Processing of the precursor to a chloroplast ribosomal protein made in the cytosol occurs in two steps, one of which depends on a protein made in the chloroplast

A conserved mechanism controls translation of Rubisco large subunit in different photosynthetic organisms

We previously proposed a mechanism for control of Rubisco expression and assembly during oxidative stress in Chlamydomonas reinhardtii. The N terminus of the large subunit (LSU) comprises an RNA recognition motif (RRM) that is normally buried in the protein, but becomes exposed under oxidizing conditions when the glutathione pool shifts toward its oxidized form. Thus, de novo translation and assembly of Rubisco LSU stop with similar kinetics and the unpaired small subunit (SSU) is rapidly degraded. Here we show that the structure of the N-terminal domain is highly conserved throughout evolution, despite its relatively low sequence similarity. Furthermore, Rubisco from a broad evolutionary range of photosynthetic organisms binds RNA under oxidizing conditions, with dissociation constant values in the nanomolar range. In line with these observations, oxidative stress indeed causes a translational arrest in land plants as well as in Rhodospirillum rubrum, a purple bacterium that lacks the SSU. We highlight an evolutionary conserved element located within alpha-helix B, which is located in the center of the RRM and is also involved in the intramolecular interactions between two LSU chains. Thus, assembly masks the N terminus of the LSU hiding the RRM. When assembly is interrupted due to structural changes that occur under oxidizing conditions or in the absence of a dedicated chaperone, the N-terminal domain can become exposed, leading to the translational arrest of Rubisco LSU. Taken together, these results support a model by which LSU translation is governed by its dimerization. In the case that regulation of type I and type II Rubisco is conserved, the SSU does not appear to be directly involved in LSU translation.

A proposed mechanism for the inhibitory effects of oxidative stress on Rubisco assembly and its subunit expression

In Chlamydomonas reinhardtii, a light-induced oxidative stress shifts the glutathione pool toward its oxidized form, resulting in a translational arrest of the large subunit (LSU) of Rubisco. We show here that the translational arrest of LSU is tightly coordinated with cessation of Rubisco assembly, and both processes take place after a threshold level of reactive oxygen species is reached. As a result, the small subunit is also eliminated by rapid degradation. We previously showed that the amino terminus of the LSU could bind RNA in a sequence-independent manner, as it shares a structural similarity with the RNA recognition motif. This domain becomes exposed only under oxidizing conditions, thus restricting the RNA-binding activity. Here we show that in vitro, thiol groups of both subunits become oxidized in the presence of oxidized glutathione. The structural changes are mediated by oxidized glutathione, whereas only very high concentrations of H2O2 confer similar results in vitro. Changes in the redox state of the LSU thiol groups are also observed in vivo, in response to a physiological light shock caused by transfer of cells from low light to high light. We propose that during a photooxidative stress, oxidation of thiol groups occurs already in nascent LSU chains, perhaps hindering their association with chaperones. As a result, their RNA recognition motif domain becomes exposed and will bind any RNA in its vicinity, including its own transcript. Due to this binding the ribosome stalls, preventing the assembly of additional ribosomes on the transcript. Polysome analysis using Suc gradients indeed shows that the rbcL RNA is associated with the polysomal fraction at all times but shifts toward fractions that contain smaller polysomes and monosomes during oxidative stress. Thus, translational arrest of the LSU most likely occurs at a postinitiation stage.

Plastid state- and light-dependent regulation of the expression of nucleus-encoded genes for chloroplast proteins in the flagellate Euglena gracilis

Interorganellar regulatory interactions in the flagellate Euglena gracilis were shown to be more complicated than in green algae and higher plants. Euglena plastids have a much more complex influence on nuclear gene expression than was previously thought. The petJ gene for cytochrome c6 represents a group of nucleus-encoded genes for chloroplast proteins, the expression of which is influenced by the state of plastids at the transcriptional level. Moroever, the regulation of these genes might be light-dependent. In contrast, for nucleus-encoded small subunit of ribulose-bisphosphate carboxylase, chlorophyll a/b binding protein, and porphobilinogen deaminase transcript levels are unchanged in wild-type cells relative to white mutants. In these cases there is no plastid-derived signal operative during transcription. Porphobilinogen deaminase appeared to be regulated even at the post-translational level.

