Maize phosphoenolpyruvate carboxylase. Cloning and characterization of mRNAs encoding isozymic forms

Multiple highly expressed phosphoenolpyruvate carboxylase genes have divergent enzyme kinetic properties in two C4 grasses

BACKGROUND AND AIMS

Phosphoenolpyruvate (PEP) carboxylase (PEPC) catalyses the irreversible carboxylation of PEP with bicarbonate to produce oxaloacetate. This reaction powers the carbon-concentrating mechanism (CCM) in plants that perform C4 photosynthesis. This CCM is generally driven by a single PEPC gene product that is highly expressed in the cytosol of mesophyll cells. We found two C4 grasses, Panicum miliaceum and Echinochloa colona, that each have two highly expressed PEPC genes. We characterized the kinetic properties of the two most abundant PEPCs in E. colona and P. miliaceum to better understand how the enzyme's amino acid structure influences its function.

METHODS

Coding sequences of the two most abundant PEPC proteins in E. colona and P. miliaceum were synthesized by GenScript and were inserted into bacteria expression plasmids. Point mutations resulting in substitutions at conserved amino acid residues (e.g. N-terminal serine and residue 890) were created via site-directed PCR mutagenesis. The kinetic properties of semi-purified plant PEPCs from Escherichia coli were analysed using membrane-inlet mass spectrometry and a spectrophotometric enzyme-coupled reaction.

KEY RESULTS

The two most abundant P. miliaceum PEPCs (PmPPC1 and PmPPC2) have similar sequence identities (>95 %), and as a result had similar kinetic properties. The two most abundant E. colona PEPCs (EcPPC1 and EcPPC2) had identities of ~78 % and had significantly different kinetic properties. The PmPPCs and EcPPCs had different responses to allosteric inhibitors and activators, and substitutions at the conserved N-terminal serine and residue 890 resulted in significantly altered responses to allosteric regulators.

CONCLUSIONS

The two, significantly expressed C4Ppc genes in P. miliaceum were probably the result of genomes combining from two closely related C4Panicum species. We found natural variation in PEPC's sensitivity to allosteric inhibition that seems to bypass the conserved 890 residue, suggesting alternative evolutionary pathways for increased malate tolerance and other kinetic properties.

Isolation, Characterization and Expression Analysis of a Leaf-specific Phosphoenolpyruvate Carboxylase Gene inOryza sativa

Suppression subtractive hybridization was carried out to enrich gene fragments over-expressed in rice leaves by subtraction to rice roots, from which two identical cDNA fragments were identified to encode putative phosphoenolpyruvate carboxylase. Then the corresponding full-length cDNA (Osppc) is isolated by RT-PCR and sequenced, which indicates an open reading frame of 2895bp is contained. Its deduced protein is encoded in 10 exons and shows high similarity to many other plant PEPCs. Comparing with maize and bacterial PEPCs, it is revealed that OSPPC shares many conserved domains and active sites that responsible for the structure, activity and regulation of this enzyme. Phylogenetic analysis demonstrates that OSPPC is grouped with C3 form PEPCs of wheat, maize and sorghum, which is consistent with the classification of rice. And a putative promoter element is predicted with DOF binding box, CAAT box and TATA box in the 5'-flanking sequence of Osppc gene. Moreover, Quantitative RT-PCR analyses are performed in hybrid rice and its parents, which show that Osppc is specifically expressed in leaf including leaf vein and sheath.

