In vivo regulatory phosphorylation of novel phosphoenolpyruvate carboxylase isoforms in endosperm of developing castor oil seeds

Our previous research characterized two phosphoenolpyruvate (PEP) carboxylase (PEPC) isoforms (PEPC1 and PEPC2) from developing castor oil seeds (COS). The association of a shared 107-kD subunit (p107) with an immunologically unrelated bacterial PEPC-type 64-kD polypeptide (p64) leads to marked physical and kinetic differences between the PEPC1 p107 homotetramer and PEPC2 p107/p64 heterooctamer. Here, we describe the production of antiphosphorylation site-specific antibodies to the conserved p107 N-terminal serine-6 phosphorylation site. Immunoblotting established that the serine-6 of p107 is phosphorylated in COS PEPC1 and PEPC2. This phosphorylation was reversed in vitro following incubation of clarified COS extracts or purified PEPC1 or PEPC2 with mammalian protein phosphatase type 2A and is not involved in a potential PEPC1 and PEPC2 interconversion. Similar to other plant PEPCs examined to date, p107 phosphorylation increased PEPC1 activity at pH 7.3 by decreasing its K(m)(PEP) and sensitivity to L-malate inhibition, while enhancing glucose-6-P activation. By contrast, p107 phosphorylation increased PEPC2's K(m)(PEP) and sensitivity to malate, glutamic acid, and aspartic acid inhibition. Phosphorylation of p107 was promoted during COS development (coincident with a >5-fold increase in the I(50) [malate] value for total PEPC activity in desalted extracts) but disappeared during COS desiccation. The p107 of stage VII COS became fully dephosphorylated in planta 48 h following excision of COS pods or following 72 h of dark treatment of intact plants. The in vivo phosphorylation status of p107 appears to be modulated by photosynthate recently translocated from source leaves into developing COS.

Metabolic Engineering of Corynebacterium glutamicum for Fuel Ethanol Production under Oxygen-Deprivation Conditions

The central metabolic pathway of Corynebacterium glutamicum was engineered to produce ethanol. A recombinant strain which expressed the Zymomonas mobilis genes coding for pyruvate decarboxylase (pdc) and alcohol dehydrogenase (adhB) was constructed. Both genes placed under the control of the C. glutamicum ldhA promoter were expressed at high levels in C. glutamicum, resulting, under oxygen-deprivation conditions, in a significant yield ofethanol from glucose in a process characterized by the absence of cellular growth. Addition of pyruvate in trace amounts to the reaction mixture induced a 2-fold increase in the ethanol production rate. A similar effect was observed when acetaldehyde was added. Disruption of the lactate dehydrogenase (ldhA) gene led to a 3-fold higher ethanol yield than wild type, with no lactate production. Moreover, inactivation of the phosphoenolpyruvate carboxylase (ppc) and ldhA genes revealed a significant amount of ethanol production and a dramatic decrease in succinate without any lactate production, when pyruvate was added. Since the reaction occurred in the absence of cell growth, the ethanol volumetric productivity increased in proportion to cell density of ethanologenic C. glutamicum in a process under oxygen-deprivation conditions. These observations corroborate the view that intracellular NADH concentrations in C. glutamicum are correlated to oxygen-deprived metabolic flows.

Phylogeny and character evolution of endemic Australian carabid beetles of the genus Pamborus based on mitochondrial and nuclear gene sequences

The phylogeny of carabid beetles in the genus Pamborus (Coleoptera: Carabidae), which is endemic to Australia, was studied using one nuclear (phosphoenolpyruvate carboxykinase) and two mitochondrial (16S ribosomal RNA and NADH dehydrogenase subunit 5) gene sequences, with a cladistic analysis of morphological data. Fourteen species that were morphologically distinguishable were used as ingroup taxa, and Maoripamborus fairburni from New Zealand was assigned as the outgroup. Simultaneous analysis of three gene sequences resulted in well-resolved trees that were largely consistent with the cladogram generated from the morphological data. Based on a clock-like tree calibrated to the New Zealand-Australia/Antarctica split 85 million years ago, it was estimated that extant Pamborus differentiated after the Oligocene, primarily since the mid-Miocene with the onset of a more arid climate and forest fragmentation in Australia. The ancestral Pamborus may have been small, whereas medium to large Pamborus species with exaggerated male genitalia constitute derived groups and are now dominant.

