Synthesis and turnover of ribulose biphosphate carboxylase and of its subunits during the cell cycle of Chlamydomonas reinhardtii

Rapid recovery of chloroplast mutations affecting ribulosebisphosphate carboxylase/oxygenase in Chlamydomonas reinhardtii

Based on the unique ability of chloroplast genes to recombine in Chlamydomonas reinhardtii, a collection of acetate-requiring mutants was screened for recombination with a mutation affecting ribulose-1,5-bisphosphate carboxylase/oxygenase [Rbu-1,5-P(2) carboxylase/oxygenase; 3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39]. This chloroplast mutation, rcl-u-1-10-6C, causes the absence of Rbu-1,5-P(2) carboxylase/oxygenase activities and alters the isoelectric point of the larger subunit. Several mutants that displayed little or no recombination with 10-6C were recovered, and two lacked carboxylase activity. These new chloroplast mutants lack both large and small Rbu-1,5-P(2) carboxylase/oxygenase subunits. The approach demonstrated here permits the routine recovery of chloroplast mutations affecting this enzyme. Multiple mutations in the Rbu-1,5-P(2) carboxylase/oxygenase large-subunit gene can be used to investigate the function and regulation of this enzyme and the regulation of chloroplast genes in general.

mRNAs encoding ribulose-1,5-bisphosphate carboxylase remain bound to polysomes but are not translated in amaranth seedlings transferred to darkness

When light-grown seedlings of amaranth are transferred to total darkness, synthesis of the large subunit (LS) and small subunit (SS) of ribulose-1,5-bisphosphate carboxylase [RbuP(2)Case; 3-phospho-D-glycerate carboxylase (dimerizing), EC 4.1.1.39] is rapidly depressed. This reduction in RbuP(2)Case synthesis occurs in the absence of any corresponding changes in levels of functional mRNA for either subunit. Four hours after light-to-dark transition little, if any, changes in the distribution of LS and SS mRNAs on polysomes could be detected. The association of these mRNAs with polysomes was authenticated by treatment with RNase A or puromycin. Furthermore, polysomes were able to synthesize LS and SS precursor in cell-free translation systems supplemented with inhibitors of initiation. Therefore, during a light-to-dark transition LS and SS mRNAs remained bound to polysomes but were not translated in vivo, suggesting that control is exercised, in part, at the translational elongation step.

The action of a range of supplementary ultraviolet (UV) wavelengths on photosynthesis in Brassica napus L. in the natural environment: effects on PS II, CO(2) assimilation and level of chloroplast proteins

The effects of different wavebands of UV radiation on photosynthesis and the expression and abundance of photosynthetic proteins in oilseed rape (Brassica napus L. cv. Rebel) were investigated. Plants were grown outdoors under natural radiation (52 degrees N, 0 degrees E) supplemented with six wavebands of UV radiation (0.4 Wm(-2)) between 313 nm and 356 nm. A control treatment was centred at 343 nm. Exposure to supplementary UV-A radiation (320-400 nm) had no significant effects, however UV-B radiation, centred at 313 nm, caused a marked reduction in photosynthesis. This decrease was related to a reduction in the initial carboxylation velocity of Rubisco which was further correlated with a large reduction in the expression and abundance of both large and small subunits of Rubisco. These results indicate a molecular mechanism behind UV-B induced reductions in photosynthesis per unit area in plants grown under field conditions.

Intron-specific RNA binding proteins in the chloroplast of the green alga Chlamydomonas reinhardtii

Mitochondria and chloroplasts both contain group II introns which are believed to be the ancestors of nuclear spliceosomal introns. We used the mitochondrial group II intron rI1 from the green alga Scenedesmus obliquus for biochemical characterization of intron-specific RNA binding proteins. rI1 is correctly spliced from a chloroplast precursor RNA when integrated into the chloroplast genome of Chlamydomonas reinhardtii. Glycerol gradients revealed the sedimentation profile of transcripts containing intron rI1 in native C. reinhardtii extracts and in deproteinized RNA preparations, thus indicating the association of rI1 containing transcripts with high molecular weight ribonucleoprotein complexes in vivo. Furthermore, the specific binding of a 61 kDa protein and a 31 kDa protein with the conserved domain IV was demonstrated using a set of intron derivatives for in vitro RNA binding experiments. We propose that we have biochemically characterized 'general splicing factors', which enable the successful splicing even of mitochondrial introns in chloroplasts.

Photoregulation of chloroplast developm ent: transcriptional, translational and post-translational controls?

Chloroplast development involves the nucleus, the cytoplasm and the chloroplast of plant cells. This may be illustrated by reference to the two most abundant proteins of the chloroplast: (i) the soluble CO 2 -fixing enzyme ribulose 1,5-bisphosphate carboxylase—oxygenase, whose large subunit (LSU) is encoded in chloroplast DNA and synthesized on chloroplast ribosomes and whose small subunit (SSU) is encoded in nuclear DNA, synthesized on cytoplasmic ribosomes in precursor form and transported into chloroplasts, and (ii) the thylakoid-bound light-harvesting chlorophyll a/b complex, whose pigment components are synthesized in the chloroplast and whose apoproteins resemble the SSU in site of coding and site of synthesis. We have examined the extent to which biosynthetic events in the nucleocytoplasmic compartments are coordinated with those inside the chloroplast during the de-etiolation of pea seedlings. We have examined the levels of LSU, SSU and the light-harvesting chlorophyll a/b protein (LHCP) by using a highly specific radioimmune assay. The steady-state levels of the corresponding mRNAs have been determined using specific cloned DNA probes. With the SSU, the mRNA and protein levels are near the limit of detection in dark-grown plants but increase markedly under continuous white light, with a lag of about 24 h. The protein appears to be under simple phytochrome control at the level of the steady-state concentration of its mRNA. The LSU also appears to be regulated through the steady-state concentration of its mRNA but in this case the mRNA is not under simple phytochrome control. The LHCP mRNA is readily detectable in dark-grown plants and accumulates further under illumination in a phytochrome-mediated manner. However, the LHCP itself (like chlorophyll) is not detectable in dark-grown plants and accumulates to high levels only under continuous illumination, with a lag of about 6 h. Post-translational control is particularly important in the accumulation of the LHCP: continuous chlorophyll synthesis is required for the stabilization of the protein within the thylakoid membrane, at least during the early stages of chloroplast development.

