Identification of the small subunit of ribulose 1,5-bisphosphate carboxylase as a product of wheat leaf cytoplasmic ribosomes

Post-translational transport into intact chloroplasts of a precursor to the small subunit of ribulose-1,5-bisphosphate carboxylase

A precursor to the small subunit of ribulose-1,5-bisphosphate carboxylase [3-phospho-D-glycerate carboxylyase (dimerizing), EC 4.1.1.39] has been identified among the products of cell-free translation of polyadenylated RNA from spinach and pea. In both cases, the precursor is larger than the mature protein by 4000-5000 daltons. Upon incubation of post-ribosomal supernatants of the in vitro protein synthesis mixtures with purified intact chloroplasts, the pea and spinach precursors are transported interchangeably into the chloroplasts and processed to the mature size and charge. Moreover, the newly transported small subunits are found to assemble with endogenous large subunits to form the holoenzyme. In contrast, a precursor to the Chlamydomonas reinhardtii small subunit is not taken up by higher plant chloroplasts, indicating the specificity of the transport events. Together, these results demonstrate that the in vitro reconstruction of the post-translational transport of the higher plant precursors is physiologically significant.

Light regulation of specific mRNA species in Lemna gibba L. G-3

Polyadenylated RNA was isolated from Lemna gibba L. G-3 and translated in a cell-free system from wheat germ. When plants were placed into complete darkness for 4 days, then returned to light for 18 hr, increased amounts of polyadenylated mRNA for at least two polypeptides were detected by in vitro translation over those amounts present in the dark. These two polypeptides have molecular weights of 32,000 and 20,000. The 20,000 dalton polypeptide was identified by immunoprecipitation as the precursor to the small subunit of the photosynthetic enzyme ribulose-1,5-bisphosphate carboxylase [3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39]. The polyadenylated mRNA that codes for the small subunit is not detectable by immunoprecipitation of translation products in the dark-treated tissue. Plants growing in the light actively synthesize both subunits of ribulose-1,5-bisphosphate carboxylase, but synthesis of these proteins was found to be greatly diminished in plants placed in darkness for 4 days. These results indicate that white light can dramatically affect the steady-state levels of specific polyadenylated mRNAs.

Cell-free synthesis of leaf protein: Identification of an apparent precursor of the small subunit of ribulose-1,5-bisphosphate carboxylase

Cytoplasmic mRNA has been isolated from the leaves of pea seedlings. Translation of this RNA in the wheat germ cell-free system produces two major products, RI and RII, with molecular weights of 33,000 and 20,000, respectively. Both of these products are considerably larger than the small subunit of ribulose-1,5-bisphosphate carboxylase [3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39], which is the major product of cytoplasmic protein synthesis in vivo and has a molecular weight of 14,000. Antiserum prepared against the small subunit of ribulose-1,5-bisphosphate carboxylase precipitates from the cell-free products, in 2-3% yield, three polypeptides of molecular weights 18,000, 16,000 and 14,000. The smallest of these polypeptides is indistinguishable, by sodium dodecyl sulfate/polyacrylamide gel electrophoresis, from the small subunit of ribulose-1,5-bisphosphate carboxylase. Although the cell-free product RII is not precipitated with antiserum prepared against the small subunit of ribulose-1,5-bisphosphate carboxylase, the two polypeptides do show extensive sequence homology, as indicated by ion exchange chromatography of their tryptic peptides. The production of RII can also be achieved in a polysome-primed cell-free system, where protein synthesis is restricted to the completion of polypeptide chains that have already been initiated in vivo. These results indicate that RII is apparently a precursor of the small subunit of ribulose-1,5-bisphosphate carboxylase. We suggest that the selective transport of cytoplasmically synthesized organelle proteins, like animal secretory proteins, may be achieved via the production of precursor polypeptides.

