The plastid envelope membranes: their structure, composition, and role in chloroplast biogenesis

Esculetin Ameliorates Psoriasis-Like Skin Disease in Mice by Inducing CD4+Foxp3+ Regulatory T Cells

Psoriasis is an autoimmune and inflammatory skin disease affecting around 2-3% of the world's population. Patients with psoriasis need extensive treatments with global immunosuppressive agents that may cause severe side effects. Esculetin, a type of coumarins, is an active ingredient extracted mainly from the bark of Fraxinus rhynchophylla, which has been used to treat inflammatory and autoimmune diseases in China. However, the antipsoriatic effects of esculetin have not been reported. In this study, we aimed to investigate the effects of esculetin on psoriatic skin inflammation in a mouse model and explored the potential molecular mechanisms underlying its action. We found that esculetin ameliorated the skin lesion and reduced PASI scores as well as weight loss in imiquimod-induced psoriasis-like mice, accompanied with weakened proliferation and differentiation of keratinocytes and T cell infiltration in esculetin-treated psoriatic mice. In addition, esculetin reduced the frequency of CD8⁺CD44^highCD62L^low effector T cells in psoriatic mice. In contrast, it increased the frequency of CD4⁺Foxp3⁺ Tregs in both lymph nodes and spleens of the psoriatic mice while promoting the differentiation of CD4⁺CD25^- T cells into CD4⁺Foxp3⁺ Tregs in vitro. Interestingly, depleting CD4⁺Foxp3⁺ Tregs largely reversed esculetin-mediated reduction in PASI scores, indicating that esculetin attenuates murine psoriasis mainly by inducing CD4⁺Foxp3⁺ Tregs. Furthermore, the mRNA levels of proinflammatory cytokines in the psoriatic mouse skin, including IL-6, IL-17A, IL-22, IL-23, TNF-α, and IFN-γ, were dramatically decreased by the treatment with esculetin. Finally, we found that esculetin inhibited the phosphorylation of IKKα and P65 in the psoriatic skin, suggesting that it inhibits the activation of NF-κB signaling. Thus, we have demonstrated that esculetin attenuates psoriasis-like skin lesion in mice and may be a potential therapeutic candidate for the treatment of psoriasis in clinic.

Human VDAC isoforms differ in their capability to interact with minocycline and to contribute to its cytoprotective activity

It has been previously demonstrated that cytoprotective activity displayed by minocycline in the case of the yeast Saccharomyces cerevisiae cells pretreated with H2O2 requires the presence of functional VDAC (YVDAC1). Thus, we decided to transform YVDAC1-depleted yeast cells (Δpor1 cells) with plasmids expressing human VDAC isoforms (HVDAC1, HVDAC2, HVDAC3) to estimate their involvement in the minocycline cytoprotective effect. We observed that only expression of HVDAC3 in Δpor1 cells provided minocycline-mediated cytoprotection against H2O2 although all human isoforms are functional in Δpor1 cells. The observation appears to be important for on-going discussion concerning VDAC isoform roles in mitochondria and cell functioning.

Oep23 forms an ion channel in the chloroplast outer envelope

BACKGROUND

Metabolite, ion and protein translocation into chloroplasts occurs across two membranes, the inner and the outer envelope. Solute and metabolite channels fulfill very important functions in integrating the organelles into the metabolic network of the cell. However so far only a few have been identified. Here we describe the identification and the characterization of the outer envelope protein of 23 kDa, Oep23 from garden pea.

RESULTS

Oep23 is found in the entire plant lineage from green algae to flowering plants. It is expressed in all organs and developmental states tested so far. The reconstituted recombinant protein Oep23 from pea forms a high conductance ion channel with a maximal conductance in the fully open state of 466 ± 14pS at a holding potential of +100 mV (in 250 mM KCl). The Oep23 channel is cation selective (PK+ : PCl- = 15 : 1) with a voltage dependent open probability of maximal Vmem = 0 mV.

CONCLUSION

The data indicate that the Oep23 activity represents a single channel unit and does not assemble into a multiple pore complex like bacterial type porins or mitochondrial voltage dependent anion channel. Thus, Oep23 represents a new member of ion channels in the outer envelope of chloroplasts involved in solute exchange.