A mechanism for intergenomic integration: abundance of ribulose bisphosphate carboxylase small-subunit protein influences the translation of the large-subunit mRNA

Multimeric protein complexes in chloroplasts and mitochondria are generally composed of products of both nuclear and organelle genes of the cell. A central problem of eukaryotic cell biology is to identify and understand the molecular mechanisms for integrating the production and accumulation of the products of the two separate genomes. Ribulose bisphosphate carboxylase (Rubisco) is localized in the chloroplasts of photosynthetic eukaryotic cells and is composed of small subunits (SS) and large subunits (LS) coded for by nuclear rbcS and chloroplast rbcL genes, respectively. Transgenic tobacco plants containing antisense rbcS DNA have reduced levels of rbcS mRNA, normal levels of rbcL mRNA, and coordinately reduced LS and SS proteins. Our previous experiments indicated that the rate of translation of rbcL mRNA might be reduced in some antisense plants; direct evidence is presented here. After a short-term pulse there is less labeled LS protein in the transgenic plants than in wild-type plants, indicating that LS accumulation is controlled in the mutants at the translational and/or posttranslational levels. Consistent with a primary restriction at translation, fewer rbcL mRNAs are associated with polysomes of normal size and more are free or are associated with only a few ribosomes in the antisense plants. Effects of the rbcS antisense mutation on mRNA and protein accumulation, as well as on the distribution of mRNAs on polysomes, appear to be minimal for other chloroplast and nuclear photosynthetic genes. Our results suggest that SS protein abundance specifically contributes to the regulation of LS protein accumulation at the level of rbcL translation initiation.

Synthesis of EF-Tu and distribution of its mRNA between stroma and thylakoids during the cell cycle of Chlamydomonas reinhardii

In Chlamydomonas reinhardii the elongation factor EF-Tu is encoded in the chloroplast DNA. We identified EF-Tu in the electrophoretic product pattern of chloroplast-made proteins and showed that this protein is only synthesized in the first half of the light period in synchronized cells. The newly synthesized EF-Tu contributed little to the almost invariable content of EF-Tu in chloroplasts during the light period of the cell cycle. However, increasing cell volume and the lack of EF-Tu synthesis in the second half of the light period led to a decrease in the concentration of EF-Tu in chloroplasts. At different times in the vegetative cell cycle, the RNA was extracted from whole chloroplasts and from free and thylakoid-bound chloroplast polysomes. The content of mRNA of EF-Tu in chloroplasts and the distribution between stroma and thylakoids were determined. During the light period, the content of the mRNA for EF-Tu varied in parallel to the rate of EF-Tu synthesis. However, in the dark, some mRNA was present even in the absence of EF-Tu synthesis. Most of the mRNA was bound to thylakoids during the whole cell cycle. This suggests that synthesis of EF-Tu is associated with thylakoid membranes.

Synthesis of two proteins in chloroplasts and mRNA distribution between thylakoids and stroma during the cell cycle of Chlamydomonas reinhardii

Chloroplasts contain thylakoid-bound and free ribosomes and polysomes. Whether binding of polysomes plays an immediate role in the regulation of chloroplast protein synthesis is not yet clear. In the present work, variations of protein synthesis and of mRNA content were measured not in greening, but in fully differentiated chloroplasts during the cell cycle of synchronized cultures of Chlamydomonas reinhardii. At different times of the vegetative cell cycle, the RNA was extracted from free and thylakoid-bound chloroplast polysomes and the partition of mRNAs between stroma and thylakoids was measured for two proteins, i.e. the 32-kDa herbicide-binding membrane protein and the soluble large subunit of the ribulose-1,5-bisphosphate carboxylase. At the same time the rates of synthesis of these two proteins were also determined. At 2 h after the onset of light, the content of both mRNAs in chloroplasts had doubled and 75-90% of each of these mRNAs were found to be bound to the thylakoids. The rate of protein synthesis, however, increased 10-fold, but reached its maximum only after about 6 h in the light. The differences in the time courses, in the stimulation of the rate of protein synthesis, and in the mRNA-binding to thylakoids point to a translational regulation of protein synthesis. Furthermore, since a very high proportion of polysomes were bound to thylakoids, containing mRNA for both a membrane and a soluble protein, this light-induced binding of polysomes to thylakoids seems to be an essential, but not the only, prerequisite for protein synthesis in chloroplasts.

Coordinate, Organ-Specific and Developmental Regulation of Ribulose 1,5-Bisphosphate Carboxylase Gene Expression in Amaranthus hypochondriacus

The expression of the genes encoding the large subunit (LSU) and small subunit (SSU) of ribulose 1,5-bisphosphate carboxylase (RuBPCase) was examined in roots, stems, cotyledons, and leaves of amaranth during the development of these tissues. The highest accumulation of LSU and SSU polypeptides occurred in cotyledons and leaves. Their steady state levels were approximately 20-fold lower in stems, while in roots neither LSU and SSU polypeptides nor their respective mRNAs could be detected. In cotyledons and leaves accumulation of these two polypeptides reached peak levels during the expansion stage of each tissue and then declined, reflecting changes in the synthesis, not turnover, of these proteins. In cotyledons and stems, the rates of synthesis of LSU and SSU polypeptides correlated with the levels of their respective mRNA, suggesting regulation primarily at the transcriptional level. In contrast, the dramatic and specific decrease in the synthesis of these two proteins during the last stages of development of the leaves could only partially be accounted for by the modest reduction in their mRNAs. Neither the translatability of these mRNAs, as assayed in cell-free systems, nor the stability of LSU and SSU polypeptides were altered, thus implying that control was being exerted at the translational level. During the development of these different organs, the expression of the LSU and SSU genes were generally coordinately regulated both at the levels of protein synthesis and mRNA accumulation.