Binding of a transition state analog to newly synthesized Rubisco

Radioactive amino acids, when added to isolated pea chloroplasts or chloroplast extracts engaged in protein synthesis, are incorporated into Rubisco large subunits that co-migrate with native Rubisco during nondenaturing electrophoresis. We have added the transition state analog 2'-carboxyarabinitol bisphosphate (CABP) to chloroplast extracts after in organello or in vitro incorporation of radioactive amino acids into Rubisco large subunits. Upon addition of CABP the radioactive bands co-migrating with native Rubisco undergo a readily detected shift in electrophoretic mobility just as the native enzyme, thus demonstrating the ability of the newly assembled molecules to interact with this transition state analog.

Assembly of Rubisco from native subunits

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

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.

Synthesis of ribulose biphosphate carboxylase in greening pea leaves. Coordination of mRNA level of two subunits

Ribulose biphosphate carboxylase/oxygenase, with its large subunit encoded in the chloroplast and the small subunit in the nucleus, is induced by light. Accumulation of the enzyme, its two mRNA levels, and the synthesis rate of enzyme protein in pea leaves were followed during induction to understand the role of mRNA levels during greening subunit synthesis. The relative mRNA levels for the large and the small subunits increased coordinately up to 3-4 days, which almost corresponded to an inflection point of the accumulation profile of RuBisCO, and then the two mRNA levels gradually decreased. To obtain information of subunit synthesis, the extent of labelling of the two subunits were determined for both assembled and unassembled subunits using specific IgG. Unassembled subunits were found for both polypeptides, with a slight excess of the small one. The observed synthesis rates of the small and the large subunits were roughly coordinated without overproduction and almost stoichiometric amounts of the two polypeptides were found. The profiles of observed synthesis rate of the two subunits and the holoenzyme were similar to those of their mRNA levels. These results suggest that the synthesis of ribulose biphosphate carboxylase/oxygenase protein and its accumulation are dependent on the coordinated change of the two mRNA levels in greening pea leaves.

Phytochrome-mediated regulation of two mRNAs, encoded by nuclei and chloroplasts of ribulose-1,5-bisphosphate carboxylase/oxygenase

Etiolated pea seedlings (5 days old) were treated with red light for 5 min and grown for 2 days more in darkness. Effects of the red light on the mRNA levels of two subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) (ribulose-P2 carboxylase), rate of enzyme synthesis and accumulation of the enzyme were examined for the plants. Increases of the two mRNA levels, encoded in nuclei and chloroplasts were observed. Red light followed by far-red light treatment for 5 min inhibited the increases. An increase parallel with the mRNA levels was found for the rate of the enzyme synthesis and accumulation of the enzyme. These results indicate that the induction of two mRNA levels is mediated by phytochrome and causes the red-light-inducible, far-red-light-reversible increase of ribulose-P2 carboxylase. When white light was irradiated to induce the enzyme, the effects of red light given in darkness were reinforced. These results indicate that light-induction of ribulose-P2 carboxylase is mediated by phytochrome and controlled by the two mRNA levels.

Biosynthesis of ribulose-1.5-bisphosphate carboxylase in greening cells of Euglena gracilis : The accumulation of ribulose-1.5-bisphosphate carboxylase and of its subunits

Light induction of chloroplast development in Euglena leads to quantitative changes in the protein composition of the soluble cell part. One major part of these is the observed accumulation of ribulose-1.5-bisphosphate carboxylase/oxygenase (RuBPCase) enzyme (EC 4.1.1.39). As measured by immunoelectrophoresis, a small amount of RuBPCase (about 10(-6) pmol) is present in a dark-grown cell, whereas a greening cell (72h) contains 10-20 pmol enzyme. Both the cytoplasmic and chloroplastic translation inhibitors, cycloheximide and spectinomycin, have a strong inhibitory effect on the synthesis of the enzyme throughout the greening process of Euglena cells. Electrophoretic and immunological analyses of the soluble phase prepared from etiolated or greening cells do not show the presence of free subunits of the enzyme. For each antibiotic-treated greening cell, the syntheses of both subunits are blocked. Our data indicate that tight reciprocal control between the syntheses of the two classes of subunits occurs in Euglena. In particular, the RuBPCase small subunit synthesis in greening Euglena seems more dependent on the protein synthesis activity of the chloroplast than the syntheses of other stromal proteins from cytoplasmic origin.

