Molecular cloning and sequencing of cDNA encoding the precursor to the small subunit of chloroplast ribulose-1,5-bisphosphate carboxylase

Genome-wide analysis and identification of light-harvesting chlorophyll a/b binding (LHC) gene family and BSMV-VIGS silencing TaLHC86 reduced salt tolerance in wheat

The discovery and identification of gene families by using wide-genome and public databases is an effective way to gain initial insight into gene function, which also is one of the current hot spots of research. Chlorophyll ab-binding proteins (LHC) are important for photosynthesis and widely involved in plant adversity stress. However, the study in wheat has not been reported. In this study, we identified 127 TaLHC members from common wheat which were unevenly distributed on all chromosomes except 3B and 3D. All members divided into three subfamilies, LHC a, LHC b and the LHC t which was only discovered in wheat. All of them had maximum expression in leaves and contained multiple light-responsive cis-acting element, which were evidence of the extensive involvement of LHC families in photosynthesis. In addition, we also analyzed their collinear relationship, targeting relationship with miRNA and their responses under different stresses. Based on these analyses, it was found that TaLHC86 was an excellent candidate gene for stress resistance. The full-length ORF of TaLHC86 was 792 bp and was localized on the chloroplasts. The salt tolerance of wheat was reduced when BSMV-VIGS silenced TaLHC86, and the photosynthetic rate and electron transport were also seriously affected. This study made a comprehensive analysis of the TaLHC family and found that TaLHC86 was a good gene for salt tolerance.

Pivotal Role of Phytohormones and Their Responsive Genes in Plant Growth and Their Signaling and Transduction Pathway under Salt Stress in Cotton

The presence of phyto-hormones in plants at relatively low concentrations plays an indispensable role in regulating crop growth and yield. Salt stress is one of the major abiotic stresses limiting cotton production. It has been reported that exogenous phyto-hormones are involved in various plant defense systems against salt stress. Recently, different studies revealed the pivotal performance of hormones in regulating cotton growth and yield. However, a comprehensive understanding of these exogenous hormones, which regulate cotton growth and yield under salt stress, is lacking. In this review, we focused on new advances in elucidating the roles of exogenous hormones (gibberellin (GA) and salicylic acid (SA)) and their signaling and transduction pathways and the cross-talk between GA and SA in regulating crop growth and development under salt stress. In this review, we not only focused on the role of phyto-hormones but also identified the roles of GA and SA responsive genes to salt stress. Our aim is to provide a comprehensive review of the performance of GA and SA and their responsive genes under salt stress, assisting in the further elucidation of the mechanism that plant hormones use to regulate growth and yield under salt stress.

Genome-wide identification of the light-harvesting chlorophyll a/b binding (Lhc) family in Gossypium hirsutum reveals the influence of GhLhcb2.3 on chlorophyll a synthesis

Light-harvesting chlorophyll a/b binding (Lhc) family proteins play a significant role in photosynthetic processes. Our objective was systematic identification and analysis of the Lhc family in cotton, as well as the relationship between Lhc family genes and chlorophyll synthesis during photosynthetic processes. We used genome-wide identification, phylogenetic analysis, chromosomal distribution and collinearity to examine potential functions of Lhc superfamily genes in upland cotton. Subcellular localization, qRT-PCR, a yeast two hybrid (Y2H) , and Virus-induced gene silencing (VIGS) experiment were used to explore function of GhLhcb2.3. Focusing on GhLhc family, gene structural analysis of G. hirsutum Lhc family genes (GhLhc) indicated the conservation of selected Lhc family members. The expression pattern of GhLhc proteins shows that Lhc family proteins are important for photosynthetic processes in leaves. Results of subcellular localization and qRT-PCR in different cotton varieties showed that GhLhcb2.3 is closely related to chloroplast chlorophyll. Y2H found extensive heteromeric interactions between the GhLhcb2.3 and GhLhcb1.4. Subcellular localization revealed that GhLhcb1.4 is located in chloroplasts. VIGS showed that GhLhcb2.3 influenced chlorophyll a synthesis. We comprehensively identified Lhc family genes in cotton, characterized these genes and reveal the influence of GhLhcb2.3 on chlorophyll a synthesis.