Translational regulation of light-induced ribulose 1,5-bisphosphate carboxylase gene expression in amaranth

The regulation of the genes encoding the large and small subunits of ribulose 1,5-bisphosphate carboxylase was examined in amaranth cotyledons in response to changes in illumination. When dark-grown cotyledons were transferred into light, synthesis of the large- and small-subunit polypeptides was initiated very rapidly, before any increase in the levels of their corresponding mRNAs. Similarly, when light-grown cotyledons were transferred to total darkness, synthesis of the large- and small-subunit proteins was rapidly depressed without changes in mRNA levels for either subunit. In vitro translation or in vivo pulse-chase experiments indicated that these apparent changes in protein synthesis were not due to alterations in the functionality of the mRNAs or to protein turnover, respectively. These results, in combination with our previous studies, suggest that the expression of ribulose 1,5-bisphosphate carboxylase genes can be adjusted rapidly at the translational level and over a longer period through changes in mRNA accumulation.

Regulation of genes encoding the large subunit of ribulose-1,5-bisphosphate carboxylase and the photosystem II polypeptides D-1 and D-2 during the cell cycle of Chlamydomonas reinhardtii

Synthesis of the major chloroplast proteins is temporally regulated in light-dark-synchronized Chlamydomonas cells. We have used cloned chloroplast DNA probes, and in vitro and in vivo protein synthesis to examine the cell cycle regulation of photosystem II polypeptides D-1 and D-2, and the large subunit of ribulose-1,5-bisphosphate carboxylase (RuBPCase LS). Synthesis and accumulation of D-1 and D-2 mRNAs occurs during the first half of the light period (G1), correlating with increasing synthesis of the polypeptides. Rifampicin, added immediately before the light period, inhibited the normal increase in D-1, D-2 polypeptide synthesis. During the dark period D-1, D-2 mRNAs persist at high levels despite reduced rates of mRNA synthesis and translation during this period. Cell-free translation analyses indicate that the D-1 mRNA present during the dark period is efficient at directing synthesis of the D-1 precursor in vitro. We conclude that expression of the psbA (D-1) and psbD (D-2) genes are regulated primarily at the transcriptional level during the light-induction period but at the translational level for the remainder of the cell cycle. Transcripts of the RuBPCase LS gene (rbcL) are also found at high levels during the light and dark periods but, unlike D-1 and D-2, LS mRNA levels do not increase until the last half of the light period and measurable synthesis and accumulation of this mRNA occurs during the dark. Furthermore, induction of LS polypeptide synthesis during the light period is insensitive to rifampicin. We conclude that LS production is regulated primarily at the translational level during the cell cycle.

Translational regulation of light-induced ribulose 1,5-bisphosphate carboxylase gene expression in amaranth

The regulation of the genes encoding the large and small subunits of ribulose 1,5-bisphosphate carboxylase was examined in amaranth cotyledons in response to changes in illumination. When dark-grown cotyledons were transferred into light, synthesis of the large- and small-subunit polypeptides was initiated very rapidly, before any increase in the levels of their corresponding mRNAs. Similarly, when light-grown cotyledons were transferred to total darkness, synthesis of the large- and small-subunit proteins was rapidly depressed without changes in mRNA levels for either subunit. In vitro translation or in vivo pulse-chase experiments indicated that these apparent changes in protein synthesis were not due to alterations in the functionality of the mRNAs or to protein turnover, respectively. These results, in combination with our previous studies, suggest that the expression of ribulose 1,5-bisphosphate carboxylase genes can be adjusted rapidly at the translational level and over a longer period through changes in mRNA accumulation.

Comparison of Patterns of Accumulation of Ribulose Bisphosphate Carboxylase Antigen and Catalytic Activity and Measurement of Antigen Half-Life during the Cell Cycle of Chlorella sorokiniana

By use of specific immunochemical procedures, ribulose-1,5-bisphosphate carboxylase (RuBPCase), antigen and catalytic activity were shown to have coincident step-patterns of accumulation during the cell cycle of Chlorella sorokiniana. Pulse-chase studies, employing radioactive sulfate, were performed during the period of rapid accumulation of enzyme activity and during the period of constant enzyme activity in the cell cycle. No degradation of RuBPCase antigen could be detected during either of these cell cycle periods. Thus, the step-pattern of accumulation of RuBPCase activity resulted from periodic synthesis of an enzyme that was stable under steady-state cell cycle conditions. Although inhibition of protein synthesis by cycloheximide, at different times in the cell cycle in the light, resulted in rapid decay of RuBPCase activity, this loss in activity occurred without detectable loss in enzyme antigen. When synchronous cells were placed into the dark, to slow the rate of protein synthesis in the absence of cycloheximide, the levels of enzyme antigen and activity decreased by 30 and 50%, respectively, during the 10-hour dark period. Thus, in C. sorokiniana changes in RuBPCase activity do not necessarily reflect parallel changes in enzyme antigen, particularly when cell growth is perturbed by changes from steady-state cultural conditions.

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.