Identification and expression analysis of two inorganic C- and N-responsive genes encoding novel and distinct molecular forms of eukaryotic phosphoenolpyruvate carboxylase in the green microalga Chlamydomonas reinhardtii

Phosphoenolpyruvate carboxylase (PEPC [Ppc]) has been previously purified and characterized in biochemical and immunological terms from two green microalgae, Chlamydomonas reinhardtii and Selenastrum minutum. The findings indicate that these algae possess at least two distinct PEPC enzyme-forms, homotetrameric Class-1 and heteromeric Class-2, that differ significantly from each other and their plant and prokaryotic counterparts. Surprisingly, however, green-algal PEPC has been unexplored to date in molecular terms. This study reports the molecular cloning of the two Ppc genes in C. reinhardtii (CrPpc1, CrPpc2), each of which is transcribed in vivo and encodes a fully active, recombinant PEPC that lacks the regulatory, N-terminal seryl-phosphorylation domain that typifies the vascular-plant enzyme. These distinct catalytic subunit-types differ with respect to their (i) predicted molecular mass ( approximately 108.9 [CrPpc1] versus approximately 131.2 kDa [CrPpc2]) and critical C-terminal tetrapeptide; and (ii) immunoreactivity with antisera against the p102 and p130 polypeptides of S. minutum PEPC1/PEPC2 and PEPC2, respectively. Only the Ppc1 transcript encodes the p102 catalytic subunits common to both Class-1 and Class-2 enzyme-forms in C. reinhardtii. The steady-state transcript levels of both CrPpc1/2 are coordinately up-/down-regulated by changes in [CO2] or [NH] during growth, and generally mirror the response of cytoplasmic glutamine synthetase (Gs1) transcript abundance to changes in inorganic [N] at 5% CO2. These collective findings provide key molecular insight into the Ppc genes and corresponding PEPC catalytic subunits in the eukaryotic algae.

Increasing the acetyl-CoA pool in the presence of overexpressed phosphoenolpyruvate carboxylase or pyruvate carboxylase enhances succinate production in Escherichia coli

An in vivo strategy to apply the activation effect of acetyl-CoA on phosphoenolpyruvate carboxylase (PEPC) and pyruvate carboxylase (PYC) to increase succinate production in Escherichia coli was studied. This approach relies on the increased intracellular acetyl-CoA and CoA levels by overexpressing E. coli pantothenate kinase (PANK). The results showed that coexpression of PANK and PEPC, and PANK and PYC, did improve succinate production compared to the individual expression of PEPC and PYC, respectively. The intracellular acetyl-CoA and CoA levels were also measured, and each showed a significant increase when the PANK was overexpressed. Another effect observed was a decrease in lactate production. The least amount of lactate was produced when PANK and PEPC, and PANK and PYC, were coexpressed. This result showed increased competitiveness of the succinate pathway at the phosphoenolpyruvate and pyruvate nodes for the carbon flux, as a result reducing the carbon flux toward the lactate pathway. The study also demonstrates a feasible method for metabolic engineering to modulate enzyme activity in vivo through specific activators and inhibitors.

Differential expression pattern of C4 bundle sheath expression genes in rice, a C3 plant

NADP-malic enzyme (NADP-ME) and phosphoenolpyruvate carboxykinase (PCK) are specifically expressed in bundle sheath cells (BSCs) in NADP-ME-type and PCK-type C4 plants, respectively. Unlike the high activities of these enzymes in the green leaves of C4 plants, their low activities have been detected in the leaves of C3 plants. In order to elucidate the differences in the gene expression system between C3 and C4 plants, we have produced chimeric constructs with the beta-glucuronidase (GUS) reporter gene under the control of the maize NADP-Me (ZmMe) or Zoysia japonica Pck (ZjPck) promoter and introduced these constructs into rice. In leaves of transgenic rice, the ZmMe promoter directed GUS expression not only in mesophyll cells (MCs) but also in BSCs and vascular cells, whereas the ZjPck promoter directed GUS expression only in BSCs and vascular cells. Neither the ZjPck nor ZmMe promoters induced GUS expression due to light. In rice leaves, the endogenous NADP-Me (OsMe1) was expressed in MCs, BSCs and vascular cells, whereas the rice Pck (OsPck1) was expressed only in BSCs and vascular cells. Taken together, the results obtained from transgenic rice demonstrate that the expression pattern of ZmMe or ZjPck in transgenic rice was reflected by that of its counterpart gene in rice.