Light Harvesting in Fluctuating Environments: Evolution and Function of Antenna Proteins across Photosynthetic Lineage

Photosynthesis is the major natural process that can harvest and harness solar energy into chemical energy. Photosynthesis is performed by a vast number of organisms from single cellular bacteria to higher plants and to make the process efficient, all photosynthetic organisms possess a special type of pigment protein complex(es) that is (are) capable of trapping light energy, known as photosynthetic light-harvesting antennae. From an evolutionary point of view, simpler (unicellular) organisms typically have a simple antenna, whereas higher plants possess complex antenna systems. The higher complexity of the antenna systems provides efficient fine tuning of photosynthesis. This relationship between the complexity of the antenna and the increasing complexity of the organism is mainly related to the remarkable acclimation capability of complex organisms under fluctuating environmental conditions. These antenna complexes not only harvest light, but also provide photoprotection under fluctuating light conditions. In this review, the evolution, structure, and function of different antenna complexes, from single cellular organisms to higher plants, are discussed in the context of the ability to acclimate and adapt to cope under fluctuating environmental conditions.

Moving toward a comprehensive map of central plant metabolism

Decades of intensive study have led to the discovery of the main pathways involved in central metabolism but only some of the pathways and regulatory networks in which they are embedded. In this review, we discuss techniques used to assemble these pathways into a systems biology framework that can enable accurate modeling of the response of central metabolism to changes, including ways to perturb metabolic systems and assemble the resulting data into a meaningful network. Critically, these networks are of such size and complexity that it is possible to derive them only if data from different groups can be comprehensively and meaningfully combined. We conclude that it is essential to establish common standards for the description of experimental conditions and data collection and to store this information in databases to which the whole community can contribute.

Chloroplast gene suppression of defective ribulosebisphosphate carboxylase/oxygenase in Chlamydomonas reinhardii: evidence for stable heteroplasmic genes

The rcl-u-1-18-5B chloroplast mutation results in the absence of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) holoenzyme in the green alga Chlamydomonas reinhardii. The 18-5B mutant strain lacks photosynthesis and displays alight-sensitive, acetate-requiring phenotype. In the present investigations, revertants of 18-5B were recovered that regained photosynthetic competence. These revertants have decreased levels of Rubisco holoenzyme relative to wild type and display heteroplasmicity, segregating wild-type (revertant) and acetate-requiring phenotypes during vegetative growth or through meiosis. One of these revertants, R10-I, was studied further. The heteroplasmicity associated with photoautotrophically-grown R10-I was found to be stable through subcloning and heritable through several crosses. During growth in acetate medium in the dark, where photosynthesis provides no selective advantage, the wild-type phenotype was lost. Acetate-requiring segregants became homoplasmic but wild-type segregants did not. Organellar intergenic-suppression is discussed in light of the observed stable heteroplasmicity.

The control of chlorophyll levels in maturing kiwifruit

Chlorophyll is present in many plant organs, including immature fruit where it is usually degraded during ripening. Mature green kiwifruit (Actinidia deliciosa) are an exception, with high concentrations of chlorophyll remaining in the fruit flesh. In gold-fleshed kiwifruit (A. chinensis), chlorophyll is degraded to colourless catabolites upon fruit ripening, leaving yellow carotenoids visible. We have identified candidate genes for the control of chlorophyll degradation in kiwifruit and examined the transcript levels of these genes in maturing kiwifruit using quantitative real-time PCR. Results indicate that the biosynthesis and degradation, or turnover, of chlorophyll is transcriptionally regulated in green- and gold-fleshed kiwifruit. Both species of kiwifruit were found to have two homologues of the stay-green gene (SGR), a small protein that is postulated to aid in the dismantling of the light-harvesting complex, allowing free chlorophyll to enter the degradation pathway. However, with the exception of very mature green fruit, where degreening was observed, SGR2 was more highly expressed in gold fruit, indicating a potential regulatory step of chlorophyll degradation. When the SGR genes were over-expressed in tobacco leaves, degreening was observed. Our results show that chlorophyll degradation is differentially regulated in kiwifruit, and suggest that gold kiwifruit transcribe more degradation genes, leading to earlier and more sustained chlorophyll degradation in this fruit than in green kiwifruit.