Processing of an apicoplast leader sequence in Plasmodium falciparum and the identification of a putative leader cleavage enzyme

The plastid (apicoplast) of the malaria-causing parasite Plasmodium falciparum was derived via a secondary endosymbiotic process. As in other secondary endosymbionts, numerous genes for apicoplast proteins are located in the nucleus, and the encoded proteins are targeted to the organelle courtesy of a bipartite N-terminal extension. The first part of this leader sequence is a signal peptide that targets proteins to the secretory pathway. The second, so-called transit peptide region is required to direct proteins from the secretory pathway across the multiple membranes surrounding the apicoplast. In this paper we perform a pulse-chase experiment and N-terminal sequencing to show that the transit peptide of an apicoplast-targeted protein is cleaved, presumably upon import of the protein into the apicoplast. We identify a gene whose product likely performs this cleavage reaction, namely a stromal-processing peptidase (SPP) homologue. In plants SPP cleaves the transit peptides of plastid-targeted proteins. The P. falciparum SPP homologue contains a bipartite N-terminal apicoplast-targeting leader. Interestingly, it shares this leader sequence with a Delta-aminolevulinic acid dehydratase homologue via an alternative splicing event.

A mammalian cytochrome fused to a chloroplast transit peptide is a functional haemoprotein and is imported into isolated chloroplasts

The small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is a major chloroplast stromal protein that is cytosolically synthesized as a precursor with an N-terminal extension, known as the transit sequence or transit peptide (Tp). The Tp is essential for the post-translational uptake of the precursor by the chloroplast. The Tp is thought to influence the conformation of the precursor protein and to facilitate polypeptide translocation across the chloroplast envelope barrier via a Tp-selective translocon. To address these issues we have devised a novel strategy to generate substrate amounts of a chloroplast targeting sequence as a fusion with the chromogenic globular domain of cytochrome b(5) (Cyt). The chimaeric protein is an ideal probe for investigating the conformation of a preprotein and events surrounding protein import into isolated chloroplasts. The Cyt of liver endoplasmic reticulum was fused at its N-terminus with the Tp of the small subunit of Rubisco of Pisum sativum (pea). To enhance its production by clearance from the cytoplasm of Escherichia coli, the chimaera was engineered by further N-terminal linkage of a prokaryotic secretory signal. Expression of this tripartite fusion resulted in mg quantities of the signal sequence-processed Tp-Cyt protein, which was eventually targeted to the membranes. The chromogenic nature of the chimaera and its localization to the bacterial membrane facilitated the biochemical isolation of the precursor in a soluble and functional form. The purified preprotein displayed spectral and enzymic properties that were indistinguishable from the native parental Cyt, implying an absence of observable influence of the Tp on the conformation of the haemoprotein. The chimaeric precursor was imported into the stroma of the isolated chloroplasts in a dose-dependent manner. Import was also strongly dependent upon exogenously supplied ATP. The stromally imported chimaeric precursor protein was processed to a size characteristic of Cyt.

Acclimation of the photosynthetic machinery to high temperature in Chlamydomonas reinhardtii requires synthesis de novo of proteins encoded by the nuclear and chloroplast genomes

The mechanism responsible for the enhancement of the thermal stability of the oxygen-evolving machinery of photosystem II during acclimation of Chlamydomonas reinhardtii to high temperatures such as 35 degrees C remains unknown. When cells that had been grown at 20 degrees C were transferred to 35 degrees C, the thermal stability of the oxygen-evolving machinery increased and within 8 h it was equivalent to that in cells grown initially at 35 degrees C. Such enhancement of thermal stability was prevented by cycloheximide and by lincomycin, suggesting that the synthesis de novo of proteins encoded by both the nuclear and the chloroplast genome was required for this process. No increase in thermal stability was observed when cells that had been grown at 35 degrees C were exposed to heat shock at 41 degrees C, optimum conditions for the induction of the synthesis of homologs of three heat shock proteins (Hsps), namely, Hsp60, Hsp70, and Hsp22. Moreover, no synthesis of these homologs of Hsps was induced at 35 degrees C. Thus it appears likely that Hsps are not involved in the enhancement of the thermal stability of the oxygen-evolving machinery.