Chloroplast Protein Synthesis in the Chromophytic Alga Olisthodiscus luteus: Cell Cycle Analysis

This study represents the first report on chloroplast protein synthesis during the synchronous cell growth of a chromophytic (chlorophyll a,c) plant. When the unicellular alga Olisthodiscus luteus is maintained on a 12-hour light:12-hour dark cycle, cell and chloroplast number double every 24 hours. A temporal separation between these two events occurs. Measurements of chloroplast and total cellular protein values suggest that polypeptide synthesis occurs mainly in the light portion of the cell cycle, and pulse chase studies demonstrate that chloroplast proteins made in the light are not degraded in the dark. Data support the following conclusions: (a) a similar complement of chloroplast DNA coded proteins is made at all phases of the light portion of the cell cycle, and (b) chloroplast protein synthesis is a light rather than a cell cycle mediated response.

Ribulose 1,5-Bisphosphate Carboxylase Synthesis during Heat Shock

Ribulose 1,5-bisphosphate carboxylase (RuBPCase) was chosen as a model protein to study how heat shock (HS) affects both chloroplast protein synthesis and the nuclear-chloroplast interaction in production of chloroplast proteins. Experiments were performed using highly chlorophyllous, soybean (Glycine max L. Merr. var Corsoy) cell suspension cultures active in chloroplast protein synthesis. Synthesis of RuBPCase large (L) and small (S) subunits was followed by in vivo labeling, and corresponding mRNA levels were examined by Northern and dot hybridization analyses. Results demonstrate that L and S synthesis declines with increasing HS temperatures (33-40 degrees C) and reaches minimum levels (20-30% of control) at temperatures of maximum HS protein synthesis (39-40 degrees C). Recovery of L and S synthesis following a 2-hour HS at 38 or 40 degrees C was also studied. The changes in S synthesis during HS and recovery correlate with the steady state levels of S mRNA. In contrast, changes in L synthesis show little relationship to the corresponding mRNA levels; levels of L mRNA remain relatively unchanged by HS. We conclude that chloroplast protein synthesis shows no greater sensitivity to HS than is observed for cytoplasmic protein synthesis and that transport of proteins into the chloroplast (e.g.S subunit) continues during HS. Furthermore, there is no apparent coordination of L and S subunit mRNA levels under the conditions examined.

Multiple transcripts for higher plantrbcL andatpB genes and localization of the transcription initiation site of therbcL gene

We have compared therbcL andatpB transcription units from spinach, maize, and pea. In most cases multiple transcripts were found for a given chloroplast gene. The 5' termini of these transcripts were determined by S1 nuclease protection and primer extension analyses. TherbcL transcripts have 5' termini 178-179 and 64 nucleotides (spinach), 300 and 59-63 nucleotides (maize), and 178 and 65 nucleotides (pea) upstream from their respective protein coding regions. TheatpB transcripts have 5' termini (453-454, 272-273, 179, and 99 nucleotides (spinach), 298-302 nucleotides (maize), and 351-355 nucleotides (pea) upstream from their respective protein coding regions. The intergenic distance between therbcL andatpB genes is relatively constant (152 to 157 base pairs) among the three chloroplast genomes. In spinach, maize, and pea, the 80 base pairs surrounding the 5' end of therbcL gene (±40 base pairs) have 85% sequence homology. Similarly, the 60 base pairs preceding theatpB gene have 48% sequence homology. Both genes have '-10' and '-35' regions that resemble the prokaryotic consensus promoter sequence. The larger, but not smaller,rbcL transcripts from spinach and pea can be labeled with alpha-(32)P-GTP by guanylyltransferase. These data suggest that DNA sequences 178-179 (spinach), 300 (maize), and 178 (pea) base pairs before therbcL protein coding regions represent sites of transcription initiation. The sequences 59-65 base pairs before therbcL protein coding regions may correspond to sites of RNA cleavage.

Kinetics of Accumulation of Ribulose-1,5-bisphosphate Carboxylase during Greening in Euglena gracilis: Photoregulation

The preparation of a rabbit antibody to ribulose-1,5-bisphosphate carboxylase (RuBPCase) from Euglena gracilis and its use to quantitate RuBPCase in dark- and light-grown cells and during light-induced chloroplast development (greening) are described. Light-grown Euglena have at least 36 times more RuBPCase than dark-grown Euglena. Light is required for both the initiation and continued increase in net synthesis of RuBPCase over the dark level: brief illumination 12 hours before exposure to continuous light eliminates the lags in the accumulation and increase in activity of RuBPCase (as well as in chlorophyll accumulation); net synthesis is blocked in greening cells returned to the dark or exposed to 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Streptomycin or cycloheximide prevents RuBPCase accumulation when added at the beginning of greening but only partially blocks accumulation when added after 25 hours of greening. After 24 hours of greening, the activity of RuBPCase per milligram chlorophyll continues to increase slowly while concentration of the enzyme per milligram chlorophyll remains constant. This increased activity may be due to activation of the enzyme as well as to net synthesis.

Differential regulation of the accumulation of the light-harvesting chlorophyll a/b complex and ribulose bisphosphate carboxylase/oxygenase in greening pea leaves

The photoregulation of chloroplast development in pea leaves has been studied by reference to three polypeptides and their mRNAs. The polypeptides were the large subunit (LSU) and the small subunit (SSU) of ribulose 1,5-bisphosphate carboxylase/oxygenase (RUBISCO), and the light-harvesting chlorophyll a/b protein (LHCP). The polypeptides were assayed by a sensitive radioimmune assay, and the mRNAs were assayed by hybridization to cloned DNA probes. LSU, LSU mRNA, and LHCP mRNA were detectable in etiolated seedlings but LHCP, SSU, and SSU mRNA were at or below the limit of detection. During the first 48 hr of de-etiolation under continuous white light, the mRNAs for LSU, SSU, and LHCP increased in concentration per apical bud by about 40-fold, at least 200-fold, and about 25-fold, respectively, while the total RNA content per apical bud increased only 3.5-fold. In the same period, the LSU, SSU, and LHCP contents per bud increased at least 60-, 100-, and 200-fold, respectively. The LHCP increased steadily in concentration during de-etiolation, whereas the accumulation LSU, SSU, and SSU mRNA showed a 24-hr lag. The accumulation of SSU, SSU mRNA, and LHCP mRNA showed classical red/far-red reversibility, indicating the involvement of phytochrome in the regulatory mechanism. LSU and LSU mRNA were induced equally well by red and far-red light. The LHCP failed to accumulate except under continuous illumination. These results indicate that the accumulation of SSU is controlled largely through the steady-state level of its mRNA, which is in turn almost totally dependent on light as an inducer and on phytochrome as one of the photoreceptors. The accumulation of LSU is largely but not totally determined by the level of its mRNA, which appears to be under strong photoregulation, which has yet to be shown to involve phytochrome. Phytochrome is involved in the regulation of LHCP mRNA levels but substantial levels of the mRNA also occur in the dark. LHCP accumulation is not primarily governed by the levels of LHCP mRNA but by posttranslational stabilization in which chlorophyll synthesis plays a necessary but not sufficient role.