Reconstructing evolution: gene transfer from plastids to the nucleus

During evolution, the genomes of eukaryotic cells have undergone major restructuring to meet the new regulatory challenges associated with compartmentalization of the genetic material in the nucleus and the organelles acquired by endosymbiosis (mitochondria and plastids). Restructuring involved the loss of dispensable or redundant genes and the massive translocation of genes from the ancestral organelles to the nucleus. Genomics and bioinformatic data suggest that the process of DNA transfer from organelles to the nucleus still continues, providing raw material for evolutionary tinkering in the nuclear genome. Recent reconstruction of these events in the laboratory has provided a unique tool to observe genome evolution in real time and to study the molecular mechanisms by which plastid genes are converted into functional nuclear genes. Here, we summarize current knowledge about plastid-to-nuclear gene transfer in the context of genome evolution and discuss new insights gained from experiments that recapitulate endosymbiotic gene transfer in the laboratory.

Discoveries in Rubisco (Ribulose 1,5-bisphosphate carboxylase/oxygenase): a historical perspective

Historic discoveries and key observations related to Rubisco (Ribulose 1,5-bisphosphate carboxylase/oxygenase), from 1947 to 2006, are presented. Currently, around 200 papers describing Rubisco research are published each year and the literature contains more than 5000 manuscripts on the subject. While trying to ensure that all the major events over this period are recorded, this analysis will inevitably be incomplete and will reflect the areas of particular interest to the authors.

Postimport methylation of the small subunit of ribulose-1,5-bisphosphate carboxylase in chloroplasts

Electron impact mass spectronomy analysis of the amino-terminal amino acid of the small subunit (SSU) of ribulose-1,5-bisphosphate carboxylase (Rubisco) showed that the amino-terminal methionine residue is post-translationally modified to N-methyl-methionine. Modification of the amino-terminal methionine residue was found in mature SSU proteins from the dicotyledonous plants pea and spinach as well as the monocotyledonous plants barley and corn. SSU methyltransferase is a soluble protein in the chloroplast stroma and accepts heterologously expressed non-methylated SSU as a substrate using S-adenosylmethionine as methyl-group donor. We show that this modification occurs after post-translational uptake of the precursor form of SSU into chloroplasts and processing to its mature size. This reaction represents a new step in the import and assembly pathway of Rubisco holoenzyme.

Phytochrome: molecular properties and biogenesis

Native Avena phytochrome, recently shown to have a monomeric molecular mass of 124 kDa, has molecular properties that differ significantly from those of the extensively characterized ‘ 120’ kDa or ‘large’ phytochrome preparations now known to contain a mixture of proteolytically degraded 118 and 114 kDa polypeptides. For example, 124 kDa phytochrome has a blocked N-terminus, a P fr λ max of 730 nm, a higher photostationary state in red light (86% P fr ), exhibits no dark reversion and shows no differential reactivity of P r and P fr toward a chemical probe of hydrophobic domains. The data indicate that the proteolytically removed 6-10 kDa polypeptide segment (s) is critical to the spectral and structural integrity of the photoreceptor; that at least part of the cleaved domain is located at the N-terminus of the molecule; that this domain influences the chemical reactivity of the chromophore with the external medium; and that a current hypothesis that P r —P fr photoconversion results in the exposure of a hydrophobic domain on the molecule is inconsistent with the properties of native phytochrome. Phytochrome has been found to exert rapid negative feedback control over the level of its own translatable mRNA. P fr formation in etiolated tissue causes a decline in translatable phytochrome mRNA that is detectable within 15—30 min and that results in more than a 95 % reduction within 2 h. Less than 1 % P fr is sufficient to induce 60 % of the maximum response, which is saturated at 20 % P fr or less. The rapidity of this autoregulatory control makes phytochrome itself an attractive system for investigating phytochrome-regulated gene expression. A project to clone phytochrome complementary DNA (cDNA) has been initiated. A major obstacle in this work has been the unexpectedly low abundance of phytochrome mRNA, less than 0.005 % of the poly (A) RNA in etiolated tissue. cDNA made from poly (A) RNA enriched ca. 200-fold in phytochrome mRNA has been cloned and bacterial colonies have been screened with a synthetic oligodeoxynucleotide hybridization probe. The sequence of this probe was derived from a known partial amino acid sequence of the phytochrome protein. Difficulties encountered with this approach are discussed.