Glutathione redox potential modulated by reactive oxygen species regulates translation of Rubisco large subunit in the chloroplast

Previous work showed a transient but dramatic arrest in the synthesis of Rubisco large subunit (LSU) upon transfer of Chlamydomonas reinhardtii cells from low light (LL) to high light (HL). Using dichlorofluorescin, a short-term increase in reactive oxygen species (ROS) was demonstrated, suggesting that their excessive formation could signal LSU down-regulation. A decrease in LSU synthesis occurred at LL in the presence of methyl viologen and was prevented at HL by ascorbate. Interfering with D1 function by mutations or by incubation with DCMU prevented the increase in ROS formation at HL and the concomitant down-regulation of LSU synthesis. If the electron transport was blocked further downstream, by mutation in the cytochrome b(6)/f or by incubation with 2, 5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, ROS formation increased, and LSU synthesis ceased. The elevation of ROS occurred concurrently with a change in the redox state of the glutathione pool, which shifted toward its oxidized form immediately after the transfer to HL and returned to its original value after 6 h. The decrease in the reduced/oxidized glutathione ratio at HL was prevented by ascorbate and could be induced at LL by methyl viologen. We suggest that excess ROS mediate a decrease in the reduced/oxidized glutathione ratio that in turn signals the translational arrest of the rbcL transcript.

Processing of a composite large subunit rRNA. Studies with chlamydomonas mutants deficient in maturation of the 23s-like rrna

(Cr.LSU). Little is known of the cis and trans requirements or of the processing pathway for this essential RNA. Previous work showed that the ribosome-deficient ac20 mutant overaccumulates an unspliced large subunit (LSU) RNA, suggesting that it might be a splicing mutant. To elucidate the molecular basis of the ac20 phenotype, a detailed analysis of the rrn transcripts in ac20 and wild-type cells was performed. The results indicate that processing of the ITSs, particularly ITS-1, is inefficient in ac20 and that ITS processing occurs after splicing. Deletion of the Cr.LSU intron from ac20 also did not alleviate the mutant phenotype. Thus, the primary defect in ac20 is not splicing but most likely is associated with ITS processing. A splicing deficiency was studied by transforming wild-type cells with rrnL genes containing point mutations in the intron core. Heteroplasmic transformants were obtained in most cases, except for P4 helix mutants; these strains grew slowly, were light sensitive, and had an RNA profile indicative of inefficient splicing. Transcript analysis in the P4 mutants also indicated that ITS processing can occur on an unspliced precursor, although with reduced efficiency. These latter results indicate that although there is not an absolutely required order for LSU processing, there does seem to be a preferred order that results in efficient processing in vivo.

Differential regulation of chloroplast gene expression in Chlamydomonas reinhardtii during photoacclimation: light stress transiently suppresses synthesis of the Rubisco LSU protein while enhancing synthesis of the PS II D1 protein

Transfer of Chlamydomonas reinhardtii cells grown photoautotrophically in low light to higher light intensities has a dramatic transient effect on the differential expression of the two major chloroplast encoded photosynthetic proteins. Synthesis of the D1 protein of Photosystem II increases more than 10-fold during the first six hours in high light (HL), whereas synthesis of the large subunit (LSU) of Rubisco drops dramatically within 15 min and only gradually resumes at about 6 h. Synthesis of the chloroplast-encoded ATP synthase beta subunit, the nuclear-encoded Rubisco small subunit and the nuclear-encoded beta-tubulin is not noticeably affected. Up regulation of psbA mRNA translation accounts for a substantial fraction of the increased D1 synthesis, since accumulation of psbA mRNA increases 4.2- and 6.3-fold less than D1 synthesis at 6 and 18 h in HL. Down-regulation of LSU synthesis is not correlated with a reduction in the steady-state level of the rbcL transcript. Primer extension mapping of the 5' ends of the rbcL mRNAs reveals transcripts with start points located at -93 and -168 relative to the first translated ATG. Transfer of low light (LL)-grown cells to HL temporarily decreases the ratio of the -93 to -168 transcripts, but this ratio normalizes after 6 h in HL, coincident with the recovery in the synthesis of LSU. These several distinct effects of temporary light stress were correlated with a rapid, sustained increase in the reduction state of QA, a transient decline in photosynthetic efficiency, a less rapid drop in total chlorophyll content and a delay in cell division.