Cytoplasmic regulation of chloroplast translation in Euglena gracilis

A regulatory role for cytoplasmically derived proteins in chloroplast translation in organello was examined by analyzing protein synthesis in plastids isolated from cells of Euglena gracilis which had been treated with cycloheximide (CHI). Incorporation of [35S]methionine by chloroplasts from CHI-inhibited Euglena was reduced approximately 40 and 90% by exposure of the cells to the antibiotic for 2 and 4 h, respectively. The chloroplast translation products were then analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography. The synthesis of polypeptides in the soluble compartment of the plastid was substantially diminished by as little as 15 min of CHI pretreatment. No qualitative alterations of the polypeptide pattern were detected. Qualitative changes were seen in the thylakoid fraction, however. Comparison of the stainable polypeptides and fluorographs of thylakoid membranes from CHI-treated cells with those of controls showed several instances in which the more slowly migrating member of a doublet accumulated with a concomitant depletion of a more rapidly migrating component. A pair of polypeptides at 28 and 30 kDa, which we believe are the Euglena homologs of the photogene product and its precursor, respectively, are representative of this phenomenon. Additionally, thylakoids from cells pretreated with CHI sometimes synthesized novel polypeptides larger than 65 kDa. Finally, when intact chloroplasts from CHI-inhibited Euglena were incubated with a postchloroplast supernatant from normal cells, there was a partial reversion of the anomalies seen in the fluorographs. These data are interpreted to indicate the cytoplasmic origin of one or more proteins whose function is to process chloroplast translation products.

Regulation of light-harvesting chlorophyll-binding protein (LHCP) mRNA accumulation during the cell cycle in Chlamydomonas reinhardi

Light-harvesting chlorophyll a/b protein (LHCP) synthesis is highly regulated during the cell cycle in light-dark synchronized C. reinhardi cells. LHCPs are a family of cytoplasmically synthesized proteins which are imported into the chloroplast. LHCPs are derived from at least two precursor proteins (32 kd and 30 kd) that are synthesized in vitro and immunoprecipitated by antiserum against chlorophyll-protein complex II proteins. A DNA copy of the mRNA encoding a 32 kd LHCP precursor was cloned from cDNA synthesized from poly(A) RNA obtained from mid-light-phase synchronous cells. Using cloned cDNA (pHS16) as a hybridization probe, we found that a single 1.2 kb RNA complementary to pHS16 accumulates in a wave-like manner during the mid-light phase of the 12 hr light-12 hr dark cycle and correlates with the pattern of chlorophyll synthesis. Light, during the light phase in the light-dark cycle, is required for accumulation of this RNA.

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.

Thylakoid membrane biogenesis in Chlamydomonas reinhardtii 137+. II. Cell-cycle variations in the synthesis and assembly of pigment

Synthesis of the chlorophyll and the major carotenoid pigments and their assembly into thylakoid membrane have been studied throughout the 12-h light/12-h dark vegetative cell cycle of synchronous Chlamydomonas reinhardtii 137+ (wild-type). Pulse exposure of cells to radioactive acetate under conditions in which labeling accurately reflects lipogenesis, followed by cellular fractionation to purify thylakoid membrane, allowed direct analysis of the pigment synthesis and assembly attendant to thylakoid biogenesis. All pigments are synthesized and assembled into thylakoids continuously, but differentially, with respect to cell-cycle time. Highest synthesis and assembly rates are confined to the photoperiod (mid-to-late G1) and support chlorophyll and carotenoid accretion before M-phase. The lower levels at which these processes take place during the dark period (S, M, and early-to-mid G1) have been ascribed to pigment turnover. Within this general periodic pattern, pigment synthesis and assembly occur in a "multi-step" manner, i.e., by a temporally-ordered, stepwise integration of the various pigments into the thylakoid membrane matrix. The cell-cycle kinetics of pigment assembly at the subcellular level mirror the kinetics of pigment synthesis at the cellular level, indicating that pigment synthesis not only provides chlorophyll and carotenoid for thylakoid biogenesis but may also serve as a critical rate-determinant to pigment assembly.

Thylakoid membrane biogenesis in Chlamydomonas reinhardtii 137+: cell cycle variations in the synthesis and assembly of polar glycerolipid

The synthesis and assembly of thylakoid membrane polar glycerolipid (glycolipid, phospholipid, and ether lipid) have been monitored in synchronous cultures of the green alga Chlamydomonas reinhardtii 137+. A "pulse" protocol using radioactive acetate as the lipogenic precursor was devised to allow assessment of both processes during the 24-h (12-h light/12-h dark) vegetative cell cycle. Under these conditions, acetate incorporation into each chromatographically resolved lipid at the cellular level reliably reflects lipid synthesis, and the appearance of radiolabeled lipid in purified photosynthetic membrane is indicative of the lipid assembly attendant to thylakoid biogenesis. Our results demonstrate that polar glycerolipid is synthesized by the alga and is assembled into its thylakoid membrane continuously, but differentially, with respect to cell cycle time. Synthesis and assembly are most rapid during the photoperiod (mid-to-late G1), reach maximum rates at mid-photoperiod, and are comparatively negligible in the dark (S, M, and early-to-mid G1). The extent to which synthesis and assembly vary within this general kinetic pattern, though, is characteristic of each thylakoid lipid, suggesting that the processes take place in a multistep manner with some temporal coordination among the different lipid types. Parallelism between the cyclic patterns of polar lipid synthesis at the cellular level and of polar lipid assembly into photosynthetic membrane at the subcellular level indicates that lipid production is not only essential to continuing thylakoid biogenesis but is also the critical determinant of the kinetics of thylakoid lipid assembly.