Molecular characterization of transketolase (EC 2.2.1.1) active in the Calvin cycle of spinach chloroplasts

A cDNA encoding the Calvin cycle enzyme transketolase (TKL; EC 2.2.1.1) was isolated from Sorghum bicolor via subtractive differential hybridization, and used to isolate several full-length cDNA clones for this enzyme from spinach. Functional identity of the encoded mature subunit was shown by an 8.6-fold increase of TKL activity upon induction of Escherichia coli cells that overexpress the spinach TKL subunit under the control of the bacteriophage T7 promoter. Chloroplast localization of the cloned enzyme is shown by processing of the in vitro synthesized precursor upon uptake by isolated chloroplasts. Southern blot-analysis suggests that TKL is encoded by a single gene in the spinach genome. TKL proteins of both higher-plant chloroplasts and the cytosol of non-photosynthetic eukaryotes are found to be unexpectedly similar to eubacterial homologues, suggesting a possible eubacterial origin of these nuclear genes. Chloroplast TKL is the last of the demonstrably chloroplast-localized Calvin cycle enzymes to have been cloned and thus completes the isolation of gene probes for all enzymes of the pathway in higher plants.

A 64-kDa protein is a substrate for phosphorylation by a distinct thylakoid protein kinase

Solubilization of spinach thylakoids with the nonionic detergent n-octyl-β-D-glucopyranoside (OG) releases active protein kinase from the membrane. Further purification was reported to demonstrate that a 64-kDa protein is the origin of this kinase activity (Coughlan S J and Hind G (1986) J Biol Chem 261: 11378-11385). The N-terminal sequence of this protein was subsequently determined (Gal A, Herrmann R, Lottspiech F and Ohad I (1992) FEBS Lett 298: 33-35). Liquid phase isoelectric focusing of the OG extract and an hydroxylapatite-purified fraction, derived from the OG preparation, reveals that the 64-kDa protein with this documented N-terminal sequence can be separated from the protein kinase activity. Experimental conditions were optimised by manipulation of ampholyte and detergent concentrations to maximise protein solubility and enzyme activity. The kinase-containing fraction was able to catalyze the phosphorylation of several proteins including the 64-kDa which was identified using antibodies raised against a synthetic peptide corresponding to the N-terminal sequence. The results described indicate that this 64-kDa protein is not the protein kinase responsible for the phosphorylation of the light-harvesting complex associated with Photosystem II.

Uniform designation for genes of the Calvin-Benson-Bassham reductive pentose phosphate pathway of bacteria

Structural and regulatory genes encoding enzymes and proteins of the reductive pentose phosphate pathway have been isolated from a number of bacteria recently. In the phototroph Rhodobacter sphaeroides, and in two chemoautotrophic bacteria, Alcaligenes eutrophus and Xanthobacter flavus, these genes have been found in distinct operons. However, in these three organisms and in other bacteria where certain of these genes have been discovered, a uniform nomenclature to designate these genes has been lacking. This report represents an effort to provide uniformity to the designation of these genes from all bacteria.

Chlorophyll a/b-binding protein genes are differentially expressed during soybean development

The levels of chlorophyll a/b-binding protein (Cab) gene polysomal poly(A)+ mRNA were quantitated throughout the development of Glycine max L. Cab mRNAs were abundant in young expanding leaves, representing 6.1% of the leaf mRNA population. Lower Cab mRNA levels were present in embryos, stems, and cotyledons of developing seedlings; the lowest levels were found in roots where they accounted for 0.04% of the polysomal poly(A)+ mRNA of this organ. To determine the contribution of different members of the Cab gene family to the Cab mRNA populations, a quantitative S1 nuclease reconstruction assay was developed. Cab3, Cab4, and Cab5 mRNAs were detected in all stages examined during soybean development but their levels underwent differential changes. Cab3 encodes the most abundant Cab mRNA in young leaves, developing embryos, and in Stage VII cotyledons from the developing soybean seedling. The levels of Cab mRNAs were compared to the levels of ribulose-1,5-bisphosphate carboxylase small subunit gene mRNA and differences in their patterns of accumulation were noted. Collectively these data indicate that during soybean embryogenesis developmental control mechanisms supersede light-regulatory signals.