Site-directed mutations at residue 251 of the photosystem II D1 protein of Chlamydomonas that result in a nonphotosynthetic phenotype and impair D1 synthesis and accumulation

In Cyanobacteria and Chlamydomonas reinhardtii, substitution of valine for alanine at position 251 of the photosystem II D1 protein in the loop between transmembrane helices IV and V confers resistance to herbicides that reduce photosystem II function and increases sensitivity to photoinhibition. Using site-directed mutagenesis and chloroplast transformation in Chlamydomonas we have examined further the role of residue 251 in relation to D1 structure, function, and photosynthetic performance. Of the 12 different amino acid substitutions for Ala251 introduced at this position, five (Arg, Asp, Gln, Glu, and His) resulted in a nonphotosynthetic phenotype. Transformants with the Arg251 substitution synthesize a normal sized 32-kDa D1 protein with greatly reduced stability. The Gln, Glu, His, and Asp transformants make a 33-34-kDa form of the D1 protein of varying stability as well as an immunologically related polypeptide of 24-25 kDa corresponding to the N-terminal portion of D1 that is unstable and appears to be an aborted D1 translation product. All mutant forms of the D1 protein are intrinsic to the thylakoids. In contrast to previous studies in Cyanobacteria showing that residues in the IV-V loop can be mutated or deleted without loss of photosynthetic competence, our results suggest that Ala251 has a key role in the structure and function of the IV-V loop region.

Chloroplast import and sequential maturation of pea carbonic anhydrase: the roles of various parts of the transit peptide

Chloroplast pea carbonic anhydrase is synthesised in the cytosol with an unusually long bipartite N-terminal extension of the mature sequence previously proposed to serve as a transit peptide. Studies of import into pea chloroplasts show that the N-terminal 69 amino acids of the previously proposed transit peptide is sufficient for translocation and localisation to the stroma, while the acidic C-terminal part does not seem to have any function in these processes. Processing of the in vitro imported precursors is shown to be at a new cleavage site located in the middle of the actual transit peptide. The results indicate that maturation occurs in more than one step. The time-course does not seem to be dependent on the age of the chloroplast but on the age of the translocated precursor.

Chloroplast ribosomes and protein synthesis

Consistent with their postulated origin from endosymbiotic cyanobacteria, chloroplasts of plants and algae have ribosomes whose component RNAs and proteins are strikingly similar to those of eubacteria. Comparison of the secondary structures of 16S rRNAs of chloroplasts and bacteria has been particularly useful in identifying highly conserved regions likely to have essential functions. Comparative analysis of ribosomal protein sequences may likewise prove valuable in determining their roles in protein synthesis. This review is concerned primarily with the RNAs and proteins that constitute the chloroplast ribosome, the genes that encode these components, and their expression. It begins with an overview of chloroplast genome structure in land plants and algae and then presents a brief comparison of chloroplast and prokaryotic protein-synthesizing systems and a more detailed analysis of chloroplast rRNAs and ribosomal proteins. A description of the synthesis and assembly of chloroplast ribosomes follows. The review concludes with discussion of whether chloroplast protein synthesis is essential for cell survival.

A nuclear mutation conferring thiostrepton resistance in Chlamydomonas reinhardtii affects a chloroplast ribosomal protein related to Escherichia coli ribosomal protein L11

We have isolated a nuclear mutant (tsp-1) of Chlamydomonas reinhardtii which is resistant to thiostrepton, an antibiotic that blocks bacterial protein synthesis. The tsp-1 mutant grows slowly in the presence or absence of thiostrepton, and its chloroplast ribosomes, although resistant to the drug, are less active than chloroplast ribosomes from the wild type. Chloroplast ribosomal protein L-23 was not detected on stained gels or immunoblots of total large subunit proteins from tsp-1 probed with antibody to the wild-type L-23 protein from C. reinhardtii. Immunoprecipitation of proteins from pulse-labeled cells showed that tsp-1 synthesizes small amounts of L-23 and that the mutant protein is stable during a 90 min chase. Therefore the tsp-1 phenotype is best explained by assuming that the mutant protein synthesized is unable to assemble into the large subunit of the chloroplast ribosome and hence is degraded over time. L-23 antibodies cross-react with Escherichia coli r-protein L11, which is known to be a component of the GTPase center of the 50S ribosomal subunit. Thiostrepton-resistant mutants of Bacillus megaterium and B. subtilis lack L11, show reduced ribosome activity, and have slow growth rates. Similarities between the thiostrepton-resistant mutants of bacteria and C. reinhardtii and the immunological relatedness of Chlamydomonas L-23 to E. coli L11 suggest that L-23 is functionally homologous to the bacterial r-protein L11.