Regulation of the synthesis of ribulose-1,5-bisphosphate carboxylase and its subunits in the flagellate Chlorogonium elongatum. I. The effect of light and acetate on the synthesis and the degradation of the enzyme

Shifting of heterotrophically cultured cells of Chlorogonium elongatum to autotrophic culture conditions results in a more than tenfold increase in the level of the chloroplastic enzyme ribulose-1,5-bisphosphate carboxylase. This increase in enzyme amount is caused by an increased synthesis of the enzyme de novo under both growth conditions. The synthesis of the enzyme is mainly under the control of two factors. Light promotes the synthesis, while acetate, the carbon source of this organisms, has a strong inhibitory effect on this process. Since a mutant lacking chlorophyll shows a normal response to the change of culture conditions the chlorophylls can be ruled out as photoreceptors.

Light Effects on the Synthesis of Ribulose-1,5-Bisphosphate Carboxylase in Lemna gibba L. G-3

Placing light-grown Lemna gibba L. G-3 into the dark results in a changed pattern of protein synthesis. Although the amount of protein in the tissue and the over-all rate of incorporation of [(35)S]methionine into protein does not significantly decline during four days of darkness, the rate of synthesis of three polypeptides declines dramatically. One of these polypeptides is the chlorophyll a/b-binding protein and the two others are the large and small subunits of ribulose-1,5-bisphosphate carboxylase. The changed rates of synthesis of the two subunits were examined after transitions of plants from light to dark and dark to light. The in vivo synthesis of both subunits, while declining to a low level during four days of darkness, increases rapidly upon returning the plants to white light. In addition, the level of poly(A) mRNA coding for the precursor polypeptide of the small subunit of the enzyme falls to a low level in the dark and increases rapidly in response to white light. The increase in translatable mRNA for the small subunit is rapid enough to account for a major part of the increased synthesis of this subunit.

The biosynthesis of ribulose bisphosphate carboxylase. Uncoupling of the synthesis of the large and small subunits in isolated soybean leaf cells

Isolated leaf cells from soybean (Glycine max) incorporate [35S]methionine into protein at a linear rate for at least 5h. Analysis of the products of incorporation by one-dimensional and two-dimensional polyacrylamide gel electrophoresis shows that major products are the large and small subunits of the chloroplast enzyme, ribulose bisphosphate carboxylase. The large subunit is synthesized by chloroplast ribosomes and the small subunit by cytoplasmic ribosomes. Addition of chloramphenicol to the cells reduces incorporation into the large subunit without affecting incorporation into the products of cytoplasmic ribosomes. Addition of cycloheximide or 2-(4-methyl-2,6-dinitroanilino)-N-methylpropionamide stops incorporation into the small subunit, but large subunit continues to be made for at least 4 h. For accurate estimates of incorporation into the large subunit, it is essential to use two-dimensional gel electrophoresis, because the large subunit region on one-dimensional gels is contaminated with the products of cytoplasmic ribosomes. Newly synthesized large subunits continue to enter complete molecules of ribulose bisphosphate carboxylase in the absence of small subunit synthesis. These results suggest that, in contrast to the situation in algal cells, the synthesis of the two subunits of ribulose bisphosphate carboxylase in the different subcellular compartments of higher plant cells is not tightly coupled over short time periods, and that a pool of small subunits exists in these cells. The results are disucssed in relation to possible mechanisms for the integration of the synthesis of the large and small subunits of ribulose bisphosphate carboxylase.

Ribulose bisphosphate carboxylase from a mutant strain of Chlamydomonas reinhardii deficient in chloroplast ribosomes : The absence of both subunits and their pattern of synthesis during enzyme recovery

The ac-20 mutant strain of the unicellular green alga, Chlamydomonas reinhardii, lacks both chloroplast ribosomes and ribulose bisphosphate carboxylase activity when grown on organic medium. Under these conditions, the cells do not posses pools of either the large or small subunit of this enzyme. When transferred to inorganic medium, the carboxylase activity recovers. During this recovery, de novo synthesis of both subunits occurs. Synthesis of both subunits is inhibited by chloramphenicol even when possible free subunit pools rather than just the subunits incorporated into whole enzyme are examined.

Biosynthesis of ribulose-1,5-bisphosphate carboxylase in spinach leaf protoplasts

Spinach leaf (Spinacia oleracea L. var. Kyoho) protoplasts sustain protein-synthesizing activity as measured by the incorporation of [(14)C]-leucine into the protein fraction both in the light and in the dark. By the immunoprecipitation of ribulose-1,5-bisphosphate (RuP(2)) carboxylase with rabbit antibody raised against the purified spinach enzyme preparation, it was found that approximately 7% of the total radiocarbon incorporated into the protein fraction in the light was in the carboxylase molecules. However, there was no measurable net increase observed in the content of the enzyme protein in the experimental conditions employed. It was found that both chloramphenicol and cycloheximide inhibited the incorporation of [(14)C]leucine into RuP(2) carboxylase and its constituent subunits, as measured by the immunoprecipitation of the enzyme molecule and its subunits, A and B.