Analyses of Phaseolus vulgaris L. and P. coccineus Lam. hybrids by RFLP: preferential transmission of P. vulgaris alleles

Restriction fragment length polymorphism (RFLP) was determined among P. vulgaris genotypes and Phaseolus species using 19 probes. The incidence of polymorphism was high (70-86%) between species, but relatively low (22-26%) between genotypes of P. vulgaris. Suitable probes were identified for the analysis of P. vulgaris and P. coccineus hybrids. The segregation pattern in F2 populations was Mendelian for two probes (LHB and VEE20) and non-Mendelian for GS-g, CHS, and CHI. Statistical analyses indicated gametic selection with preferential transmission of the P. vulgaris alleles, which may account for the selective recovery of P. vulgaris progeny types observed earlier. The available hybrids of P. vulgaris and P. coccineus and the high degree of interspecific RFLP will facilitate the construction of a linkage map for Phaseolus.

Molecular biology of the C3 photosynthetic carbon reduction cycle

In recent years the enzymes of the C3 photosynthetic carbon reduction (PCR) cycle have been studied using the techniques of molecular biology. In this review we discuss the primary protein sequences and structural predictions that have been made for a number of these enzymes, which, with the input of crystallographic analysis, gives the opportunity to understand the mechanisms of enzyme activity.The genome organisation and gene structure of the PCR enzymes is another area which has recently expanded, and we discuss the regulation of the genes encoding these enzymes and the complex interaction of various factors which influence their expression.

Photoreceptors controlling transcription of rbcS genes in green leaf tissue of Pisum sativum

We have investigated the photoreceptors controlling transcription of genes encoding the small subunit (rbcS) of ribulose 1,5-bisphosphate carboxylase-oxygenase in green leaf tissue of pea (Pisum sativum). RbcS transcription was measured by hybridising labelled transcripts of isolated nuclei to rbcS cDNA clones. Transfer of green Pisum leaf tissue to darkness for 5 h causes a substantial decrease in the rate of rbcS transcription and the rate is restored rapidly when the plants are returned to white light. Low fluence rates of red light are ineffective in restoring the rate of rbcS transcription, suggesting that phytochrome alone does not fully mediate the response. Blue light is similarly effective to white light of an equal fluence rate (120 mumol m-2 s-1) in restoring the rate of rbcS transcription in the dark-treated plants, indicating that a blue light photoreceptor is involved. However, red light at the same fluence rate produces about 65% of the effect of blue or white light, showing that the blue light photoreceptor is not the only photoreceptor controlling rbcS transcription in the green leaf tissue. The identity of the photoreceptor responsible for the red light effect is discussed. Similar effects of blue and red light are observed at the level of transcript abundance in dark-grown pea leaf tissue given a brief illumination with red light, which potentiates the tissue for rapid transcript accumulation in white light.

Minimal sequence requirements for the regulated expression of rbcS-3A from Pisum sativum in transgenic tobacco plants

RbcS-3A, the most highly expressed member of the pea multigene family encoding the small subunit of ribulose 1,5-bisphosphate carboxylase, is expressed in a light-dependent and organ-specific manner. In order to further delineate the sequences which mediate this complex pattern of regulation, putative regulatory sequences were assayed for function in transgenic tobacco plants in the context of an inactive 5' deleted rbcS-3A test gene. We have identified a minimal functional unit of 58 bp which is able to confer organ-specific transcriptional activity. It contains two sequences conserved among the pea rbcS family members, namely box II (-151 to -138; GTGTGGTTAATATG) and box III (-125 to -114; ATCATTTTCACT). These sequences bind the nuclear factor termed GT-1 in vitro. Substitution mutations within this 58 bp element have demonstrated that sequences upstream of, or located between, boxes II and III are not required for the transcriptional activity conferred by this element. Distance and orientation of these sequences from the gene are not critical for activity within the limits tested. DNA fragments upstream of nucleotide -170 of rbcS-3A that contain other GT-1 binding sites can also confer regulated expression upon the rbcS-3A promoter deleted to -50. Multimers of individual motifs, namely four tandem copies of boxes II and III, are unable to drive expression of the deleted promoter. These observations suggest that while GT-1 binding is necessary for promoter activity it is by itself not sufficient.