Phosphoenolpyruvate carboxylase genes in C3, crassulacean acid metabolism (CAM) and C3/CAM intermediate species of the genus Clusia: rapid reversible C3/CAM switches are based on the C3 housekeeping gene

The genus Clusia includes species that exhibit either the C3 or crassulacean acid metabolism (CAM) mode of photosynthesis, or those that are able to switch between both modes according to water availability. In order to screen for species-specific genetic variability, we investigated the key carboxylase for CAM, phosphoenolpyruvate carboxylase (PEPC). Sequence analysis of DNA isolated from the obligate CAM species, Clusia hilariana, the obligate C3 species, Clusia multiflora, and an intermediate species that can switch between C3 and CAM photosynthesis, Clusia minor, revealed three different isoforms for C. hilariana and one each for the other two species. Sequence alignments indicated that PEPC from the intermediate species had high homology with the C3 protein and with one of CAM plant proteins. These were assumed to constitute 'housekeeping' proteins, which can also support CAM in intermediate species. The other two isoforms of the CAM plant C. hilariana were either CAM-specific or showed homologies with PEPC from roots. Phylogenetic trees derived from neighbour-joining analysis of amino acid sequences from 13 different Clusia species resulted in two distinct groups of plants with either 'housekeeping' PEPC only, or additionally CAM-related isoforms. Only C. hilariana showed the third, probably root-specific isoform. The high homology of the PEPC from the intermediate species with the C3 protein indicates that for the reversible transition from the C3 to CAM mode of photosynthesis, the C3 type of PEPC is sufficient. Its expression, however, is strongly increased under CAM-inducing conditions. The use of the C3 isoform could have facilitated the evolution of CAM within the genus, which occurred independently for several times.

Integration and expression of Sorghum C(4) phosphoenolpyruvate carboxylase and chloroplastic NADP(+)-malate dehydrogenase separately or together in C(3) potato plants(1)

We have integrated two cDNAs expressing Sorghum photosynthetic phosphoenolpyruvate carboxylase (C(4)-PEPC) and NADP-malate dehydrogenase (cpMDH), two key enzymes involved in the primary carbon fixation pathway of NADP-malic enzyme-type C(4) plants, separately or together into a C(3) plant (potato). Analysis of the transgenic plants showed a 1.5-fold increase in PEPC and cpMDH activities compared to untransformed plants. Immunolocalization confirmed an increase at the protein level of these two enzymes in the transgenic plants and indicated that the Sorghum cpMDH was specifically addressed to the chloroplasts of potato mesophyll cells. However, integration of either or both of the cDNAs into the potato genome did not appear to significantly modify either tuber starch grain content or the rate of photosynthetic O(2) production compared to control untransformed plants. The low level of transgene expression probably explains the lack of influence on carbon metabolism and photosynthetic rates. This general observation suggests that some complex mechanism may regulate the level of production of foreign C(4) metabolism enzymes in C(3) plants.

Simultaneous expression of NAD-dependent isocitrate dehydrogenase and other krebs cycle genes after nitrate resupply to short-term nitrogen-starved tobacco

Mitochondrial NAD-dependent (IDH) and cytosolic NADP-dependent isocitrate dehydrogenases have been considered as candidates for the production of 2-oxoglutarate required by the glutamine synthetase/glutamate synthase cycle. The increase in IDH transcripts in leaf and root tissues, induced by nitrate or NH4+ resupply to short-term N-starved tobacco (Nicotiana tabacum) plants, suggested that this enzyme could play such a role. The leaf and root steady-state mRNA levels of citrate synthase, acotinase, IDH, and glutamine synthetase were found to respond similarly to nitrate, whereas those for cytosolic NADP-dependent isocitrate dehydrogenase and fumarase responded differently. This apparent coordination occurred only at the mRNA level, since activity and protein levels of certain corresponding enzymes were not altered. Roots and leaves were not affected to the same extent either by N starvation or nitrate addition, the roots showing smaller changes in N metabolite levels. After nitrate resupply, these organs showed different response kinetics with respect to mRNA and N metabolite levels, suggesting that under such conditions nitrate assimilation was preferentially carried out in the roots. The differential effects appeared to reflect the C/N status after N starvation, the response kinetics being associated with the nitrate assimilatory capacity of each organ, signaled either by nitrate status or by metabolite(s) associated with its metabolism.