Contribution of galactoglycerolipids to the 3-dimensional architecture of thylakoids

Thylakoid membranes, the universal structure where photosynthesis takes place in all oxygenic photosynthetic organisms from cyanobacteria to higher plants, have a unique lipid composition. They contain a high fraction of 2 uncharged glycolipids, the galactoglycerolipids mono- and digalactosyldiacylglycerol (MGDG and DGDG, respectively), and an anionic sulfolipid, sulfoquinovosediacylglycerol (SQDG). A remarkable feature of the evolution from cyanobacteria to higher plants is the conservation of MGDG, DGDG, SQDG, and phosphatidylglycerol (PG), the major phospholipid of thylakoids. Using neutron diffraction on reconstituted thylakoid lipid extracts, we observed that the thylakoid lipid mixture self-organizes as a regular stack of bilayers. This natural lipid mixture was shown to switch from hexagonal II toward lamellar phase on hydration. This transition and the observed phase coexistence are modulated by the fine-tuning of the lipid profile, in particular the MGDG/DGDG ratio, and by the hydration. Our analysis highlights the critical role of DGDG as a contributing component to the membrane stacking via hydrogen bonds between polar heads of adjacent bilayers. DGDG interactions balance the repulsive electrostatic contribution of the charged lipids PG and SQDG and allow the persistence of regularly stacked membranes at high hydration. In developmental contexts or in response to environmental variations, these properties can contribute to the highly dynamic flexibility of plastid structure.

Chloroplast envelope membranes: a dynamic interface between plastids and the cytosol

Chloroplasts are bounded by a pair of outer membranes, the envelope, that is the only permanent membrane structure of the different types of plastids. Chloroplasts have had a long and complex evolutionary past and integration of the envelope membranes in cellular functions is the result of this evolution. Plastid envelope membranes contain a wide diversity of lipids and terpenoid compounds serving numerous biochemical functions and the flexibility of their biosynthetic pathways allow plants to adapt to fluctuating environmental conditions (for instance phosphate deprivation). A large body of knowledge has been generated by proteomic studies targeted to envelope membranes, thus revealing an unexpected complexity of this membrane system. For instance, new transport systems for metabolites and ions have been identified in envelope membranes and new routes for the import of chloroplast-specific proteins have been identified. The picture emerging from our present understanding of plastid envelope membranes is that of a key player in plastid biogenesis and the co-ordinated gene expression of plastid-specific protein (owing to chlorophyll precursors), of a major hub for integration of metabolic and ionic networks in cell metabolism, of a flexible system that can divide, produce dynamic extensions and interact with other cell constituents. Envelope membranes are indeed one of the most complex and dynamic system within a plant cell. In this review, we present an overview of envelope constituents together with recent insights into the major functions fulfilled by envelope membranes and their dynamics within plant cells.

The bioenergetic role of dioxygen and the terminal oxidase(s) in cyanobacteria

Owing to the release of 13 largely or totally sequenced cyanobacterial genomes (see and ), it is now possible to critically assess and compare the most neglected aspect of cyanobacterial physiology, i.e., cyanobacterial respiration, also on the grounds of pure molecular biology (gene sequences). While there is little doubt that cyanobacteria (blue-green algae) do form the largest, most diversified and in both evolutionary and ecological respects most significant group of (micro)organisms on our earth, and that what renders our blue planet earth to what it is, viz. the O(2)-containing atmosphere, dates back to the oxygenic photosynthetic activity of primordial cyanobacteria about 3.2x10(9) years ago, there is still an amazing lack of knowledge on the second half of bioenergetic oxygen metabolism in cyanobacteria, on (aerobic) respiration. Thus, the purpose of this review is threefold: (1) to point out the unprecedented role of the cyanobacteria for maintaining the delicate steady state of our terrestrial biosphere and atmosphere through a major contribution to the poising of oxygenic photosynthesis against aerobic respiration ("the global biological oxygen cycle"); (2) to briefly highlight the membrane-bound electron-transport assemblies of respiration and photosynthesis in the unique two-membrane system of cyanobacteria (comprising cytoplasmic membrane and intracytoplasmic or thylakoid membranes, without obvious anastomoses between them); and (3) to critically compare the (deduced) amino acid sequences of the multitude of hypothetical terminal oxidases in the nine fully sequenced cyanobacterial species plus four additional species where at least the terminal oxidases were sequenced. These will then be compared with sequences of other proton-pumping haem-copper oxidases, with special emphasis on possible mechanisms of electron and proton transfer.