The nucleotide sequence of the tobacco chloroplast gene for the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase

The gene for the large subunit (LS) of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCase/Oase) from tobacco has been cloned in pBR322 and sequenced. The coding region contains 1431 bp (477 codons). The deduced amino acid sequence of tobacco LS protein shows 90% homology with those of maize and spinach LS. The positions in the gene corresponding to the 5' and the 3' ends of tobacco LS mRNA have been located on the DNA sequence by the S1 nuclease mapping procedure. The LS gene promoter sequence has homology with Escherichia coli promoter sequences; its terminator sequence is capable of forming a stem-and-loop structure. A sequence GGAGG, which is complementary to a sequence near the 3' end of tobacco chloroplast 16S rRNA and a putative ribosome binding site, occurs 6-10 bp upstream from the initiation codon.

Suitable conditions for characterization, identification, and isolation of the mRNA of the small subunit of ribulose-1,5-bisphosphate carboxylase from Nicotiana sylvestris

The products synthesized in vitro by messenger RNA (mRNA) extracted from Nicotiana sylvestris were analyzed by electrophoresis on polyacrylamide slab gels. Only three of the major polypeptides synthesized are considered here: P55, P32, and P20. P55 and P32 were translated from chloroplast mRNA. P55 corresponds to the large subunit of ribulose-1,5-bisphosphate (RuP2) carboxylase; P32 is probably a chloroplast membrane protein. P20, the polypeptide synthesized from cytoplasmic poly(A)(+) RNA, is the precursor of the small subunit of RuP2 carboxylase. The balance between P20 and P32, in which their relative proportions varied inversely, was regulated by the age of the leaves and the time of illumination; we took advantage of this phenomenon to isolate the mRNA from the small subunit in relatively large amounts. This mRNA has a molecular weight of 350,000.

Cell-free synthesis of active ribulose-1,5-bisphosphate carboxylase in the mesophyll chloroplasts of Sorghum vulgare

Chloroplast and whole leaf cell RNA from Vigna sinensis, a C3 plant were used as exogenous templates for translation in a cell-free light-dependent system of isolated chloroplasts from Sorghum vulgare, a C4-type plant. Analysis of immunoprecipitates of the translated products with the total cellular RNA on sodium dodecyl sulfate polyacrylamide gels revealed the synthesis of both the subunits of ribulose-1,5-bisphosphate carboxylase. Similar analysis of the product translated with the RNA from Vigna chloroplasts, indicated the synthesis of only the large subunit of the carboxylase. Apparently the chloroplast protein synthetic machinery is capable of translating the mRNA for the smaller subunit of this protein as well, which is known to be translated in the cytoplasmic ribosomal system. Sufficient quantities of ribulose-1,5-bisphosphate carboxylase were synthesised in vitro in the preincubated chloroplast system with the whole cell RNA from the C3 plant to assay the ribulose 1,5-bisphosphate-dependent carboxylation. The newly synthesised protein in the cell-free system is identical in many ways to the native enzyme including the Mg2+ concentration-dependent shift in pH optima towards neutral side. It is specifically inhibited by anti-native ribulose-1,5-bisphosphate carboxylase and pyridoxal 5'-phosphate.

Ribulose-1,5-bisphosphate carboxylase as a nuclear and chloroplast marker

The data on the primary structure of ribulose-1,5-bisphosphate carboxylase/oxygenase are reviewed. Examples of their use as markers and in the elucidation of the evolution, adaptation and function of this key enzyme are given.