Molecular characterization of higher plant NAD-dependent isocitrate dehydrogenase: evidence for a heteromeric structure by the complementation of yeast mutants

NAD-dependent isocitrate dehydrogenase (IDH) is a key enzyme controlling the activity of the citric acid cycle. Despite more than 30 years of work, the plant enzyme remains poorly characterized. In this paper, a molecular characterization of the plant IDH is presented. Starting from probes defined according to sequence comparisons, three full-length cDNAs named Ntidha, Ntidhb and Ntidhc encoding different IDH subunits have been isolated from a Nicotiana tabacum cell suspension library. Sequence comparisons of the tobacco IDH subunits with the E. coli NADP-dependent enzyme, and the yeast IDH1 and IDH2 subunits suggested that only IDHa had the capacity to be catalytic as IDHb and IDHc were lacking certain residues implied in catalysis. The ability of antibodies raised against the recombinant IDHa protein to preferentially cross-react with IDH2 indicated that IDHa was more closely related to IDH2 than to IDH1. Complementation of yeast single IDH mutants showed that IDHb and IDHc could replace the function of the yeast regulatory IDH1 subunit. Although IDHa was unable to complement the IDH2 mutant, its catalytic function was revealed by the ability of two heteromeric enzymes, composed of either IDHa with IDHb or IDHa with IDHc, to replace IDH function in a yeast double mutant lacking both subunits. Expression studies at the protein and mRNA levels show that each subunit is present in both root and leaf tissues and that the three IDH genes respond in the same way to nitrate addition. Taken together, such observations suggest that the physiologically active enzyme is composed of the three different subunits. These results show for the first time that the plant IDH is heteromeric and that IDH subunit composition appears to be conserved between plant and animal kingdoms.

Analysis of a 14-kb fragment containing a putative cell wall gene and a candidate for the ARA1, arabinose kinase, gene from chromosome IV of Arabidopsis thaliana

An Arabidopsis thaliana genomic DNA fragment of 14kb has been characterized in the framework of the E.S.S.A. programme. Computational and molecular approaches identified three novel gene sequences coding, respectively, for a protein of unknown function, a putative membrane-anchored cell wall protein and an arabinose kinase gene corresponding to the locus ARA1. The latter two genes named AtSEB1 and AtISA1 have been characterized in detail. They are very different in their organization, codon usage and level of expression. Homologues of AtSEB1 and AtISA1 have been identified. Sequence comparisons showed that the former genes contained a long 5' extension coding for an N-terminal domain probably specifying subcellular localization. Cloning and sequencing of the cognate cDNA for the AtISA1 homologue in A. thaliana, named GAL1, indicate that it encodes for a galactokinase-like protein. Our results highlight the integrative outcome of a systematic sequencing project in which links between biochemically and genetically characterized mutants, ESTs and genomic sequence data are generated.

Identification of a tobacco cDNA encoding a cytosolic NADP-isocitrate dehydrogenase

A cDNA which encodes a specific member of the NADP-dependent isocitrate dehydrogenase (ICDH) multi-isoenzyme family has been isolated from a tobacco cell suspension library, and the expression pattern of ICDH transcripts examined in various plant tissues. To assign this cDNA to a specific ICDH isoenzyme, the major, cytosolic ICDH isoenzyme of tobacco leaves (ICDH1) was purified to homogeneity and its N-terminus as well as several tryptic peptides, representing 30% of the protein, were sequenced. The comparison of these amino acid sequences with the deduced protein sequence of the cDNA confirmed that this clone encodes for ICDH1. The total ICDH specific activity and protein content were higher in vascular-enriched tobacco leaf tissue than in deveined (depleted in midrib and first-order veins) leaves. Taking advantage of antibodies raised against either ICDH1 or the chloroplastic ICDH2 isoenzyme from tobacco cell suspensions, an immuno-cytochemical approach indicated that the ICDH1 isoenzyme, located in the cytosolic compartment of tobacco leaf cells, is responsible for this expression pattern. This observation was confirmed by northern blot analyses, using a specific probe obtained from the 3' non-coding region of the ICDH1 cDNA. A comparison of ICDH protein sequences shows a large degree of similarity between eukaryotes (> 60%) but a poor homology is observed when compared to Escherichia coli ICDH (< 20%). However, it was found that the amino acids implicated in substrate binding, deduced from the 3-dimensional structure of the E. coli NADP-ICDH, appear to be conserved in all the deduced eukaryotic ICDH proteins reported until now.