Endogenous hormones and expression of senescence-related genes in different senescent types of maize

Levels of cytokinins and abscisic acid (ABA) and the expression of senescence-related genes were investigated in two maize (Zea mays L.) cultivars of different senescence type, cv. P3845 (stay-green) and cv. Hokkou 55 (earlier senescent), in a field study. The delay in leaf senescence in P3845 was correlated with increased levels of chlorophyll and nitrogen and a higher photon-saturated photosynthetic rate (P(sat)). Compared with the earlier senescent Hokkou 55, P3845 showed enhanced contents of cytokinins (trans-zeatin riboside, t-ZR; dihydrozeatin riboside, DHZR; isopentenyladenosine, iPA) and reduced levels of ABA in its leaves. In roots, P3845 had increased levels of t-ZR, DHZR, and ABA, but decreased concentrations of iPA. It was concluded that a higher rate of cytokinin transport from roots to leaves contributes to the delay of senescence in P3845. By contrast, the translocation of ABA from roots to shoots may be blocked in the stay-green cultivar, which also results in retarded leaf senescence. P3845 ear leaves contained more malondialdehyde (MDA) and higher catalase (CAT) and superoxide dismutase (SOD) activities than Hokkou 55. Since the accumulation of the mRNAs for Rubisco small subunit (rbcS), phosphoenolpyruvate carboxylase (PEPC), and SOD peaked after Chl content and P(sat) had reached their maxima, it is speculated that when leaf senescence is initiated, Chl contents decrease first, followed by the degradation of the photosynthetic apparatus and of photosynthesis-related enzymes. See1 and See2 encode senescence-related cysteine proteases; their mRNAs were most abundant in yellowing leaves, suggesting that these proteins are involved in the process of senescence rather than its initiation. mRNAs of both genes were more abundant in Hokkou 55 than in P3845, which suggests a regulation of leaf senescence at the transcriptional level.

Phosphate deficiency regulates phosphoenolpyruvate carboxylase expression in proteoid root clusters of white lupin

Proteoid roots play a major role in enabling white lupin (Lupinus albus L.) to adapt to phosphate (Pi) deficiency. Such roots release citrate from proteoid rootlets, which allows this species to mobilize Pi from sparingly soluble Pi sources. Release of citrate is preceded by a significant accumulation of organic acids, in which a Pi deficiency-inducible phosphoenolpyruvate carboxylase (PEPC) activity has been involved. To gain an insight into this adaptive mechanism, the expression of three different transcripts coding for PEPC was examined in proteoid rootlets of Pi-starved and Pi-starved-and-rescued white lupin. Semi-quantitative reverse transcriptase (RT)-PCR experiments performed with gene-specific primers targeted to the 3'-end region of the corresponding cDNAs revealed that the transcripts for these three PEPCs differentially accumulate in both Pi-starved and Pi-starved-and-rescued proteoid rootlets. Semi-quantitative RT-PCR analysis in Pi-starved proteoid rootlets sampled at different times after being rescued from Pi deficiency showed that Pi levels differentially down-regulated the three PEPC transcripts. RT-PCR experiments were further extended to Pi-starved and Pi-fed whole roots, cotyledons, and leaves on which a tissue-specific, Pi-dependent PEPC expression was observed. These results indicate that there exists at least three different transcripts coding for PEPC in proteoid root clusters of white lupin, whose expression are differentially regulated by Pi.

Identification and functional verification of archaeal-type phosphoenolpyruvate carboxylase, a missing link in archaeal central carbohydrate metabolism

Despite the fact that phosphoenolpyruvate carboxylase (PEPC) activity has been measured and in some cases even purified from some Archaea, the gene responsible for this activity has not been elucidated. Using sensitive sequence comparison methods, we detected a highly conserved, uncharacterized archaeal gene family that is distantly related to the catalytic core of the canonical PEPC. To verify the predicted function of this archaeal gene family, we cloned a representative from the hyperthermophilic acidophile Sulfolobus solfataricus and functionally produced the corresponding enzyme as a fusion with the Escherichia coli maltose-binding protein. The purified fusion protein indeed displayed highly thermostable PEPC activity. The structural and biochemical properties of the characterized archaeal-type PEPC (atPEPC) from S. solfataricus are in good agreement with previously reported biochemical analyses of other archaeal PEPC enzymes. The newly identified atPEPC, with its distinct properties, constitutes yet another example of the versatility of the enzymes of the central carbon metabolic pathways in the archaeal domain.