Functional assembly of thylakoid ΔpH-dependent/Tat protein transport pathway componentsin vitro

Assembly of the components of the thylakoid deltapH-dependent/Tat protein transport machinery was analyzed in vitro. Upon incubation with intact chloroplasts, precursors to all three components, Hcf106, cpTatC and Tha4, were imported into the organelle and assembled into characteristic endogenous complexes. In particular, all of the imported cpTatC and approximately two-thirds of the imported Hcf106 functionally assembled into 700 kDa complexes capable of binding Tat pathway precursor proteins. The amounts assembled into thylakoids by this procedure were moderate. However, physiological quantities of mature forms of Tha4 and Hcf106 were integrated into isolated thylakoids and a significant percentage of the Hcf106 so integrated was assembled into the 700 kDa complex. Interestingly, a mutant form of Hcf106 in which an invariant transmembrane glutamate was changed to glutamine integrated into the membrane but did not assemble into the receptor complex. Analysis of energy and known pathway component requirements indicated that Hcf106 and Tha4 integrate by an unassisted or 'spontaneous' mechanism. The functionality of in vitro integrated Tha4 was verified by its ability to restore transport to thylakoid membranes from the maize tha4 mutant, which lacks the Tha4 protein. Development of this functional in vitro assembly assay will facilitate structure-function studies of the thylakoid Tat pathway translocation machinery.

Characterization of leachianone G 2"-dimethylallyltransferase, a novel prenyl side-chain elongation enzyme for the formation of the lavandulyl group of sophoraflavanone G in Sophora flavescens Ait. cell suspension cultures

Leachianone G (LG) 2"-dimethylallyltransferase, a novel prenyl side-chain elongation enzyme, was identified in Sophora flavescens Ait. cultured cells. The enzyme transfers a dimethylallyl group to the 2" position of another dimethylallyl group attached at position 8 of LG to form sophoraflavanone G, a branched monoterpenoid-conjugated flavanone characteristic to this plant. This membrane-bound dimethylallyltransferase required Mg2+ (optimum concentration was 10 mm) for the reaction and had an optimum pH of 8.8. It utilized dimethylallyl diphosphate as the sole prenyl donor, and the 2'-hydroxy function in LG was indispensable to the activity. The apparent Km values for dimethylallyl diphosphate and LG were 59 and 2.3 microm, respectively. Subcellular localization of three enzymes that participated in the formation of the lavandulyl group was also investigated by sucrose density gradient centrifugation. Two prenyltransferases, naringenin 8-dimethylallyltransferase and LG 2"-dimethylallyltransferase, were localized in the plastids, whereas 8-dimethylallylnaringenin 2'-hydroxylase, which catalyzes the crucial step in the lavandulyl-group formation, was associated with the endoplasmic reticulum. These results suggest the close cooperation between the plastids and the endoplasmic reticulum in the formation of lavandulyl groups.

On the translocation of proteins across the chloroplast envelope

Most of the chloroplast proteins are coded for in the nucleus and are synthesized in the cytosol from where they are subsequently transported into the different chloroplast compartments. The structural properties of the N-terminal extensions (transit peptides) of these nuclear-coded precursor proteins are discussed as well as the energy requirements for their translocation and the involvement of receptor proteins and that of other (ATP-dependent) factors.

Sequence analysis and protein import studies of an outer chloroplast envelope polypeptide

A chloroplast outer envelope membrane protein was cloned and sequenced and from the sequence it was possible to deduce a polypeptide of 6.7 kDa. It has only one membrane-spanning region; the C terminus extends into the cytosol, whereas the N terminus is exposed to the space between the two envelope membranes. The protein was synthesized in an in vitro transcription-translation system to study its routing into isolated chloroplasts. The import studies revealed that the 6.7-kDa protein followed a different and heretofore undescribed translocation pathway in the respect that (i) it does not have a cleavable transit sequence, (ii) it does not require ATP hydrolysis for import, and (iii) protease-sensitive components that are responsible for recognition of precursor proteins destined for the inside of the chloroplasts are not involved in routing the 6.7-kDa polypeptide to the outer chloroplast envelope.