Evolution of the C4 phosphoenolpyruvate carboxylase promoter of the C4 dicot Flaveria trinervia: an expression analysis in the C3 plant tobacco

The key enzymes of photosynthetic carbon assimilation in C4 plants have evolved from C3 isoforms which were present in the C3 ancestral species. We are interested in the molecular changes responsible for the novel expression pattern of C4 genes and are focussing on phosphoenolpyruvate carboxylase (PEPCase) of the genus Flaveria. The C4 isoform of PEPCase in the C4 plant F. trinervia is encoded by the ppcA subgroup of the PEPcase gene family and is abundantly expressed in the mesophyll cells of leaves. The orthologous ppcA genes of the C3 plant F. pringlei are only weakly expressed and their transcripts do not accumulate in a leaf-specific manner but, rather, are present in all plant organs. To answer the question whether the differences in the expression levels of the ppcA genes from F. pringlei and F. trinervia are caused by changes in the 5' upstream regions of the genes or by C4-specific trans-regulatory factors, varying parts of the 5' flanking region of the ppcA1 genes of both species were fused to the beta-glucuronidase (GUS) gene and inserted in the tobacco genome. GUS expression analysis of transgenic plants revealed that the level of expression of the Flaveria ppcA1 genes are recapitulated in the heterologous C3 plant tobacco. Hence, the 5' upstream region of the ppcA1 gene of F. trinervia contains regulatory cis-elements that are responsible for the C4-specific, abundant expression of this gene. These sequences are located upstream of position -500 relative to the transcription initiation site.(ABSTRACT TRUNCATED AT 250 WORDS)

Gene density and organization in a small region of the Arabidopsis thaliana genome

We have characterized a 6.4 kb genomic fragment from Arabidopsis thaliana ecotype Columbia overlapping the 5' end of the AKin10 gene which encodes a protein Ser/Thr kinase. Using, as probes, various restriction fragments located upstream of AKin10, two cDNA clones have been isolated from a cDNA library prepared from young shoot tissue. A comparison between the cDNA and the above genomic sequences allowed us to locate two novel genes, Atcys1 and Athyp1 (for Arabidopsis thaliana cystathionine gamma-synthase 1 and hypothetical protein 1). The coding sequences of both genes are interrupted by introns and the exons match the sequences of the corresponding cDNAs. Further analysis of the genomic fragment revealed the presence of an open reading frame (ORF) of 609 nucleotides situated between the two genes. Atcys1, Athyp1, AKin10 and the ORF are very close to each other and organized in the same polarity; hence, the intergenic regions probably contain, within less than 0.5 kb, all the regulatory elements necessary to govern initiation and termination of transcription. The deduced protein sequence of Atcys1 shows a high degree of similarity with the cystathionine gamma-synthase from Escherichia coli. The putative product of the Athyp1 gene contains seven hydrophobic regions flanked by hydrophilic domains, reminiscent of membrane-spanning proteins. Southern blot hybridization experiments suggest the presence of one copy of Atcys1, Athyp1 and AKin10 per haploid genome, and Northern blot analysis demonstrates that the three genes are differentially expressed in roots, shoots and leaves.