Structure of the Rubisco operon from the unicellular red alga Cyanidium caldarium: evidence for a polyphyletic origin of the plastids

The genes for both subunits of ribulose-1,5-bisphosphate-carboxylase/oxygenase (Rubisco) were located on the plastid DNA (ptDNA) of the unicellular red alga Cyanidium caldarium. Both genes are organized together in an operon. The sequence homology of both genes to the corresponding genes from the unicellular red alga Porphyridium aerugineum is remarkably high, whereas homology to Rubisco genes from chloroplasts and two recent cyanobacteria is significantly lower. These data provide strong evidence for a polyphyletic origin of chloroplasts and rhodoplasts. In addition the genes for the small subunit of Rubisco (rbcS) from red algae show about 60% homology to rbcS genes from cryptophytes and chromophytes. Thus, homologies in the rbcS gene indicate a close phylogenetic relationship between rhodoplasts and the plastids of Chromophyta.

Differential expression of two types of sucrose synthase-encoding genes in wheat in response to anaerobiosis, cold shock and light

The expression of two types of sucrose synthase-encoding genes, Ss1 and Ss2, in hexaploid wheat (Triticum aestivum, L.), has been investigated using type-specific probes, corresponding to the 250-270 bp C-terminal portions of the respective cDNA clones. Both types of genes are highly expressed in developing endosperm, where the expression of the Ss2 type slightly precedes in time that of the Ss1 type. Expression of Ss genes is lower in etiolated leaves and in roots than in endosperm. In the first two tissues, the Ss1 mRNA is much more abundant than the Ss2 mRNA, and the Ss1 mRNA level sharply increases in response to anaerobiosis and to cold shock (6 degrees C), while the level of Ss2 mRNA is not significantly affected. Upon illumination of etiolated leaves, the Ss1 level mRNA decreases significantly and the Ss2 mRNA level increases.

The genes of both subunits of ribulose-1,5-bisphosphate carboxylase constitute an operon on the plastome of a red alga

Plastid (pt) DNA from the red alga Porphyridium aerugineum was purified by CsCl gradient centrifugation. An EcoRI library of the ptDNA was screened with a gene probe specific for the gene encoding the large subunit (LSU) of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco EC 4.1.1.39) from spinach. A 5.8 kb EcoRI clone containing the LSU gene (rbcL) was isolated and the DNA sequence of the Porphyridium rbcL gene and its flanking regions was determined. An open reading frame was found 130 bp downstream from the rbcL gene that shows homology to genes coding for the small subunit of Rubisco (rbcS) from higher plants and cyanobacteria. Both genes (rbcL + rbcS) are cotranscribed. Comparison of rbcL and rbcS sequences from Porphyridium, higher plants and cyanobacteria seems to reveal a remarkable evolutionary distance between the plastids of the red algae (rhodoplasts), chloroplasts and cyanobacteria.

Cloning and sequencing of a cDNA encoding the small subunit precursor of ribulose-1,5-bisphosphate carboxylase from Chlamydomonas moewusii. Evolution of RUBISCO SS polypeptide

We have isolated and characterized a full-length cDNA clone encoding the precursor of the small subunit (pSU) of ribulose-1,5-bisphosphate carboxylase/oxygenase (RUBISCO) from the green alga, Chlamydomonas moewusii. Comparison with the C. reinhardtii rbcS1 gene sequence reveals that both small subunit (SS) coding regions are 75% homologous and that their predicted mature polypeptide chains are each composed of 140 amino acids. In contrast, their transit peptides appear to be divergent. We also show that transcription of the C. moewusii rbcS gene(s) which generates a 1,230 and a 930 base mRNA species are light-stimulated/or accumulated during the light period of the cell cycle. Finally, the SS polypeptide sequences of fifteen different photosynthetic organisms are compared; this analysis reveals at least five well-conserved polypeptide domains.