Lipid transfer in plants

Plant cells contain cytosolic proteins, called lipid transfer proteins (LTP), which are able to facilitate in vitro intermembrane transfer of phospholipids. Proteins of this kind from three plants, purified to homogeneity, have several properties in common: molecular mass around 9 kDa, high isoelectric point, lack of specificity for phospholipids, and binding ability for fatty acids. The comparison of their amino acid sequences revealed striking homologies and conserved domains which are probably involved in their function as LTPs. These proteins could play a major role in membrane biogenesis by conveying phospholipids from their site of biosynthesis to membranes unable to form these lipids. Immunochemical methods were used to establish an in vivo correlation between membrane biogenesis and the level of LTP or the amount of LTP synthesized in vitro from mRNAs. The recent isolation of a full-length cDNA allows novel approaches to studying the participation of LTPs in the biogenesis of plant cell membranes.

Intracellular localization of prenyltransferases of isoflavonoid phytoalexin biosynthesis in bean and soybean

The intracellular localization of prenyltransferases involved in the biosynthesis of the phytoalexins glyceollin in soybean (Glycine max L.) and phaseollin in French bean (Phaseolus vulgaris L.) has been investigated. By sucrose- and Percoll-gradient centrifugation of microsomes of an elicitor-challenged soybean cell culture, the membranes containing prenyltransferase were separated from the endoplasmic reticulum and shown to be lighter in density. In a continuous Percoll gradient the peak of prenyltransferase activity coincided with the peak of galactolipid synthesis, as determined by incorporation of uridine 5'-diphospho-[(14)C]galactose (UDP-[(14)C]galactose). Intact chloroplasts isolated from cupricchloride-treated bean leaves contained both prenyltransferase and UDP-galactose transferase activity. Both activities increased during chloroplast isolation. Fractionation of swollen chloroplasts on a discontinuous sucrose gradient showed prenyltransferase and UDP-galactose transferase activity in the envelope membrane subfraction. It is concluded that in both plants prenyltransferase is located in the envelope membrane of plastids.

Light-independent NADPH-protochlorophyllide oxidoreductase activity in purified plasma membrane from the cyanobacterium Anacystis nidulans

A light plasma membrane fraction corresponding to a buoyant density of 1.087 +/- 0.005 g/cm3 and devoid of chlorophyll was prepared and purified from Anacystis nidulans according to a recently published procedure (G.A.Peschek, V.Molitor, M.Trnka, M. Wastyn and W. Erber (1988) Methods Enzymol. 167, 437-449). Besides major amounts of carotenoids the plasma membranes contained a small but significant pool of chlorophyllide a and protochlorophyllide a as verified by room temperature and 77K spectrofluorimetry and analytical separation and identification by high performance liquid chromatography using authentic standards. Incubation of the plasma membranes in strict darkness in the presence of NADPH was accompanied by the gradual and stoichiometric replacement of protochlorophyllide by chlorophyllide, NADP+ effecting the reverse transition. The reaction was completely insensitive to illumination (5-20 w/m2 tungsten light) but abolished after heating of the membranes (90 degrees C, 5 min) or in the presence of 10 mM EGTA, and was specifically stimulated by calcium ions. Our results indicate the occurrence of light-independent NADPH:protochlorophyllide oxidoreductase activity in the plasma membrane of Anacystis nidulans.

Regulation and expression of the multigene family coding light-harvesting chlorophyll a/b-binding proteins of photosystem II

The current state of knowledge concerning the expression of the nuclear genes that code the light-harvesting chlorophyll a/b-binding polypeptides of photosystem II is presented. This review covers the structure of these genes, the complex multistep pathway involved in their expression, and the environmental and other factors which regulate their expression. Some of the effects of these factors are mediated, at least in part, at the level of transcription, but other effects can be explained only by the existence of multiple posttranscriptional regulatory steps.

Detection of chlorophyllide in chlorophyll-free plasma membrane preparations from Anacystis nidulans