Partial purification and properties of enzymes involved in the processing of a chloroplast import protein from Chlamydomonas reinhardii

Two stromal peptidases (SPP-1 and SPP-2) were partially purified from chloroplasts of Chlamydomonas reinhardii. They specifically processed in vitro the precursor of the small subunit of ribulose-1,5-bisphosphate carboxylase (pSS), which had been synthesized by using the cloned rbcS-2 gene of Chlamydomonas. SPP-1 shortened pSS to an intermediate-sized form (iSS), while SPP-2 cut pSS and iSS to the mature small subunit SS. N-terminal amino acid sequencing demonstrated that the reaction product obtained with SPP-2 had an N-terminus identical to natural SS, and that iSS derived from pSS by hydrolysis at the amino side of the methionine located within the transit sequence. By gel filtration, apparent molecular masses of 340 kDa and 90 kDa were determined for SPP-1 and SPP-2, respectively. The comparison of these molecular masses with the protein patterns obtained by SDS/PAGE of the partially purified enzymes suggested that at least SPP-1 was a multimeric protein. The enzymes differed also in their pH optima of about 8 (SPP-1) and 9 (SPP-2) and in their sensitivity to different inhibitors. However, both enzymes seem to be serine proteases as they were completely blocked by N-alpha-tosyl-L-lysinechloromethane or tosylphenylalaninechloromethane, respectively. Competition experiments, using either mature SS or a synthetic hexadecapeptide with 15 amino acids similar to the C-terminal end of the transit sequence of pSS, indicated that SPP-2 had some affinities not only to the transit sequence of pSS, but especially to sequences in the mature protein part. We conclude that SPP-2 in Chlamydomonas is the enzyme involved in import of pSS into chloroplasts and responsible for its processing by a one-step mechanism.

Processing of the chloroplast transit peptide of pea carbonic anhydrase in chloroplasts and in Escherichia coli. Identification of two cleavage sites

The chloroplast transit peptide (cTP) of pea carbonic anhydrase was shown to be processed at two different sites, giving protein subunits of two sizes. The cleavage sites were identified and found to be localized immediately before and after a highly charged part, containing 8 acidic and 6 basic residues, of the cTP. Properties of pea carbonic anhydrase produced in Escherichia coli show that folding, oligomerization and catalytic activity do not depend on the presence of the acidic part or the rest of the cTP. The pattern of processing of the cTP in E. coli indicates that cleavage at site I is specific for a chloroplastic stromal peptidase and that cleavage at site I prevents processing at site II.

Nuclear-encoded chloroplast ribosomal protein L27 of Nicotiana tabacum: cDNA sequence and analysis of mRNA and genes

A tobacco (Nicotiana tabacum cv. Petite Havana) leaf cDNA library was constructed in the expression vector lambda gt11. Immunological and nucleic acid hybridization screening yielded several cDNAs encoding an M(r) 19,641 precursor to an M(r) 14,420 mature protein which is homologous to Escherichia coli ribosomal protein L27. One cDNA (L27-1; 882 nucleotides long) contains 104 bp of 5'-noncoding sequence, 51 codons for a transit peptide, 128 codons for the predicted mature L27 polypeptide, and 241 bp of 3'-noncoding sequence, including the poly(A)29 tail. A beta-galactosidase-L27 fusion protein was bound to nitrocellulose filters, expressed, and used as an affinity matrix to purify monospecific antibody to L27 protein from an antiserum of rabbits immunized with 50S chloroplast ribosomal proteins. Using this monospecific antibody, protein L27 was identified among HPLC-purified tobacco chloroplast ribosome 50S subunit proteins. The predicted amino terminus of the mature L27 protein was confirmed by partial sequencing of the HPLC-purified L27 protein. The mature L27 protein has 66%, 61%, 56%, and 48% amino acid sequence identity with the L27-type ribosomal proteins of Bacillus subtilis, E. coli, Bacillus stearo-thermophilus, and yeast mitochondria (MRP7), respectively, in the homologous overlapping regions. The transit peptide of tobacco chloroplast ribosomal protein L27 has 41% amino acid sequence similarity with the MRP7 mitochondrial targeting sequence. Tobacco chloroplast L27 protein also has a 40 amino acid long carboxyl-terminal extension (compared to its bacterial counterparts) which is similar to the corresponding portion of yeast MRP7.(ABSTRACT TRUNCATED AT 250 WORDS)

mRNAs for two ribosomal proteins are preferentially translated in the chloroplast of Chlamydomonas reinhardtii under conditions of reduced protein synthesis