In-vitro translation of messenger-RNA from developing bean leaves. Evidence for the existence of stored messenger-RNA and its light-induced mobilisation into polyribosomes

Poly(A)-containing messenger RNA was purified from polyribosomes isolated from the primary leaves of 7-day-old dark-grown seedlings of Phaseolus vulgaris var. Masterpiece. Analysis of the messenger RNA on 2.4% polyacrylamide gels showed that it consists of a heterogeneous population of molecules with an average molecular weight of 500,000. The nucleotide composition of the RNA was 16.0% cytidylic acid, 39.4% adenylic acid, 21.3% guanylic acid and 23.2% uridylic acid. Based on the degree of resistance of the RNA to digestion with ribonucleases A and T1 the average length of the poly(A) sequence was calculated to be 120 nucleotides. No significant differences in mobility in polyacrylamide gels, nucleotide composition or polyadenylic acid content were found between the poly(A)-containing mRNA from polyribosomes of primary leaves of dark-grown plants and those given a 16 h white light treatment. Purified poly(A)-containing mRNA was shown to direct the incorporation of [(35)S]methionine into proteins in an in vitro protein-synthesising system from wheat germ. The protein products were fractionated according to molecular size by electrophoresis in 15% polyacrylamide/urea/SDS gels and the protein bands were detected by fluorography. Messenger RNAs directing the synthesis of three polypeptides with molecular weights of 34,000, 32,000 and 25,000 were detected in polyribosomes of plants following white light treatment. These messenger RNAs were absent, or present in much lower amounts, in polyribosomal messenger RNA from leaves of dark-grown plants, although they were present in total cell poly(A)-containing RNA. This indicates that certain messenger RNAs may be stored in the dark and that light stimulates these RNAs to engage in polyribosome formation. Continuous far-red (730 nm) irradiation for 4 h also caused the appearance of these messenger RNAs in the polyribosomes although 5 min red light followed by 4 h darkness had little effect. This suggests that phytochrome acting in the "high energy" mode, may be the photoreceptor responsible for initiating the response.

Light-induced enzyme synthesis in cell suspension cultures of Petroselinum hortense. Demonstration in a heterologous cell-free system of rapid changes in the rate of phenylalanine ammonia-lyase synthesis

The conditions for protein synthesis in vitro with polyribosomes from cell suspension cultures of parsel (Petroselinum hortense) and a wheat-germ extract were investigated. Two different criteria were used as estimated of the translational activity: (a) the total rate of incorporation of [35S]methionine into acid-insoluble material; (b) the ratio of large (molecular weight greater than 25000) to small (molecular weight less than 25000) peptide products. Depending on which of the criteria was employed, the pH optimum and the optimal concentrations for Tris=acetate, magnesium acetate, KCL, methionine and the wheat-germ extract differed considerably. The translational activity of the polyribosomes (both criteria) was effciently protected by 0.1 M Mg2+ against degradation during the isolation procedure. The rate of synthesis of phenylalanine ammonia-lyase in vitro with the polyribosomes was determined by measuring the incorporation rate of L-[35S]methionine into protein which was precipitable by a rabbit antiserum prepared for the purified enzyme. The immunoprecipitate was analyzed by disc gel electrophoresis in the presence of dodecylsulfate and was shown to contain small amounts of the complete enzyme subunits and relatively large amounts of shorter peptides which were also characteristic for the enzyme. The time course of light-induced changes in the rate of phenylalanine ammonia-lyase synthesis in vitro were investigated during a period of 15 h under two different conditions of induction: the cell cultures were irradiated with ultraviolet light eith (A) continuously or (B) for 2.5 h and then returned to darkness. Although the highest rate of enzyme synthesis was observed somewhat later inexperiment A than in experiment B, the periods of time during which the rate of synthesis increased rapidly were limited in both cases to only a few hours. The results obtained in vitro were identical within the limits of the experimental error with theoretical calculations of the changes in the rate constant of phenylalanine ammonia-lyase synthesis in vivo. These changes were calculated from the corresponding curves for the changes in the enzyme activity under the conditions of induction. The results are in agreement with previous observations suggesting that the induction of phenylalanine ammonia-lyase by light in the parsley cells was a short-term effect whose efficiency was greatly reduced within the 15 h of experimentation, even under continuous irradiation.