Regulation of gene expression of ribulose bisphosphate carboxylase in greening pea leaves

Ribulose bisphosphate carboxylase/oxygenase (RuBisCO) is composed of two subunits, the chloroplastcoded large subunit (LS) and the nuclear-coded small subunit (SS). The effects of different light doses on the levels of the large subunit gene (rbcL), the rbcL and the rbcS mRNAs, and the rate of synthesis of RuBisCO were followed during greening. The rbcL gene dosage changed in response to light whereas the level of the rbcL mRNA changed independently of the gene dosage. This suggests that the expression of the rbcL gene is transcriptionally regulated and that the change in gene dosage only partially contributes to the increase in the mRNA. It appeared that the RuBisCO synthesis rate was proportional to the rbcS mRNA level rather than rbcL mRNA level. These results, taken together with the earlier observations of many researchers, suggest that RuBisCO synthesis in greening pea leaves is controlled primarily at the level of transcription of both genes and is fine-tuned at the post-transcriptional level in chloroplasts, so that the amount of LS is almost stoichiometric to that of SS.

Changes in legumin messenger RNAs throughout seed development in Pisum sativum L

The patterns of accumulation of three classes of legumin mRNA from Pisum sativum have been followed through seed development by cell-free translation and hybridization to complementary DNAs. Maximum amounts of mRNA were found at 19 days after flowering (DAF) for two classes and at 24 DAF for the third class. The proportions of the three classes varied through development: the RNA species which was 40% of the total legumin mRNA at 14 DAF represented 15-20% of the total at 25-27 DAF, whereas a second mRNA species represented approx. 30% and 70% at the same stages, respectively. Based on these results, some deductions about the possible contribution of individual genes within classes are made.

Nuclear DNA regulates the level of ribulose 1,5-bisphosphate carboxylase oxygenase in Medicago sativa L

The response to selection for leaf proteins was studied during three selection cycles. Selection for high total nitrogen content showed 75% heritability, and the levels of both ribulose 1,5-bisphosphate carboxylase oxygenase (Rubisco) and cytoplasmic protein were strongly under nuclear DNA control. High and low protein content were correlated with chloroplast area. Although the amounts of nuclear DNA were similar, the ratio of Rubisco/DNA and chlorophyll/DNA changed during the selection process. It can be concluded that the levels of Rubisco achieved in mature plants of M. sativa are under nuclear DNA control. The possible involvement of small subunit (SSU) genes in controlling these levels is discussed.

Site-directed mutagenesis of the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase from Anacystis nidulans

Using oligonucleotide-directed mutagenesis of the gene encoding the small subunit (rbcS) from Anacystis nidulans mutant enzymes have been generated with either Trp-54 of the small subunit replaced by a Phe residue, or with Trp-57 replaced by a Phe residue, whereas both Trp-54 and Trp-57 have been replaced by Phe residues in a double mutant. Trp-54 and Trp-57 are conserved in all amino acid sequences or the small subunit (S) that are known at present. The wild-type and mutant forms of Rubisco have all been purified to homogeneity. The wild-type enzyme, purified from Escherichia coli is indistinguishable from enzyme similarly purified from A. nidulans in subunit composition, subunit molecular mass and kinetic parameters (Vmax CO2 = 2.9 U/mg, Km CO2 = 155 microM). The single Trp mutants are indistinguishable from the wild-type enzyme by criteria (a) and (b). However, whereas, Km CO2 is also unchanged, Vmax CO2 is 2.5-fold smaller than the value for the wild-type enzyme for both mutants, demonstrating for the first time that single amino acid replacements in the non-catalytic small subunit influence the catalytic rate of the enzyme. The specificity factor tau, which measures the partitioning of the active site between the carboxylase and oxygenase reactions, was found to be invariant. Since tau is not affected by these mutations we conclude that S is an activating not a regulating subunit.