Two mutants of the green alga Chlamydomonas reinhardtii, one deficient in the small subunit of the chloroplast ribosome and the other having chloroplast ribosomes with reduced function under certain conditions, show a characteristic syndrome of photosynthetic defects resulting from reduced chloroplast protein synthesis. These include subnormal levels of ribulose 1,5-bisphosphate carboxylase (Rubisco), reduced Hill reaction activity, diminished capacity to fix CO2, and abnormal thylakoid stacking. However, these mutants accumulate normal appearing chloroplast ribosome monomers or large subunits containing normal ribosomal protein components. In this paper, we demonstrate that pulse-labeled cells of these mutants synthesize two large subunit chloroplast ribosomal proteins at about 60% of the wild-type rate, whereas Rubisco large subunit (LSU) and the alpha subunit of CF1 are made at only 4 to 8% of the wild-type rate. No difference in the rate of turnover between ribosomal proteins and Rubisco LSU in mutant and wild-type cells was observed during a subsequent 60 min chase. Differences between the mutants and wild-type cells in the relative synthesis rates of these proteins were not reflected in the relative levels of mRNA (either hybridizable or in vitro translatable). In aggregate, these data suggest that C. reinhardtii preferentially translates chloroplast ribosomal protein mRNAs under conditions of reduced total chloroplast protein synthesis.

Domain structure of mitochondrial and chloroplast targeting peptides

Representative samples of mitochondrial and chloroplast targeting peptides have been analyzed in terms of amino acid composition, positional amino acid preferences and amphiphilic character. No highly conserved 'homology blocks' are found in either class of topogenic sequence. Mitochondrial-matrix-targeting peptides are composed of two domains with different amphiphilic properties. Arginine is frequently found either at position -10 or -2 relative to the cleavage site, suggesting that some targeting peptides may be cleaved twice in succession by two different matrix proteases. In stroma-targeting chloroplast transit peptides three distinct regions are evident: an uncharged amino-terminal domain, a central domain lacking acidic residues and a carboxy-terminal domain with the potential to form an amphiphilic beta-strand. Targeting peptides that route proteins to the mitochondrial intermembrane space or the lumen of chloroplast thylakoids have a mosaic design with an amino-terminal matrix- or stroma-targeting part attached to a carboxy-terminal extension that shares many characteristics with secretory signal peptides.

Unassembled polypeptides of the plastidic ribosomes in heat-treated 70S-ribosome-deficient rye leaves

The polypeptides of the subunits of 70S ribosomes isolated from rye (Secale cereale L.) leaf chloroplasts were analyzed by two-dimensional polyacrylamide gel electrophoresis. The 50S subunit contained approx. 33 polypeptides in the range of relative molecular mass (Mr) 13000-36000, the 30S subunit contained approx. 25 polypeptides in the range of Mr 13000-40500. Antisera raised against the individual isolated ribosomal subunits detected approx. 17 polypeptides of the 50S and 10 polypeptides of the 30S subunit in the immunoblotting assay. By immunoblotting with these antisera the major antigenic ribosomal polypeptides (r-proteins) of the chloroplasts were clearly and specifically visualized also in separations of leaf extracts or soluble chloroplast supernatants. In extracts from rye leaves grown at 32° C, a temperature which is non-permissive for 70S-ribosome formation, or in supernatants from ribosome-deficient isolated plastids, six plastidic r-proteins were visualized by immunoblotting with the anti-50S-serum and two to four plastidic r-proteins were detected by immunoblotting with the anti-30S-serum, while other r-proteins that reacted with our antisera were missing. Those plastidic r-proteins that were present in 70S-ribosome-deficient leaves must represent individual unassembled ribosomal polypeptides that were synthesized on cytoplasmic 80S ribosomes. For the biogenesis of chloroplast ribosomes the mechanism of coordinate regulation appear to be less strict than those known for the biogenesis of bacterial ribosomes, thus allowing a marked accumulation of several unassembled ribosomal polypeptides of cytoplasmic origin.