Metabolic analysis of Corynebacterium glutamicum during lactate and succinate productions under oxygen deprivation conditions

Lactate and succinate were produced from glucose by Corynebacterium glutamicum under oxygen deprivation conditions without growth. Addition of bicarbonate to the reaction mixture led not only to a 3.6-fold increase in succinate production rate, but also to a 2.3- and 2.5-fold increase, respectively, of the rates of lactate production and glucose consumption, compared to the control. Furthermore, when small amounts of pyruvate were added to the reaction mixture, acid production rates and the glucose consumption rate were multiplied by a factor ranging from 2 to 3. These phenomena were paralleled by an increase in the NAD(+)/NADH ratio, thus corroborating the view that the efficient regeneration of NAD(+) could be triggered by the addition of either bicarbonate or pyruvate. To investigate the global metabolism of corynebacteria under oxygen deprivation conditions, we engineered several strains where the genes coding for key metabolic enzymes had been inactivated by gene disruption and replacement. A lactate dehydrogenase (LDH)-deficient mutant was not able to produce lactate, suggesting this enzyme has no other isozyme. Although a pyruvate carboxylase (pyc) mutant exhibited similar behavior to that of the wild type, phosphoenolpyruvate carboxylase (ppc) mutants were characterized by a dramatic decrease in succinate production, which was concomitant to decreased lactate production and glucose consumption rates. This set of observations corroborates the view that in coryneform bacteria under oxygen deprivation conditions the major anaplerotic reaction is driven by the ppc gene product rather than by the pyc gene product. Moreover, intracellular NADH concentrations in C. glutamicum were observed to correlate to oxygen-deprived metabolic flows.

Metabolic flux analysis for a ppc mutant Escherichia coli based on 13C-labelling experiments together with enzyme activity assays and intracellular metabolite measurements

The physiology and central metabolism of a ppc mutant Escherichia coli were investigated based on the metabolic flux distribution obtained by (13)C-labelling experiments using gas chromatography-mass spectrometry (GC-MS) and 2-dimensional nuclear magnetic resonance (2D NMR) strategies together with enzyme activity assays and intracellular metabolite concentration measurements. Compared to the wild type, its ppc mutant excreted little acetate and produced less carbon dioxide at the expense of a slower growth rate and a lower glucose uptake rate. Consequently, an improvement of the biomass yield on glucose was observed in the ppc mutant. Enzyme activity measurements revealed that isocitrate lyase activity increased by more than 3-fold in the ppc mutant. Some TCA cycle enzymes such as citrate synthase, aconitase and malate dehydrogenase were also upregulated, but enzymes of glycolysis and the pentose phosphate pathway were downregulated. The intracellular intermediates in the glycolysis and the pentose phosphate pathway, therefore, accumulated, while acetyl coenzyme A and oxaloacetate concentrations decreased in the ppc mutant. The intracellular metabolic flux analysis uncovered that deletion of ppc resulted in the appearance of the glyoxylate shunt, with 18.9% of the carbon flux being channeled via the glyoxylate shunt. However, the flux of the pentose phosphate pathway significantly decreased in the ppc mutant.

The phosphoenolpyruvate carboxylase from Methanothermobacter thermautotrophicus has a novel structure

In Methanothermobacter thermautotrophicus, oxaloacetate synthesis is a major and essential CO(2)-fixation reaction. This methanogenic archaeon possesses two oxaloacetate-synthesizing enzymes, pyruvate carboxylase and phosphoenolpyruvate carboxylase. The phosphoenolpyruvate carboxylase from this organism was purified to homogeneity. The subunit size of this homotetrameric protein was 55 kDa, which is about half that of all known bacterial and eukaryotic phosphoenolpyruvate carboxylases (PPCs). The NH(2)-terminal sequence identified this enzyme as the product of MTH943, an open reading frame with no assigned function in the genome sequence. A BLAST search did not show an obvious sequence similarity between MTH943 and known PPCs, which are generally well conserved. This is the first report of a new type of phosphoenolpyruvate carboxylase that we call PpcA ("A" for "archaeal"). Homologs to PpcA were present in most archaeal genomic sequences, but only in three bacterial (Clostridium perfringens, Oenococcus oeni, and Leuconostoc mesenteroides) and no eukaryotic genomes. PpcA was the only recognizable oxaloacetate-producing enzyme in Methanopyrus kandleri, a hydrothermal vent organism. Each PpcA-containing organism lacked a PPC homolog. The activity of M. thermautotrophicus PpcA was not influenced by acetyl coenzyme A and was about 50 times less sensitive to aspartate than the Escherichia coli PPC. The catalytic core (including His(138), Arg(587), and Gly(883)) of the E. coli PPC was partly conserved in PpcA, but three of four aspartate-binding residues (Lys(773), Arg(832), and Asn(881)) were not. PPCs probably evolved from PpcA through a process that added allosteric sites to the enzyme. The reverse is also equally possible.