Arabidopsis homologs of the shaggy and GSK-3 protein kinases: molecular cloning and functional expression in Escherichia coli

The conservation in evolution of fundamental signal transduction modules offers a means of isolating genes likely to be involved in plant development. We have amplified by PCR Arabidopsis cDNA and genomic sequences related to the product of the shaggy/zeste-white 3 (sgg) segment polarity gene of Drosophila. This regulatory protein is functionally homologous to glycogen synthase kinase-3 in mammals (GSK-3), which regulates, among others, the DNA-binding activity of the c-jun/AP1 transcription factor. Analysis of PCR products led to the identification of five genes; for two of which, corresponding full-length cDNAs, ASK-alpha and gamma (for Arabidopsis shaggy-related protein kinase), were characterized. The encoded proteins were 70% identical to GSK-3 and sgg over the protein kinase catalytic domain and, after production in Escherichia coli, autophosphorylated mainly on threonine and serine residues, but phosphotyrosine was also detected. ASK-alpha and ASK-gamma also phosphorylated phosphatase inhibitor-2 and myelin basic protein, on threonine and serine, respectively. The high conservation of the protein kinases of GSK-3 family, and their action at the transcriptional level, suggest that the ASK proteins have important functions in higher plants.

Structure and expression of a gene from Arabidopsis thaliana encoding a protein related to SNF1 protein kinase

The AKin10 gene from Arabidopsis thaliana encoding a putative Ser/Thr protein kinase (PK) has been isolated and characterized. The AKin10-encoding gene is located on a genomic 5.4-kb BamHI fragment and contains ten introns, one being located in the 5' untranslated region. The deduced amino acid sequence of AKin10 is 65% identical over the catalytic domain to the yeast PK (SNF1). SNF1 is essential for the derepression of many glucose-repressible genes, including Suc2 which encodes invertase. Southern blot hybridization experiments suggested the presence of one copy of the gene per haploid genome of A. thaliana. Northern hybridization experiments indicated that this gene is expressed in roots, shoots and leaves. AKin10 may play an important role in a signal transduction cascade regulating gene expression and carbohydrate metabolism in higher plants.

MNF1, a leaf tissue-specific DNA-binding protein of maize, interacts with the cauliflower mosaic virus 35S promoter as well as the C4 photosynthetic phosphoenolpyruvate carboxylase gene promoter

When gel shift assays were performed with maize nuclear extract and a DNA fragment containing the cauliflower mosaic virus (CaMV) 35S promoter, three DNA-protein complexes were observed. Analyses with nuclear extracts prepared from green leaves, etiolated leaves, stems and roots showed that the complexes resulted from the existence of at least two nuclear factors. One of them is presumably a constitutive nuclear factor found in all tissues tested, and another is a leaf-specific factor present both in green and etiolated leaves. This leaf-specific nuclear factor seemed to be identical to MNF1, previously identified as a factor interacting with the promoter of the maize gene for phosphoenolpyruvate carboxylase involved in the C4 photosynthesis. Deletion analysis revealed that MNF1 binds to the sequence from -281 to -235 relative to the transcription start site of the CaMV 35S promoter. MNF1-like nuclear protein was also found in tobacco nuclear extracts. The possibility that MNF1 participates as a positive trans-acting factor in the expression of genes in maize leaves is discussed.

Multiple cDNAs of phosphoenolpyruvate carboxylase in the C4 dicot Flaveria trinervia

We have isolated and characterized cDNA clones for the leaf-specific C4-phosphoenolpyruvate carboxylase (PEPCase) from the dicotyledonous C4 plant Flaveria trinervia. The isolation of multiple cDNAs indicates that in this plant the C4 isoform is encoded by a small subgroup of the PEPCase gene family. The deduced amino acid sequence reveals a higher degree of similarity to the CAM and C3 isozymes of the dicotyledonous, facultative CAM plant Mesembryanthemum crystallinum than to the C4 PEPCases of monocotyledonous origin.