Plasma and thylakoid membranes were isolated and purified from the cyanobacterium Anacystis nidulans. Spectrophotometric examination of acetone extracts gave major absorption bands resulting from carotenoids and chlorophyll a in plasma and thylakoid membranes, respectively. Only a very small absorption peak at 663 nm was detected in acetone extracts of plasma membranes which, in contrast to the corresponding peak from thylakoid membranes, could not be extracted into n-hexane; methanol, on the other hand, was effective with both plasma and thylakoid membranes. Aqueous membrane suspensions excited at 435 nm gave strong fluorescence emission at 662 nm for plasma membranes, but only a very small one for thylakoid membranes which had been adjusted to equal absorbance at 678 nm. Excitation spectra of the 668 nm fluorescence emission peak in acetone extracts of plasma and thylakoid membranes were strikingly different from each other. Finally, high performance liquid chromatography afforded clear-cut preparative separation of the two "chlorophyll-like" pigments in plasma and thylakoid membranes, respectively, and identification by comparison with retention characteristics known from the literature, together with a pure chlorophyll a standard. Our results indicate that the highly fluorescent and polar "chlorophyll-like" pigment in plasma membranes of Anacystis is a chlorophyll precursor, viz. chlorophyllide a.

Localization of a 64-kDa phosphoprotein in the lumen between the outer and inner envelopes of pea chloroplasts

The identification and localization of a marker protein for the intermembrane space between the outer and inner chloroplast envelopes is described. This 64-kDa protein is very rapidly labeled by [gamma-32P]ATP at very low (30 nM) ATP concentrations and the phosphoryl group exhibits a high turnover rate. It was possible to establish the presence of the 64-kDa protein in this plastid compartment by using different chloroplast envelope separation and isolation techniques. In addition comparison of labeling kinetics by intact and hypotonically lysed pea chloroplasts support the localization of the 64-kDa protein in the intermembrane space. The 64-kDa protein was present and could be labeled in mixed envelope membranes isolated from hypotonically lysed plastids. Mixed envelope membranes incorporated high amounts of 32P from [gamma-32P]ATP into the 64-kDa protein, whereas separated outer and inner envelope membranes did not show significant phosphorylation of this protein. Water/Triton X-114 phase partitioning demonstrated that the 64-kDa protein is a hydrophilic polypeptide. These findings suggest that the 64-kDa protein is a soluble protein trapped in the space between the inner and outer envelope membranes. After sonication of mixed envelope membranes, the 64-kDa protein was no longer present in the membrane fraction, but could be found in the supernatant after a 110,000 x g centrifugation.

On the mechanism of rapid plasma membrane and chloroplast envelope expansion in Dunaliella salina exposed to hypoosmotic shock

Dunaliella salina cells rapidly diluted from their normal 1.71 M NaCl-containing growth medium into medium containing 0.86 M NaCl swelled within 2--4 min to an average volume 1.76 X larger and a surface area 1.53 X larger than found in control cells. Morphometric analysis of thin section electron micrographs revealed that certain organelles, including the chloroplast, nucleus, and some types of vacuoles, also expanded in surface area as much or more than did the entire cell. It is likely that glycerol, the most important osmotically active intracellular solute, was present in high concentration within these organelles as well as in the cytoplasm itself. Thin section and freeze-fracture electron microscopy were utilized to trace the origin of membrane material whose addition permitted the large increase in plasma membrane surface area and the equally large growth of the chloroplast outer envelope. The findings indicated that the plasma membrane's expansion resulted from its selective fusion with numerous small (less than or equal to 0.25 micron diam) vesicles prevalent throughout the cytoplasm. In contrast, new membrane added to the chloroplast outer envelope was drawn from an entirely different source, namely, elements of the endoplasmic reticulum.

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

In pulse-chase experiments in which log-phase cells of Chlamydomonas reinhardtii were labeled in vivo for 5 min with H2(35)SO4, fluorographs of immunoprecipitates from whole cell extracts revealed that chloroplast ribosomal proteins L-2, L-6, L-21, and L-29, which are made in the cytosol and imported, appeared in their mature forms. However, in the case of chloroplast ribosomal protein L-18, which is also made in the cytoplasm and imported, a prominent precursor with an apparent molecular weight of 17,000 was found at the end of a 5-min pulse. This precursor was processed to its mature size (apparent molecular weight of 15,500) within the first 5 min of the subsequent chase. As determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the precursor to L-18 formed in vivo was 1.5 kilodaltons smaller than the primary product detected in translations of Chlamydomonas polyadenylated RNA in vitro. Upon a 10-min incubation with a postribosomal supernatant from Chlamydomonas, the 18,500-dalton precursor detected in vitro could be partially converted into a polypeptide that comigrated with the 17,000-dalton precursor detected in extracts of cells labeled in vivo. Under conditions in which the total amounts of chloroplast proteins had been reduced and cells were made to synthesize ribosomes rapidly, the apparent half-life of the 17,000-dalton precursor was extended over that seen in log-phase cells. When chloroplast protein synthesis was inhibited with lincomycin for 3 h before labeling under these conditions, the 17,000-dalton L-18 precursor but not the mature form was found, and the precursor was slowly degraded during a 60-min chase. When cells were placed in the dark for 3 h before labeling, processing of this precursor to the mature form appeared unaffected, but the chloroplast-synthesized ribosomal protein L-26 was detected, indicating that chloroplast protein synthesis was still occurring. We interpret these results to indicate that the maturation of protein L-18 in vivo involves at least two processing steps, one of which depends on a protein made on chloroplast ribosomes.

