactivated enzyme [Eia] (in the absence of Mg2+ and HCO3-) and as enzyme locked in an in vitro transition state [CABP-E] (Ea fully saturated with the transition state analogue 2-carboxy-D-arabinitol-1,5-bisphosphate [CABP-E]). In conjunction with X-ray crystallography, X-ray small angle scattering and other biophysical and biochemical data, the results obtained by electron microscopy support the idea that drastic configurational changes occur. Upon transition from Ea to the CABP-E the upper and lower L4S4 halves of the molecule consisting of eight large and eight small subunits (L8S8; MW = 536,000 Da) are assumed to be laterally shifted by as much as 3.6 nm relative to one another while the location of the small subunits on top of the large subunits, and relative to them, remains the same. For the Eia a similar sliding-layer configurational change in the range of 2-2.5 nm is proposed and in addition it is suggested that other configurational/conformational changes take place. The proposed structural changes are discussed with respect to the current model for the tobacco enzyme and correlated with data obtained for various other plant and (cyano) bacterial L8S8 RuBisCOs leading to speculations about structure-function relationships.

Assembly of Rubisco from native subunits

Large subunits of ribulosebisphosphate carboxylase/oxygenase (Rubisco) (3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39) from prokaryotic sources can assemble into intact enzyme either in vitro or in Escherichia coli cells. Large subunits of higher plant Rubisco do not assemble into Rubisco in E. coli cells, nor is it possible to reconstitute higher plant Rubisco from its dissociated subunits in vitro. This behavior represents an obstacle to any practical attempts at engineering the higher plant enzyme, and it suggests that the in vivo assembly mechanism of higher plant Rubisco must be more complex than is commonly expected for oligomeric proteins of organelles. In pea chloroplasts, a binding protein interacts with newly synthesized large subunits, in quantities expected for an intermediate in the assembly process, as judged by Western blotting. Radiotracer-labeled large subunits which interact with this binding protein can be shown to assemble into Rubisco in reactions which lead to changes in the aggregation state of the binding protein. Antibody to this binding protein specifically inhibits the assembly of these subunits into Rubisco. Rubisco synthesis appears to be subject to many types of control: gene dosage, transcription rate, selective translation of message, post-translational degradation and threshold concentration effects have been observed in various organisms' synthesis of Rubisco. The biochemical mechanisms underlying most of these effects have not been elucidated. The post-translational assembly mechanism in particular appears to require further study.

Cloning of the Alcaligenes eutrophus genes for synthesis of poly-beta-hydroxybutyric acid (PHB) and synthesis of PHB in Escherichia coli

Eight mutants of Alcaligenes eutrophus defective in the intracellular accumulation of poly-beta-hydroxybutyric acid (PHB) were isolated after transposon Tn5 mutagenesis with the suicide vector pSUP5011. EcoRI fragments which harbor Tn5-mob were isolated from pHC79 cosmid gene banks. One of them, PPT1, was used as a probe to detect the intact 12.5-kilobase-pair EcoRI fragment PP1 in a lambda L47 gene bank of A. eutrophus genomic DNA. In six of these mutants (PSI, API, GPI, GPIV, GPV, and GPVI) the insertion of Tn5-mob was physically mapped within a region of approximately 1.2 kilobase pairs in PP1; in mutant API, cointegration of vector DNA has occurred. In two other mutants (GPII and GPIII), most probably only the insertion element had inserted into PP1. All PHB-negative mutants were completely impaired in the formation of active PHB synthase, which was measured by a radiometric assay. In addition, activities of beta-ketothiolase and of NADPH-dependent acetoacetyl coenzyme A (acetoacetyl-CoA) reductase were diminished, whereas the activity of NADPH-dependent acetoacetyl-CoA reductase was unaffected. In all PHB-negative mutants the ability to accumulate PHB was restored upon complementation in trans with PP1. The PHB-synthetic pathway of A. eutrophus was heterologously expressed in Escherichia coli. Recombinant strains of E. coli JM83 and K-12, which harbor pUC9-1::PP1, pSUP202::PP1, or pVK101::PP1, accumulated PHB up to 30% of the cellular dry weight. Crude extracts of these cells had significant activities of the enzymes PHB synthase, beta-ketothiolase, and NADPH-dependent acetoacetyl-CoA reductase. Therefore, PP1 most probably encodes all three genes of the PHB-synthetic pathway in A. eutrophus. In addition to PHB-negative mutants, we isolated mutants which accumulate PHB at a much lower rate than the wild type does. These PHB-leaky mutants exhibited activities of all three PHB-synthetic enzymes; Tn5-mob had not inserted into PP1, and the phenotype of the wild type could not be restored with fragment PP1. The rationale for this mutant type remains unknown.