Sequence-specific interactions of a maize factor with a GC-rich repeat in the phosphoenolpyruvate carboxylase gene

A plant nuclear protein PEP-I, which binds specifically to the promoter region of the phosphoenolpyruvate carboxylase (PEPC) gene, was identified. Methylation interference analysis and DNA binding assays using synthetic oligonucleotides revealed that PEP-I binds to GC-rich elements. These elements are directly repeated sequences in the promoter region of the PEPC gene and we have suggested that they may be cis-regulatory elements of this gene. The consensus sequence of the element is CCCTCTCCACATCC and the CTCC is essential for binding of PEP-I. PEP-I is present in the nuclear extracts of green leaves, where the PEPC gene is expressed. However, no binding was detected in tissues where the PEPC gene is not expressed in vivo, such as roots or etiolated leaves. Thus, PEP-I is the first factor identified in plants which has different binding activity in light-grown compared with dark-grown tissue. PEP-I binding is also tissue-specific, suggesting that PEP-I may function to coordinate PEPC gene expression with respect to light and tissue specificity. This report describes the identification and characterization of the sequences required for PEP-I binding.

Analysis of expression and evolutionary relationships of phosphoenolpyruvate carboxylase genes in Flaveria trinervia (C4) and F. pringlei (C3)

Phosphoenolpyruvate carboxylase (PEPCase) was shown to be encoded by a multigene family in various Flaveria species analysed. Several clones were isolated from genomic libraries of F. pringlei (C3 species) and F. trinervia (C4 species) and classified into four distinct groups according to their hybridization behaviour to a full-length cDNA clone encoding the PEPCase C4 isozyme of F. trinervia. A detailed cross-hybridization analysis demonstrated that the closest relative of most of the PEPCase genes isolated from F. trinervia and F. pringlei was not found in the same but in the other species. Northern analysis, using stringent conditions, allowed discrimination of class-specific PEPCase transcripts and revealed characteristic organ-specific expression patterns.

Multiple interactions between tissue-specific nuclear proteins and the promoter of the phosphoenolpyruvate carboxylase gene for C4 photosynthesis in Zea mays

The expression of the phosphoenolpyruvate carboxylase (PEPC) gene involved in C4 photosynthesis is regulated in a highly organized manner. Nuclear factors interacting with DNA fragments from the 5' flanking region (from positions -1012 to +88 relative to the transcription start site) of the maize gene were identified by gel shift assays. Among the three kinds of such nuclear proteins (MNF1, MNF2a and MNF2b) found in the extract from maize leaves, MNF2a and MNF2b, which were distinguishable by their chromatographic behavior, interacted with the same motif of the repeated sequence (RS2) in the region from -432 to -201. MNF1 interacted with the region from -905 to -818 in which two copies of another kind of repeated sequence (RS1) reside. All of these nuclear factors were found only in the extracts from green and etiolated leaves but not in those from stems and roots. The relative content of MNF1 and MNF2b was almost equal in green and etiolated leaves, while that of MNF2a was significantly higher in etiolated leaves than green leaves. It is suggested that expression of the PEPC gene is controlled by the combined effects of these nuclear factors.

Primary structure of sorghum malate dehydrogenase (NADP) deduced from cDNA sequence. Homology with malate dehydrogenase (NAD)

Malate dehydrogenase (NADP) (NADP-MDH) is an important enzyme of the photosynthetic CO2 fixation pathway of C4 plants. We have isolated two clones from a sorghum lambda gt11 cDNA library (CM3, 932 bp, and CM7, 1441 bp). Nucleotide sequence analysis of the cDNAs CM3 and CM7 showed the existence of two NADP-MDH mRNA species encoding different enzyme subunits. Microsequencing of the N-terminus of the mature protein indicated that a specific cleavage of 13 amino acids occurred during the purification steps of the enzyme. The full-length cDNA CM7 contains a large open reading frame encoding an NH2-terminal transit peptide of 40 amino acids and a mature protein of 389 amino acids (42.207 kDa). Alignment of the NADP-MDH sequence with those of several malate dehydrogenases revealed some similarities with NAD-MDHs.