Overexpression of a cyanobacterial phosphoenolpyruvate carboxylase with diminished sensitivity to feedback inhibition in Arabidopsis changes amino acid metabolism

Phosphoenolpyruvate carboxylase (EC 4.1.1.31) from Synechococcus vulcanus (SvPEPC) is a unique enzyme, being almost insensitive to feedback inhibition at neutral pH. In order to assess its usefulness in metabolic engineering of plants, SvPEPC was expressed in Arabidopsis thaliana (L.) Heynh. under the control of the cauliflower mosaic virus 35S promoter. About one-third of the transformants of the T1 generation showed severe visible phenotypes such as leaf bleaching and were infertile when grown on soil. However, no such phenotype was observed with Arabidopsis transformed with Zea mays L. PEPC (ZmPEPC) for C4 photosynthesis, which is normally sensitive to a feedback inhibitor, L-malate. For the SvPEPC transformants of the T2 generation, which had been derived from fertile T1 transformants, three kinds of phenotype were observed when plants were grown on an agar medium containing sucrose: Type-I plants showed poor growth and a block in true leaf development; Type-II plants produced a few true leaves, which were partially bleached; Type-III plants were apparently normal. In Type-I plants, total PEPC activity was increased about 2-fold over the control plant but there was no such increase in Type-III plants. The phenotypes of Type-I plants were rescued when the sucrose-containing agar medium was supplemented with aromatic amino acids. Measurement of the free amino acid content in whole seedlings of Type-I transformants revealed that the levels of the aromatic amino acids Phe and Tyr were lowered significantly as compared with the control plants. In contrast, the levels of several amino acids of the aspartic and glutamic families, such as Asn, Gln and Arg, were markedly enhanced (4- to 8-fold per plant fresh weight). However, when the medium was supplemented with aromatic amino acids, the levels of Asn, Gln, and Arg decreased to levels slightly higher than those of control plants, accompanied by growth recovery. Taken together, it can be envisaged that SvPEPC is capable of efficiently exerting its activity in the plant cell environment so as to cause imbalance between aromatic and non-aromatic amino acid syntheses. The growth inhibition of Type-I plants was presumed to be primarily due to a decreased availability of phosphoenolpyruvate, one of the precursors for the shikimate pathway for the synthesis of aromatic amino acids and phenylpropanoids. The possible usefulness of SvPEPC as one of the key components for installing the C4-like pathway is proposed.

Effect of overexpression of Actinobacillus succinogenes phosphoenolpyruvate carboxykinase on succinate production in Escherichia coli

Succinate fermentation was investigated in Escherichia coli strains overexpressing Actinobacillus succinogenes phosphoenolpyruvate carboxykinase (PEPCK). In E. coli K-12, PEPCK overexpression had no effect on succinate fermentation. In contrast, in the phosphoenolpyruvate carboxylase mutant E. coli strain K-12 ppc::kan, PEPCK overexpression increased succinate production 6.5-fold.

Metabolic engineering with Dof1 transcription factor in plants: Improved nitrogen assimilation and growth under low-nitrogen conditions

Utilization of transcription factors might be a powerful approach to modification of metabolism for a generation of crops having superior characteristics because a single transcription factor frequently regulates coordinated expression of a set of key genes for respective pathways. Here, we apply the plant-specific Dof1 transcription factor to improve nitrogen assimilation, the essential metabolism including the primary assimilation of ammonia to carbon skeletons to biosynthesize amino acids and other organic compounds involving nitrogen in plants. Expressing Dof1 induced the up-regulation of genes encoding enzymes for carbon skeleton production, a marked increase of amino acid contents, and a reduction of the glucose level in transgenic Arabidopsis. The results suggest cooperative modification of carbon and nitrogen metabolisms on the basis of their intimate link. Furthermore, elementary analysis revealed that the nitrogen content increased in the Dof1 transgenic plants (approximately 30%), indicating promotion of net nitrogen assimilation. Most significantly, the Dof1 transgenic plants exhibit improved growth under low-nitrogen conditions, an agronomically important trait. These results highlight the great utility of transcription factors in engineering metabolism in plants.