Chloroplast ribosomal protein L-18 in Chlamydomonas reinhardtii is processed during ribosome assembly

Chloroplast ribosomal protein L-18 is made in the cytoplasm as a precursor, imported into the chloroplast, and processed to the mature form in two steps. We report here that the intermediate produced following the first processing step associates specifically with a ribosomal complex migrating with the chloroplast ribosome large subunit peak in sucrose gradients, and is then processed into mature L-18. This processing event is slowed down in mutant cells deficient in synthesis of non-ribosomal proteins in the chloroplast. Thus the second processing step of L-18 occurs during ribosome assembly, depends on one or more nonribosomal proteins made in the chloroplast, and may be required for the maturation of the 50 S ribosome subunit. The mature L-18 protein shows extensive sequence homology at its amino-terminus to Escherichia coli ribosomal protein L27, which is located at the interface between 30 S and 50 S subunits and is involved in the formation of the peptidyl-tRNA binding site.

The chloroplast-located glutamine synthetase of Phaseolus vulgaris L.: nucleotide sequence, expression in different organs and uptake into isolated chloroplasts

Work using a full-length cDNA clone has revealed that the plastid-located glutamine synthetase (GS) of Phaseolus vulgaris is encoded by a single nuclear gene. Nucleotide sequencing has shown that this cDNA is more closely related to a cDNA encoding the plastidic GS of Pisum sativum than to cDNAs encoding three different cytosolic GS subunits of P. vulgaris. The plastid GS subunits are initially synthesized as higher M r (47000) precursors containing an N-terminal presequence of about 50 amino acids which is structurally similar to the presequences of other nuclear-encoded chloroplast proteins. The precursor has been synthesized in vitro and is imported by isolated pea chloroplasts and processed to two polypeptides of the same size as native P. vulgaris chloroplast GS subunits (M r 42000). Experiments with fusion proteins show that the N-terminal 68 amino acids of this precursor allow the cytosolic GS subunit β also to be imported and processed by isolated chloroplasts. Polyadenylated mRNA specifically related to the plastidic GS gene is most highly abundant in chloroplast-containing organs (leaves and stems) but is also detectable in roots and nodules.

Ultrastructural studies of physiologically identified electrosensory afferent synapses in the gymnotiform fish, Eigenmannia

Eigenmannia is a weakly electric fish that emits a constant-frequency electric organ discharge (EOD). Probability coder (P unit) and phase coder (T unit) electroreceptive afferents differentially encode changes in EOD amplitude and phase, respectively. physiologically identified T and P units were intracellularly labelled with HRP and their terminals were examined with electron microscopy to determine their postsynaptic targets. This technique reveals that phase and amplitude are relayed to first-order electrosensory neurons by two parallel but not independent pathways. P-type afferents terminate on granular interneurons, basilar pyramidals, and polymorphic cells, electrosensory lateral line lobe targets that monitor amplitude modulations, but P-type afferents do not contact spherical cells. T-type afferents relay phase information to spherical cells and thus form a separate afferent pathway. T unit terminals do not synapse directly on basilar pyramidal cells. Collateral branches from T-type afferents, however, were also found to terminate on granule and polymorphic cells, thereby adding phase information into the amplitude channel. P- and T-type afferents exhibit cellular specificity by forming synaptic junctions with different subsets of post synaptic targets in the deep neuropil. The afferent terminals make either asymmetric chemical or gap junction synapses depending on the identity of the post synaptic target. T units contacting granule cells or polymorphic cells had not been previously described. Two possible roles of adding phase to amplitude information are discussed in terms of electrolocation.

Isolation of nuclear encoded plastid ribosomal protein cDNAs

A pea leaf cDNA library was constructed in the expression vector lambda gt11 and screened with antisera raised against proteins extracted from 30S and 50S ribosomal subunits and 70S ribosomes prepared from isolated pea chloroplasts. Six recombinant phage were identified that encoded fusion proteins containing plastid ribosomal protein antigenic determinants. Phage-induced cell lysate proteins, containing the fusion proteins, were bound to nitrocellulose membranes and used as affinity matrices to prepare monospecific antibodies. These antibodies were then used to identify by Western blotting which plastid ribosomal protein shared antigenic determinants with the fusion proteins. cDNA inserts from the antigen-producing phage were used to hybrid-select complementary mRNAs. The cell-free translation products of these mRNAs were added to a pea chloroplast in vitro transport system and imported proteins analyzed by two-dimensional gel electrophoresis. The imported proteins comigrated with the plastid ribosomal proteins that were identified as being antigenically related to the fusion proteins produced by the corresponding recombinant phage. The imported proteins were 3,500-5,500 daltons smaller than their precursors.