Complete structure of the gene for phosphoenolpyruvate carboxylase from maize

Phosphoenolpyruvate carboxylase is a key enzyme in photosynthesis in some plants that exploit the C4 photosynthetic pathway for the fixation of CO2. We cloned the gene for this enzyme from maize genomic libraries and analyzed its complete primary structure. The sequence of the cloned gene spans 6781 bp and consists of 10 exons and 9 introns. The site of initiation of transcription is located 84 nucleotides upstream from the first nucleotide of the initiation codon (position -84), as determined by the method of primer-extension analysis. The analysis suggests that there is another initiation site located at position -81. The 5'-flanking region of the gene lacks typical TATA and CCAAT elements in the anticipated regions, but there is a TATA-similar sequence (TATTT) around the -30 regions as well as sequence homologous to the Sp-1 protein-binding site (CCGCCC). Six long, direct repeated sequences and a light-responsive element are also present in the 5'-flanking region. The results of Southern blot analysis indicated that the phosphoenolpyruvate carboxylase gene exists as a small multi-gene family, but the enzyme that is expressed at high levels in green leaves and is involved in C4 photosynthesis is encoded by a single-copy gene in the maize genome.

Expression of the CAM-form of phospho(enol)pyruvate carboxylase and nucleotide sequence of a full length cDNA from Mesembryanthemum crystallinum

We have determined the complete nucleotide sequence of a full length cDNA encoding the Crassulacean acid metabolism (CAM) isogene of phospho(enol)pyruvate carboxylase (PEPCase). The cDNA clone, 3348 bp in length, was obtained from mRNA isolated from Mesembryanthemum crystallinum (common ice plant) which had undergone salt stress and subsequent induction of CAM. The long open reading frame encodes PEPCase (EC 4.1.1.31) with a predicted molecular mass of 110533 daltons. The deduced amino acid sequence of the ice plant PEPCase is most similar to that from maize having an amino acid identity of 74.9%. Sequence identity in corresponding regions of the PEPCase proteins from Escherichia coli and the cyanobacterium Anacystis nidulans are 41.4% and 33.5%, respectively. A compilation of the four amino acid sequences permitted the identification of phylogenetically conserved regions within the proteins which may play a role in the function of this important enzyme in plant metabolism. Gene specific probes from 3' coding and noncoding regions of the cDNA clone used to probe genomic Southern blots established that this PEPCase gene is present in one copy in the nuclear genome of M. crystallinum. Transcripts arising from this gene increase dramatically when M. crystallinum is irrigated with 0.5 M NaCl, a stress which induces this plant to switch the primary fixation of CO2 from C3 (Calvin cycle) to CAM mode. The salt-induced mRNA encodes a PEPCase isoform which is undetectable in plants in the C3 mode as demonstrated by Northern hybridization.

Gene expression during CAM induction under salt stress in Mesembryanthemum: cDNA library and increased levels of mRNA for phosphoenolpyruvate carboxylase and pyruvate orthosphosphate dikinase

Mesembryanthemum crystallinum responds to high salinity in the soil by shifting the mode of carbon assimilation from the C3 mode to Crassulacean acid metabolism (CAM). Several enzymes of carbon metabolism have increased apparent activities in the CAM mode, including phosphoenolpyruvate carboxylase (PEPcase) and pyruvate orthophosphate dikinase (PPDK). We have identified cDNA clones for PEPcase and PPDK by immunological screening of a cDNA library constructed in the protein expression vector lambda gt11. The clones were characterized by immunoblotting and RNA blotting techniques. RNA blotting showed that during CAM induction the steady-state level of mRNAs for both PEP case and PPDK increased.