Expression of phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxylase kinase genes. Implications for genotypic capacity and phenotypic plasticity in the expression of crassulacean acid metabolism

In plants with crassulacean acid metabolism (CAM), dark CO2 uptake is mediated by phosphoenolpyruvate carboxylase (PEPC), an enzyme that can be regulated at transcriptional and posttranslational levels. Reversible phosphorylation of PEPC is catalyzed by a dedicated PEPC kinase, which in turn is regulated at the transcriptional level over the 24-h cycle in CAM plants. PEPC kinase controls the day/night regulation of PEPC during the CAM cycle, thus facilitating plasticity for optimizing CO2 uptake under different environmental conditions. To understand the importance of PEPC kinase in relation to its target PEPC in terms of CAM performance, the expression of the genes encoding the two enzymes was investigated in four species of Clusia that have photosynthetic patterns ranging from C3 photosynthesis to constitutive CAM. By linking changes in the expression of PEPC and PEPC kinase to day/night patterns of leaf gas exchange, organic acid, and soluble sugar contents under different environmental conditions, the genetic and metabolic limitations to CAM plasticity were assessed. The results indicate that PEPC expression is a major factor underpinning the genotypic capacity for CAM and that PEPC kinase expression does not appear to limit CAM. The day/night regulation of Ppck transcript abundance was found to be a consequence of CAM and the day/night cycling of associated metabolites, rather than the primary controlling factor for the temporal separation of carboxylation processes.

cis-Regulatory elements for mesophyll-specific gene expression in the C4 plant Flaveria trinervia, the promoter of the C4 phosphoenolpyruvate carboxylase gene

C(4) photosynthesis depends on the strict compartmentalization of CO(2) assimilatory enzymes. cis-regulatory mechanisms are described that ensure mesophyll-specific expression of the gene encoding the C(4) isoform of phosphoenolpyruvate carboxylase (ppcA1) of the C(4) dicot Flaveria trinervia. To elucidate and understand the anatomy of the C(4) ppcA1 promoter, detailed promoter/reporter gene studies were performed in the closely related C(4) species F. bidentis, revealing that the C(4) promoter contains two regions, a proximal segment up to -570 and a distal part from -1566 to -2141, which are necessary but also sufficient for high mesophyll-specific expression of the beta-glucuronidase reporter gene. The distal region behaves as an enhancer-like expression module that can direct mesophyll-specific expression when inserted into the ppcA1 promoter of the C(3) plant F. pringlei. Mesophyll expression determinants were restricted to a 41-bp segment, referred to as mesophyll expression module 1 (Mem1). Evolutionary and functional studies identified the tetranucleotide sequence CACT as a key component of Mem1.

[Inheritance and expression of the maize pepc gene in progenies of transgenic rice bred by crossing]

By cross breeding, the maize pepc gene in the pepc transgenic rice was successfully incorporated into the parents of two-, three-line hybrid rice, including sterile lines (Peiai64S, 2302S, 2304S, 2306S and Shuangjiu A) and restorer lines (5129, 02428 and Wanjing97) to breed the high-photosynthetic efficiency parents of hybrid rice and utilize heterosis between C4 and C4/C3 rice. Some lines of pepc transgenic rice (LPTR) have been developed. The study on the generations of LPTR suggests the following: (1) The segregation observed in F2 and BC1 progenies demonstrated that pepc transgene inherited as a single dominant gene in the progenies of LPTR. (2) The maize pepc gene is actively expressed at high level in LPTR, and changes of pepc gene expression in the progenies of LPTR may be related to position effect, difference of gene copy number and environmental factors. (3) Through the selection method of soaking seeds into hygromycin solution to germinate, tracing the pepc gene by PCR analysis, evaluating the performance of the rice plants in the field and examining PEPC activities, the segregation of the pepc transgene in LPTR was controlled effectively. Based on the above strategy three pepc transgene lines, H1596, H1597 and Y1470, have been selected. The result suggests that it is possible to breed practical, stable and high-expression pepc transgenic rice by conventional crossing.

Evolution of c4 phosphoenolpyruvate carboxylase. Genes and proteins: a case study with the genus Flaveria

C4 photosynthesis is characterized by a division of labour between two different photosynthetic cell types, mesophyll and bundle-sheath cells. Relying on phosphoenolpyruvate carboxylase (PEPC) as the primary carboxylase in the mesophyll cells a CO2 pump is established in C4 plants that concentrates CO2 at the site of ribulose 1,5-bisphosphate carboxylase/oxygenase in the bundle-sheath cells. The C4 photosynthetic pathway evolved polyphyletically implying that the genes encoding the C4 PEPC originated from non-photosynthetic PEPC progenitor genes that were already present in the C3 ancestral species. The dicot genus Flaveria (Asteraceae) is a unique system in which to investigate the molcular changes that had to occur in order to adapt a C3 ancestral PEPC gene to the special conditions of C4 photosynthesis. Flaveria contains not only C3 and C4 species but also a large number of C3-C4 intermediates which vary to the degree in which C4 photosynthetic traits are expressed. The C4 PEPC gene of Flaveria trinervia, which is encoded by the ppcA gene class, is highly expressed but only in mesophyll cells. The encoded PEPC protein possesses the typical kinetic and regulatory features of a C4-type PEPC. The orthologous ppcA gene of the C3 species Flaveria pringlei encodes a typical non-photosynthetic, C3-type PEPC and is weakly expressed with no apparent cell or organ specificity. PEPCs of the ppcA type have been detected also in C3-C4 intermediate Flaveria species. These orthologous PEPCs have been used to determine the molecular basis for C4 enzyme characteristics and to understand their evolution. Comparative and functional analyses of the ppcA promoters from F. trinervia and F. pringlei make it possible to identity the cis-regulatory sequences for mesophyll-specific gene expression and to search for the corresponding trans-regulatory factors.