Cloning, expression and directed mutagenesis of the genes for ribulose bisphosphate carboxylase/oxygenase

The dominant natural form of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is composed of large (L) 55-kDa and small (S) 15-kDa subunits. This enzyme (as the L8S8 form) is widely distributed among oxygenic photosynthetic species and among chemosynthetic bacteria. Another form lacking small subunits is found as an L2 dimer in Rhodospirillum rubrum or an L oligomer of uncertain aggregation state from Rhodopseudomonas spharoides. The present article reviews two basically different approaches in cloning the R. rubrum gene for RuBisCO. One results in high level expression of this gene product fused with a limited aminoterminal stretch of β-galactosidase and the other results in expression of wild-type enzyme in Escherichia coli. Also reviewed are a number of reports of cloning and assembly of the L8S8 enzyme in using E. coli L and S subunit genes from Anacystis nidulans, Anabaena 7120, Chromatium vinosum and Rps. sphaeroides.In vitro oligonucleotide-directed mutagenesis has been applied to the gene for RuBisCO from R. rubrum. In terms of contributing new information to our understanding of the catalytic mechanism for RuBisCO, the most significant replacement has been of lys 166 by a number of neutral amino acids or by arg or his. Results establish that lys 166 is a catalytically essential residue and illustrate the power of directed mutagenesis in understanding structure-function correlates for RuBisCO.Oligonucleotide-directed mutagenesis has also been applied to the first and second conserved regions of the S subunit gene for RuBisCO from A. nidulans. In the latter region, corresponding amino acid changes of trp 55 and trp 58 to phe, singly or together, had little or no effect upon enzyme activity. In contrast, mutagenesis in the first conserved region leading to the following pairs of substitutions: arg10 arg 11 to gly 10 gly11; thr14 phe 15 ser 16 to ala 14 phe 15 ala 16; ser 16 tyr 17 to ala 16 asp 17; or pro 19 pro 20 to ala 19 ala 20, are all deleterious.Advances are anticpated in the introduction and expression of interesting modifications of S (and L) subunit genes in plants. A new method of introducing and expressing foreign genes in isolated etiochloroplasts is identified.

Molecular and cellular regulation of autotrophic carbon dioxide fixation in microorganisms

Images

Sequence analysis of the Alcaligenes eutrophus chromosomally encoded ribulose bisphosphate carboxylase large and small subunit genes and their gene products

The nucleotide sequence of the chromosomally encoded ribulose bisphosphate carboxylase/oxygenase (RuBPCase) large (rbcL) and small (rbcS) subunit genes of the hydrogen bacterium Alcaligenes eutrophus ATCC 17707 was determined. We found that the two coding regions are separated by a 47-base-pair intergenic region, and both genes are preceded by plausible ribosome-binding sites. Cotranscription of the rbcL and rbcS genes has been demonstrated previously. The rbcL and rbcS genes encode polypeptides of 487 and 135 amino acids, respectively. Both genes exhibited similar codon usage which was highly biased and different from that of other organisms. The N-terminal amino acid sequence of both subunit proteins was determined by Edman degradation. No processing of the rbcS protein was detected, while the rbcL protein underwent a posttranslational loss of formylmethionyl. The A. eutrophus rbcL and rbcS proteins exhibited 56.8 to 58.3% and 35.6 to 38.5% amino acid sequence homology, respectively, with the corresponding proteins from cyanobacteria, eucaryotic algae, and plants. The A. eutrophus and Rhodospirillum rubrum rbcL proteins were only about 32% homologous. The N- and C-terminal sequences of both the rbcL and the rbcS proteins were among the most divergent regions. Known or proposed active site residues in other rbcL proteins, including Lys, His, Arg, and Asp residues, were conserved in the A. eutrophus enzyme. The A. eutrophus rbcS protein, like those of cyanobacteria, lacks a 12-residue internal sequence that is found in plant RuBPCase. Comparison of hydropathy profiles and secondary structure predictions by the method described by Chou and Fasman (P. Y. Chou and G. D. Fasman, Adv. Enzymol. 47:45-148, 1978) revealed striking similarities between A. eutrophus RuBPCase and other hexadecameric enzymes. This suggests that folding of the polypeptide chains is similar. The observed sequence homologies were consistent with the notion that both the rbcL and rbcS genes of the chemoautotroph A. eutrophus and the thus far characterized rbc genes of photosynthetic organisms have a common origin. This suggests that both subunit genes have a very ancient origin. The role of quaternary structure as a determinant of the rate of accepted amino acid substitution was examined. It is proposed that the sequence of the dimeric R. rubrum RuBPCase may be less conserved because there are fewer structural constraints for this RuBPCase than there are for hexadecameric enzymes.