Recovery of a soybean urease genomic clone by sequential library screening with two synthetic oligodeoxynucleotides

We report the first isolation of a low-copy-number gene from a complex higher plant (soybean) genome by direct screening with synthetic oligodeoxynucleotide (oligo) probes. A synthetic, mixed, 21-nucleotide (nt) oligo (21-1) based on a seven amino acid (aa) sequence from soybean seed urease, was used to screen genomic libraries of soybean (Glycine max [L.] Merr.) in the lambda Charon 4 vector. Twenty homologous clones were recovered from a screen of 500,000 plaques. These were counterscreened with embryo-specific cDNA (15-2 cDNA) made by priming with a second, mixed 15-nt oligo (15-2), based on a Jack bean (Canavalia ensiformis) urease peptide [Takishima et al., J. Natl. Def. Med. Coll. 5 (1980) 19-23]. Five out of 20 clones were homologous to 15-2 cDNA and proved to be identical. Nucleotide sequence analysis of representative clone E15 confirmed that it contained urease sequences. Subclones of E15 homologous to the oligo probes contain a deduced amino acid sequence which matches 108 of 130 aa residues of an amino acid run in a recently published [Mamiya et al., Proc. Jap. Acad. 61B (1985) 359-398] complete protein sequence for Jack-bean seed urease. Using clone E15 as a probe of soybean embryonic mRNA revealed a homologous 3.8-kb species that is the size of the urease messenger. This species is absent from mRNA of embryos of a soybean seed urease-null mutant. However, both urease-positive and urease-null genomes contain the 11-kb DNA fragment bearing urease sequences.

Expression of two nuclear genes encoding chloroplast proteins during early development of cucumber seedlings

Cloned complementary DNA probes have been used to measure steady-state transcript levels for the small subunit of ribulose bisphosphate carboxylase-oxygenase (SSU) and the chlorophyll a/b-binding protein (LHCP) in cotyledons during early development of cucumber seedlings. Initial accumulation of trancripts to SSU occurs 2d after germination and is independent of light and developmentally programmed. Although transcripts accumulate in dark-grown tissues, their levels increase rapidly in light-grown cotyledons from day 4, coinciding with emergence above the soil, so that by day 6 levels are 2.4 times higher in light-grown compared with dark-grown cotyledons. In contrast, accumulation of transcripts to LHCP occurs only in light-grown cotyledons. Southern blot analysis of genomic DNA indicates that in cucumber there are one and two genes encoding SSU and LHCP, respectively, considerably fewer than in those other plant species that have been examined. Both LHCP genes are expressed in light-grown cotyledons.

Sequence, evolution and differential expression of the two genes encoding variant small subunits of ribulose bisphosphate carboxylase/oxygenase in Chlamydomonas reinhardtii

We have sequenced the two genes for the small subunit of ribulose bisphosphate carboxylase/oxygenase (Rubisco) in Chlamydomonas reinhardtii and analyzed their expression. The two genes encode variant small subunits that differ by four amino acid residues. Both genes are expressed and each is transcribed into an RNA of distinct size. The accumulation of the two RNAs changes depending on the growth conditions, so the small subunit composition of Rubisco may be expected to differ in response to the environment. The C. reinhardtii small subunit sequence is homologous to those of vascular plants or cyanobacteria, but is longer at the amino terminus and in internal positions. The number and location of the intervening sequences in the genes from C. reinhardtii and from other plants differ. In several cases, internal length differences in the polypeptide coincide with the positions of introns in the coding sequence. Thus, changes in the exon structure of the genes during evolution may have been accompanied by substantial changes in the encoded protein. The translation and splicing signals in C. reinhardtii are similar to those of other eukaryotes, but the transcription signals are less conserved and the highly biased codon usage is very unusual.

The role of the transit peptide in the routing of precursors toward different chloroplast compartments

The role of the transit peptide in the routing of imported proteins inside the chloroplast was investigated with chimeric proteins in which the transit peptides for the nuclear-encoded ferredoxin and plastocyanin precursors were exchanged. Import and localization experiments with a reconstituted chloroplast system show that the ferredoxin transit peptide directs mature plastocyanin away from its correct location, the thylakoid lumen, to the stroma. With the plastocyanin transit peptide-mature ferredoxin chimera, a processing intermediate is arrested on its way to the lumen. We propose a two domain hypothesis for the plastocyanin transit peptide: the first domain functions in the chloroplast import process, whereas the second is responsible for transport across the thylakoid membrane. Thus, the transit peptide not only targets proteins to the chloroplast, but also is a major determinant in their subsequent localization within the organelle.