Kinetic analysis of fraction I protein biosynthesis in young protoplasts of tobacco leaves

At maximum inhibition chloramphenicol reduced [35S] methionine incorporation into acid-insoluble materials of sterile protoplasts from young tobacco leaves 5-7 cm in length by 30% compared to 70% by cycloheximide, indicating that 30% of the [35S] methionine became incorporated into chloroplast proteins and 70% into cytoplasmic proteins. [35S] Methionine became incorporated into both the large and small subunits of Fraction I protein, the major soluble protein of chloroplasts. Rifampicin and streptolydigin inhibited [3H] uridine incorporation into the 23 and 16 S rRNAs of chloroplasts to a much greater extent than into the 25 and 18 S cytoplasmic rRNAs. Rifampicin inhibited [35S] metionine incorporation into Fraction I protein after the third hour of incubation; streptolydigin after 2 h, the former evidently preventing initiation of mRNA for the large subunit of Fraction I protein and the latter its elongation. About 2.5 h was required between initiation of the large subunit mRNA synthesis, and appearance of the protein. It was estimated that 45 min is required for transcription of the mRNA which has a half-life of 2 h and that 105 min is required for its translation into approximately 350 amino acids constituting the large subunit monomeric polypeptide. The effect of chloramphenicol, cycloheximide and 2-(4-methyl-2,6-dinitroanaline)-N-methyl propionamide, the latter an inhibitor of protein initiation by 80 S ribosomes, on kinetics of Fraction I protein synthesis indicate that protoplasts contain a pool of small subunit polypeptides and that 30 min is required to polymerize the approximately 100 amino acids constituting the primary structure.

The sites of synthesis of the principal thylakoid membrane polypeptides in Chlamydomonas reinhardtii

The sites of synthesis of the major thylakoid membrane polypeptides have been studied in the green alga Chlamydomonas reinhardtii by pulse labeling of cells with [14C]acetate in the presence of inhibitors specific for chloroplast and cytoplasmic protein synthesis. The labeled membrane polypeptides were separated by an improved method of sodium dodecyl sulfate (SDS) gradient gel electrophoresis, and autoradiographs were made of the dried gels. The results demonstrate that of the 33 polypeptides resolved in the gels, at least nine are made on chloroplast ribosomes. Two of these (polypeptides 2 and 6) are associated with the reaction centers of photosystems I and II. Another polypeptide (polypeptide 5) appears from genetic data to be coded by chloroplast DNA. Experiments with a mutant whose chloroplast ribosomes are resistant to spectinomycyn (spr-u-1-6-2) show that polypeptides whose synthesis takes place on chloroplast ribosomes are made in the presence of spectinomycin in the mutant although their synthesis is blocked by this antibiotic in wild type cells.

Changes in polypeptide initiation and elongation rates during the cell cycle of Chlamydomonas reinhardi

Changes in the rate of protein synthesis during the cell cycle of Chlamydomonas reinhardi have been measured by determining changes in the separate rates of polypeptide chain initiation and elongation and in the rate of incorporation of a radioactive amino acid. The rate of polypeptide chain elongation, determined from the relative rates of labeling of two size classes of polyribosomes, varies only about twofold during the cell cycle. The rate of polypeptide chain initiation, determined from an analysis of the distribution of ribosomes in monoribosomes (and ribosomal subunits) and polyribosomes, varies more than 25-fold. Also, the overall rate of protein synthesis during the cell cycle varies to the same extent as the rate of chain initiation. Measurement of protein synthetic rates using incorporation of a radioactive amino acid ([3H]arginine) underestimates the actual change in the rate of protein synthesis during the cell cycle. The vast changes in the initiation rate during the cell cycle suggest a mechanism for selecting specific messenger RNAs for translation at different cell-cycle stages.

In vitro synthesis and processing of a putative precursor for the small subunit of ribulose-1,5-bisphosphate carboxylase of Chlamydomonas reinhardtii

Translation of polyadenylated mRNA of Chlamydomonas reinhardtii in a cell-free wheat germ system resulted in the synthesis of numerous discrete polypeptides. Among them was a species with molecular weight 20,000 that was immunoprecipitated specifically by antibodies raised against the authentic small subunit (16,500 daltons) of ribulose-1,5-bisphosphate carboxylase [3-phospho-D-glycerate carboxy-lyase(dimerizing), EC 4.1.1.39]. Since the immunoprecipitated polypeptide has a larger molecular weight by approximately 3500 than the small subunit (S) it was identified as a putative biosynthetic precursor (pS). Post-translational conversion of pS by a specific endoprotease yielded two detectable products: one apparently identical in size to S and the other, a small peptide, presumably representing the remainder of pS. The endoprotease requires sulfhydryl groups for its activity and is present in a C. reinhardtii postribosomal supernatant as well as in a free polysome fraction. The latter could account for the observation that completion of nascent chains in free polysomes yielded S but not pS. We propose that pS is an extrachloroplastic form of S and that the small peptide portion plays a role in the transfer of S into the chloroplast.