Phosphoenolpyruvate carboxylase: a new era of structural biology

There have been remarkable advances in our knowledge of this important enzyme in the last decade. This review focuses on three recent topics: the three-dimensional structure of the protein, molecular mechanisms of catalytic and regulatory functions, and the molecular cloning and characterization of PEPC kinases, which are Ser/Thr kinases involved specifically in regulatory phosphorylation of vascular plant PEPC. Analysis by X-ray crystallography and site-directed mutagenesis for E. coli and maize PEPC identified the catalytic site and allosteric effector binding sites, and revealed the functional importance of mobile loops. We present the reaction mechanism of PEPC in which we assign the roles of individual amino acid residues. We discuss the unique molecular property of PEPC kinase and its possible regulation at the post-translational level.

Dexamethasone and colostrum feeding affect hepatic gluconeogenic enzymes differently in neonatal calves1,2,3

Plasma glucose concentrations in neonates are influenced by colostrum feeding and by glucocorticoids. We have tested whether a high-glucocorticoid status after birth, as well as colostrum feeding, influences glucose metabolism in association with changes of hepatic expression and activities of gluconeogenic enzymes phosphoenolpyruvate carboxykinase (PEPCK; EC 4.1.1.32) and pyruvate carboxylase (PC; EC 6.4.1.1) in neonatal calves. Calves (n = 14 per group) were fed either colostrum or a milk-based formula with nutrient and energy contents similar to colostrum. Half the calves in each feeding group were treated with dexamethasone (DEXA; 30 microg/[kg BW x d]). Pre- and postprandial blood samples were taken on d 1, 2, 4, and 5 and liver samples were collected on d 5 of life. Dexamethasone treatment increased (P < or = 0.05) plasma concentrations of glucose, insulin, and glucagon more in colostrum-fed than in formula-fed calves but increased (P < or = 0.05) urea concentrations and decreased (P < or = 0.05) concentrations of NEFA, ACTH, and cortisol independent of colostrum vs. formula feeding. Colostrum feeding increased (P < 0.05) plasma glucose, but decreased (P < 0.05) plasma urea concentrations. Glucagon-to-insulin ratios in DEXA-treated and colostrum-fed calves were decreased (P < 0.05). Dexamethasone treatment decreased hepatic mRNA levels and activities of PC (P < 0.001 and P < 0.10) and activities of PEPCK (P < 0.001) but increased (P < 0.001) the glycogen content. Colostrum feeding increased (P < 0.05) mitochondrial PEPCK mRNA levels and PEPCK activities in calves not treated with DEXA but decreased (P < 0.1) amounts of PC mRNA. In conclusion, increased plasma glucose concentrations after DEXA treatment were not associated with a stimulation of hepatic gluconeogenic enzyme activities; however, colostrum feeding probably raised plasma glucose concentrations because of increased hepatic gluconeogenic activities.

Inorganic carbon limitation induces transcripts encoding components of the CO(2)-concentrating mechanism in Synechococcus sp. PCC7942 through a redox-independent pathway