In vivo phosphorylation of sorghum leaf phosphoenolpyruvate carboxylase

The use of immunological techniques allowed us to purify close to homogeneity phosphoenolpyruvate carboxylase (PEPc, EC 4.1.1.31) from sorghum leaf. It was thus established that: 1) this protein is phosphorylated in vivo on seryl residues; 2) in C4-type photosynthesis, the phosphorylation process mainly concerns the PEPC isozyme form G; 3) enzyme phosphorylation displays significant variations through a day-night alternation which therefore suggests light control of the process.

Further analysis of cDNA clones for maize phosphoenolpyruvate carboxylase involved in C4 photosynthesis. Nucleotide sequence of entire open reading frame and evidence for polyadenylation of mRNA at multiple sites in vivo

Four clones of cDNA for phosphoenolpyruvate carboxylase [EC 4.1.1.31] were obtained from a maize green leaf cDNA library by colony hybridization. The largest cDNA was of full-length (3335 nucleotides), being 243 nucleotides longer than the cDNA cloned previously [(1986) Nucleic Acids Res. 14, 1615-1628]. Alignment of the sequence for the N-terminal coding region found in two of the four clones with the sequence reported previously, established the sequence of the entire coding region for the enzyme. The sequencing of 3'-untranslated region of the clones revealed that the poly(A) tract is attached at multiple sites in vivo.

Cellular pattern of photosynthetic gene expression in developing maize leaves

Leaf development in C4 plants such as maize involves the differentiation of two photosynthetic cell types [bundle sheath (BS) and mesophyll (M)] to form Kranz-type leaf anatomy. This cellular dimorphism partitions photosynthetic activities so that each enzyme of the C4 pathway accumulates only in the appropriate cell type. We have exploited this property to study BS and M cell interactions in developing maize leaves. Our previous studies showed that C4 proteins appear concurrently with the appearance of Kranz anatomy. To look at earlier events in BS and M cell development we have used three of the corresponding C4 mRNAs as cell-specific markers. We have followed, in situ, the accumulation of malic enzyme (ME), phosphoenolpyruvate carboxylase (PEPCase), and ribulose bisphosphate carboxylase (RuBPCase) mRNAs in developing leaves of both normal and mutant argentia (ar) maize. We have isolated a partial cDNA clone for maize ME to examine ME mRNA expression. We show that throughout the development of light-grown seedlings, all three mRNAs accumulate in a cell-specific fashion in both normal and ar leaves. The pattern of C4 mRNA accumulation longitudinally along the veins, laterally across the leaf, and locally around individual veins reveals the spatial and temporal sequence of BS and M cell development. BS cell-specific mRNAs accumulate around developing veins before Kranz anatomy is evident morphologically. Our analysis of the ar mutant, in which C4 mRNA appearance is delayed relative to the appearance of Kranz anatomy, demonstrates first that BS and M cells develop in clusters across the leaf blade and second that BS cells surrounding any individual vein are activated asynchronously. We discuss our results in relation to models and mechanisms of BS and M cell development.

Photoregulation process of sorghum leaf phosphoenolpyruvate carboxylase: study with monoclonal antibodies

Monoclonal antibodies were produced against the G isozyme subunit of PEP carboxylase (PEPC) from Sorghum leaves by the hybridoma technique. More than 400 antibodies-producing hybridomas to PEPC were produced from the fusion of spleen cells from immunized mice with NS1 myeloma cells. By using an ELISA, three hybridomas (91-G, 83-G, 49-EG) were selected. Monoclonal antibodies were subsequently characterized in a Western experiment; Mabs 83-G and 91-G were found to be highly specific to the G isozyme whereas Mab 49-EG recognized both forms (E and G isozymes) of the enzyme. Addition of Mabs to the enzyme preparation did not modify its catalytic activity nor its activation by glycine. Use of these probes provided direct and definite evidence of the specific enhancing effect of light on the G form and on its corresponding mRNA.