The cell cycle program of polypeptide labeling in Chlamydomonas reinhardtii

The cell cycle program of polypeptide labeling in syndhronous cultures of wild-type Chlamydomonas reinhardtii was analyzed by pulse-labeling cells with 35SO4 = or [3H]arginine at different cell cycle stages. Nearly 100 labeled membrane and soluble polypeptides were resolved and studied using one-dimensional sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The labeling experiments produced the following results. (a) Total 35SO4 = and [3H]arginine incorporation rates varied independently throughout the cell cycle. 35SO4 = incorporation was highest in the mid-light phase, while [3H]arginine incorporation peaked in the dark phase just before cell division. (b) The relative labeling rate for 20 of 100 polypeptides showed significant fluctuations (3-12 fold) during the cell cycle. The remaining polypeptides were labeled at a rate commensurate with total 35SO4 = or [3H]arginine incorporation. The polypeptides that showed significant fluctuations in relative labeling rates served as markers to identify cell cycle stages. (c) The effects of illumination conditions on the apparent cell cycle stage-specific labeling of polypeptides were tested. Shifting light-grown asynchronous cells to the dark had an immediate and pronounced effect on the pattern of polypeptide labeling, but shifting dark-phase syndhronous cells to the light had little effect. The apparent cell cycle variations in the labeling of ribulose 1,5-biphosphate (RUBP)-carboxylase were strongly influenced by illumination effects. (d) Pulse-chase experiments with light-grown asynchronous cells revealed little turnover or inter-conversion of labeled polypeptides within one cell generation, meaning that major polypeptides, whether labeled in a stage-specific manner or not, do not appear transiently in the cell cycle of actively dividing, light-grown cells. The cell cycle program of labeling was used to analyze effects of a temperature-sensitive cycle blocked (cb) mutant. A synchronous culture of ts10001 was shifted to restrictive temperature before its block point to prevent it from dividing. The mutant continued its cell cycle program of polypeptide labeling for over a cell generation, despite its inability to divide.

Periodic variations in the ratio of free to thylakoid-bound chloroplast ribosomes during the cell cycle of Chlamydomonas reinhardtii

The ratio of free to thylakoid-bound chloroplast ribosomes in Chlamydomonas reinhardtii undergoes periodic changes during the synchronous light-dark cycle. In the light, when there is an increase in the chlorophyll content and synthesis of thylakoid membrane proteins, about 20-30% of the chloroplast ribosomes are bound to the thylakoid membranes. On the other hand, only a few or no bound ribosomes are present in the dark when there is no increase in the chlorophyll content. The ribosome-membrane interaction depends not only on the developmental stage of the cell but also on light. Thus, bound ribosomes were converted to the free variety after cultures at 4 h in the light had been transferred to the dark for 10 min. Conversely, a larger number of chloroplast ribosomes became attached to the membranes after cultures at 4 h in the dark had been illuminated for 10 min. Under normal conditions, when there was slow cooling of the cultures during cell harvesting, chloroplast polysomal runoff occurred in vivo leading to low levels of thylakoid-bound ribosomes. This polysomal runoff could be arrested by either rapid cooling of the cells or the addition of chloramphenicol or erythromycin. Each of these treatments prevented polypeptide chain elongation on chloroplast ribosomes and thus allowed the polyosomes to remain bound to the thylakoids. Addition of lincomycin, an inhibitor of chain initiation on 70S ribosomes, inhibited the assembly of polysome-thylakoid membrane complex in the light. These results support a model in which initiation of mRNA translation begins in the chloroplast stroma, and the polysome subsequently becomes attached to the thylakoid membrane. Upon natural chain termination, the chloroplast ribosomes are released from the membrane into the stroma.

Incorporation of polypeptides into thylakoid membranes of Chlamydomonas reinhardtii. Cyclic variations

A purified fraction of unstacked thylakoid membranes (TMF1u) has been obtained from homogenates of Chlamydomonas reinhardtii (wild type 137+) by using repeated centrifugates in sucrose density gradients and low salt treatment. The contaminants of the fraction are reduced to a few mitochondria (approximately 3% of the total mitochondrial population), a few osmiophilic granules, and fragments of chloroplast envelopes. By SDS-polyacrylamide gel electrophoresis the polypeptide components of TMF1u were resolved into at least 30 bands. To determine the relative rates of assembly of newly synthesized polypeptides into thylakoid membranes, synchronized algal cells were doubly labeled in vivo with L-[14C] and L-[3H]arginine--used for long- and short-term labeling, respectively. TMF1u's were isolated from the labeled cells at selected time points during the cycle and the distribution of radioactivity was assayed in the gel electrophoretograms of their solubilized polypeptides. Incorporation of newly synthesized polypeptides into the bands of the gels was found to occur continuously but differentially throughout the cycle. Maximal rates of incorporation for the majority of the polypeptides were detected shortly after cell division (6D-7D; equivalent to early G1 phase). The rates of radioactive labeling decreased gradually to a low level at the end of the dark period and then rose slightly at the beginning of the next light period. The findings suggest that, in addition to the light/dark control postulated in the past, assembly of newly synthesized proteins into thylakoid membranes is activated by signals at work in the early G1 phase.

Synchronous Growth and Plastid Replication in the Naturally Wall-less Alga Olisthodiscus luteus

Olisthodiscus luteus is a unicellular biflagellate alga which contains many small discoidal chloroplasts. This naturally wall-less organism can be axenically maintained on a defined nonprecipitating artificial seawater medium. Sufficient light, the presence of bicarbonate, minimum mechanical turbulence, and the addition of vitamin B(12) to the culture medium are important factors in the maintenance of a good growth response. Cells can be induced to divide synchronously when subject to a 12-hour light/12-hour dark cycle. The chronology of cell division, DNA synthesis, and plastid replication has been studied during this synchronous growth cycle. Cell division begins at hour 4 in the dark and terminates at hour 3 in the light, whereas DNA synthesis initiates 3 hours prior to cell division and terminates at hour 10 in the dark. Synchronous replication of the cell's numerous chloroplasts begins at hour 10 in the light and terminates almost 8 hours before cell division is completed. The average number of chloroplasts found in an exponentially growing synchronous culture is rather stringently maintained at 20 to 21 plastids per cell, although a large variability in plastid complement (4-50) is observed within individual cells of the population. A change in the physiological condition of an Olisthodiscus cell may cause an alteration of this chloroplast complement. For example, during the linear growth period, chloroplast number is reduced to 14 plastids per cell. In addition, when Olisthodiscus cells are grown in medium lacking vitamin B(12), plastid replication continues in the absence of cell division thereby increasing the cell's plastid complement significantly.