The cyanobacterial CO2-concentrating mechanism (CCM) allows photosynthesis to proceed in CO2-limited aquatic environments, and its activity is modulated in response to inorganic carbon (Ci) availability. Real-time reverse transcriptase-PCR analysis was used to examine the transcriptional regulation of more than 30 CCM-related genes in Synechococcus sp. strain PCC7942 with an emphasis on genes encoding high-affinity Ci transporters and carboxysome-associated proteins. This approach was also used to test hypotheses about sensing of Ci limitation in cyanobacteria. The transcriptional response of Synechococcus sp. to severe Ci limitation occurs rapidly, being maximal within 30 to 60 min, and three distinct temporal responses were detected: (a). a rapid, transient induction for genes encoding carboxysome-associated proteins (ccmKLMNO, rbcLS, and icfA) and the transcriptional regulator, cmpR; (b). a slow sustained induction of psbAII; and (c). a rapid sustained induction of genes encoding the inducible Ci transporters cmpABCD, sbtA, and ndhF3-D3-chpY. The Ci-responsive transcripts investigated had half-lives of 15 min or less and were equally stable at high and low Ci. Through the use of a range of physiological conditions (light and Ci levels) and inhibitors such as 3-(3,4-dichlorophenyl)-1,1dimethylurea, glycolaldehyde, dithiothreitol, and ethoxyzolamide, we found that no strict correlation exists between expression of genes known to be induced under redox stress, such as psbAII, and the expression of the Ci-responsive CCM genes. We argue that redox stress, such as that which occurs under high-light stress, is unlikely to be a primary signal for sensing of Ci limitation in cyanobacteria. We discuss the data in relation to current theories of CO2 sensing in cyanobacteria.

Comparison of Al-induced gene expression in sensitive and tolerant soybean cultivars

In order to identify genes involved in soybean resistance to aluminium (Al) stress differential gene expression patterns of Al-stressed and non-stressed tolerant and sensitive soybean cultivars were compared. Out of eight described genes, potentially related to mechanisms of aluminium stress, only phosphoenolpyruvate carboxylase (PEPC) revealed enhanced expression in roots of tolerant as compared to sensitive soybean cultivars under stress conditions. Additionally, two novel full-length cDNA sequences, homologous to translationally controlled tumour proteins (TCTP, clone 58, GenBank accession number AF421558) and inosine-5'-monophosphate dehydrogenases (IMPDH, clone 633, GenBank accession number AF421559) with enhanced expression of the corresponding genes only in roots of Al-tolerant soybean cultivar under stress conditions were isolated and characterized. For functional analysis full-length cDNA 633 was transferred in Arabidopsis thaliana. Only 6% of the seedlings from the wild type survived Al stress, whereas 86% of transgenics were vital demonstrating superiority in stress protection. Compared with the wild type, transgenic plants showed diminished Al penetration into the roots after the stress treatment especially in the division and elongation zones of the roots. Formation of numerous lateral roots in transgenic plants with low elicited callose accumulation under stress conditions indicated ability of the IMPDH homologue to mediate aluminium tolerance in transgenic plants. Possible functional activities of Al up-regulated genes in resistance mechanisms are discussed.

Molecular evolution of C4 phosphoenolpyruvate carboxylase in the genus Flaveria--a gradual increase from C3 to C4 characteristics

In order to elucidate the discrete steps in phospho enolpyruvate carboxylase (PEPC) evolution concerning K(m)-PEP and malate tolerance a comparison was made between C3, C3-C4 and C4 species of the dicot genus Flaveria. The PEPCs of this genus are encoded by a gene family comprising three classes: ppcA, ppcB and ppcC [J. Hermans and P. Westhoff (1990) Mol Gen Genet 224:459-468, (1992) Mol Gen Genet 234:275-284]. The ppcA of F trinervia (C4) codes for the C4 PEPC isoform but other plants of the genus contain ppcA orthologues too. The C3 plant F. pringlei showed the lowest levels of ppcA PEPC mRNA followed by F. pubescens (C3-C4) while the C4-like plant F. brownii displayed RNA amounts close to the C4 species F. trinervia. In contrast to the similar expression profiles of F. brownii (C4-like) and F. trinervia (C4) the PEPC amino acid sequence of F. brownii was more similar to the C3 and C3-C4 ppcA PEPCs than to the C4 PEPC. Similarly, the C3, C3-C4 and C4-like ppcA PEPCs showed almost identical PEP saturation kinetics when activated by glucose-6-phosphate ( K(m)-PEP: 17-20 microM) while the K(m)-PEP for the C4 PEPC was determined to be 53 microM. However, without activation the ppcA PEPCs of F. pubescens and F. brownii displayed C3-C4 intermediate values. A similar picture was obtained when the malate sensitivities were compared. In the non-activated state the F. trinervia (C4) enzyme was 10 times more tolerant to malate than the F. pringlei counterpart. The ppcA enzymes of F. pubescens (C3-C4) and F. brownii (C4-like) displayed intermediate values. In contrast, the inclusion of 5 mM glucose-6-phosphate in the reaction mixture changed the order totally. Interestingly, the activation rendered the C4 enzyme about 50% less tolerant to malate than the C3 PEPC. The activation had a positive effect on malate tolerance of the F. pubescens (C3-C4) PEPC while the ppcA PEPC of F. brownii (C4-like) was almost unaffected.