Localization of phosphatidylcholine in outer envelope membrane of spinach chloroplasts

We have examined the effects of phospholipase C from Bacillus cereus on the extent of phospholipid hydrolysis in envelope membrane vesicles and in intact chloroplasts. When isolated envelope vesicles were incubated in presence of phospholipase C, phosphatidylcholine and phosphatidylglycerol, but not phosphatidylinositol, were totally converted into diacylglycerol if they were available to the enzyme (i.e., when the vesicles were sonicated in presence of phospholipase C). These experiments demonstrate that phospholipase C can be used to probe the availability of phosphatidylcholine and phosphatidylglycerol in the cytosolic leaflet of the outer envelope membrane from spinach chloroplasts. When isolated, purified, intact chloroplasts were incubated with low amounts of phospholipase C (0.3 U/mg chlorophyll) under very mild conditions (12 degrees C for 1 min), greater than 80% of phosphatidylcholine molecules and almost none of phosphatidylglycerol molecules were hydrolyzed. Since we have also demonstrated, by using several different methods (phase-contrast and electron microscopy, immunochemical and electrophoretic analyses) that isolated spinach chloroplasts, and especially their outer envelope membrane, remained intact after mild treatment with phospholipase C, we can conclude that there is a marked asymmetric distribution of phospholipids across the outer envelope membrane of spinach chloroplasts. Phosphatidylcholine, the major polar lipid of the outer envelope membrane, is almost entirely accessible from the cytosolic side of the membrane and therefore is probably localized in the outer leaflet of the outer envelope bilayer. On the contrary, phosphatidylglycerol, the major polar lipid in the inner envelope membrane and the thylakoids, is probably not accessible to phospholipase C from the cytosol and therefore is probably localized mostly in the inner leaflet of the outer envelope membrane and in the other chloroplast membranes.

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

In pulse-chase experiments in which log-phase cells of Chlamydomonas reinhardtii were labeled in vivo for 5 min with H2(35)SO4, fluorographs of immunoprecipitates from whole cell extracts revealed that chloroplast ribosomal proteins L-2, L-6, L-21, and L-29, which are made in the cytosol and imported, appeared in their mature forms. However, in the case of chloroplast ribosomal protein L-18, which is also made in the cytoplasm and imported, a prominent precursor with an apparent molecular weight of 17,000 was found at the end of a 5-min pulse. This precursor was processed to its mature size (apparent molecular weight of 15,500) within the first 5 min of the subsequent chase. As determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the precursor to L-18 formed in vivo was 1.5 kilodaltons smaller than the primary product detected in translations of Chlamydomonas polyadenylated RNA in vitro. Upon a 10-min incubation with a postribosomal supernatant from Chlamydomonas, the 18,500-dalton precursor detected in vitro could be partially converted into a polypeptide that comigrated with the 17,000-dalton precursor detected in extracts of cells labeled in vivo. Under conditions in which the total amounts of chloroplast proteins had been reduced and cells were made to synthesize ribosomes rapidly, the apparent half-life of the 17,000-dalton precursor was extended over that seen in log-phase cells. When chloroplast protein synthesis was inhibited with lincomycin for 3 h before labeling under these conditions, the 17,000-dalton L-18 precursor but not the mature form was found, and the precursor was slowly degraded during a 60-min chase. When cells were placed in the dark for 3 h before labeling, processing of this precursor to the mature form appeared unaffected, but the chloroplast-synthesized ribosomal protein L-26 was detected, indicating that chloroplast protein synthesis was still occurring. We interpret these results to indicate that the maturation of protein L-18 in vivo involves at least two processing steps, one of which depends on a protein made on chloroplast ribosomes.