Substrate-dependent transport of the NADPH:protochlorophyllide oxidoreductase into isolated plastids

Gossypium hirsutum gene of unknown function Gohir.A03G0737001 encodes a potential Chaperone-like Protein of protochlorophyllide oxidoreductase (CPP1)

A gene of unknown function identified in Gossypium hirsutum , Gohir.A03G0737001.1, was studied using sequence and bioinformatic tools. The encoded protein (referred to here as GhCPP1-A0A1U8HKT6) was predicted to function as a Chaperone-like protein of protochlorophyllide oxidoreductase (CPP1), which is involved with initiation of photochemical reactions of chlorophyll biosynthesis. Sequence analysis indicates it is embedded in the chloroplast envelope membrane through four transmembrane regions and contains a J-like domain that is structurally similar to the J domain of DnaJ/Hsp40 "holdase" chaperone proteins.

A Protochlorophyllide (Pchlide) a Oxygenase for Plant Viability

Higher plants contain a small, 5-member family of Rieske non-heme oxygenases that comprise the inner plastid envelope protein TIC55, phaeophorbide a oxygenasee (PAO), chlorophyllide a oxygenase (CAO), choline monooxygenase, and a 52 kDa protein (PTC52) associated with the precursor NADPH:protochlorophyllide (Pchlide) oxidoreductase A (pPORA) A translocon (PTC). Some of these chloroplast proteins have documented roles in chlorophyll biosynthesis (CAO) and degradation (PAO and TIC55), whereas the function of PTC52 remains unresolved. Biochemical evidence provided here identifies PTC52 as Pchlide a oxygenase of the inner plastid envelope linking Pchlide b synthesis to pPORA import. Protochlorophyllide b is the preferred substrate of PORA and its lack no longer allows pPORA import. The Pchlide b-dependent import pathway of pPORA thus operates in etiolated seedlings and is switched off during greening. Using dexamethasone-induced RNA interference (RNAi) we tested if PTC52 is involved in controlling both, pPORA import and Pchlide homeostasis in planta. As shown here, RNAi plants deprived of PTC52 transcript and PTC52 protein were unable to import pPORA and died as a result of excess Pchlide a accumulation causing singlet oxygen formation during greening. In genetic studies, no homozygous ptc52 knock-out mutants could be obtained presumably as a result of embryo lethality, suggesting a role for PTC52 in the initial greening of plant embryos. Phylogenetic studies identified PTC52-like genes amongst unicellular photosynthetic bacteria and higher plants, suggesting that the biochemical function associated with PTC52 may have an ancient evolutionary origin. PTC52 also harbors conserved motifs with bacterial oxygenases such as the terminal oxygenase component of 3-ketosteroid 9-alpha-hydroxylase (KshA) from Rhodococcus rhodochrous. 3D-modeling of PTC52 structure permitted the prediction of amino acid residues that contribute to the substrate specificity of this enzyme. In vitro-mutagenesis was used to test the predicted PTC52 model and provide insights into the reaction mechanism of this Rieske non-heme oxygenase.

NADPH:protochlorophyllide oxidoreductase B (PORB) action in Arabidopsis thaliana revisited through transgenic expression of engineered barley PORB mutant proteins

NADPH:protochlorophyllide oxidoreductase (POR) is a key enzyme for the light-induced greening of etiolated angiosperm plants. It belongs to the 'RED' family of reductases, epimerases and dehydrogenases. All POR proteins characterized so far contain evolutionarily conserved cysteine residues implicated in protochlorophyllide (Pchlide)-binding and catalysis. cDNAs were constructed by site-directed mutagenesis that encode PORB mutant proteins with defined Cys→Ala exchanges. These cDNAs were expressed in transgenic plants of a PORB-deficient knock-out mutant (porB) of Arabidopsis thaliana. Results show that porB plants expressing PORB mutant proteins with Ala substitutions of Cys276 or Cys303 are hypersensitive to high-light conditions during greening. Hereby, failure to assemble higher molecular weight complexes of PORB with its twin isoenzyme, PORA, as encountered with (Cys303→Ala)-PORB plants, caused more severe effects than replacing Cys276 by an Ala residue in the active site of the enzyme, as encountered in (Cys276→Ala)-PORB plants. Our results are consistent with the presence of two distinct pigment binding sites in PORB, with Cys276 establishing the active site of the enzyme and Cys303 providing a second, low affinity pigment binding site that is essential for the assembly of higher molecular mass light-harvesting PORB::PORA complexes and photoprotection of etiolated seedlings. Failure to assemble such complexes provoked photodynamic damage through the generation of singlet oxygen. Together, our data highlight the importance of PORB for Pchlide homoeostasis and greening in Arabidopsis.

Common functions of the chloroplast and mitochondrial co-chaperones cpDnaJL (CDF1) and mtDnaJ (PAM16) in protein import and ROS scavenging in Arabidopsis thaliana

As semi-autonomous cell organelles that contain only limited coding information in their own DNA, chloroplasts and mitochondria must import the vast majority of their protein constituents from the cytosol. Respective protein import machineries have been identified that mediate the uptake of chloroplast and mitochondrial proteins and interact with molecular chaperones of the HEAT-SHOCK PROTEIN (HSP) 70 family operating as import motors. Recent work identified unexpected new functions of 2 DnaJ co-chaperones in mitochondrial and chloroplast protein translocation and suggest a common mechanism of reactive oxygen species (ROS) scavenging that shall be discussed here.

Plant Protochlorophyllide Oxidoreductases A and B: CATALYTIC EFFICIENCY AND INITIAL REACTION STEPS

The enzyme protochlorophyllide oxidoreductase (POR, EC 1.3.1.33) has a key role in plant development. It catalyzes one of the later steps in chlorophyll synthesis, the light-induced reduction of protochlorophyllide (PChlide) into chlorophyllide (Chlide) in the presence of NADPH. Two isozymes of plant POR, POR A and POR B from barley, which differ in their function during plant life, are compared with respect to their substrate binding affinity, catalytic efficiency, and catalytic mechanism. POR B as compared with POR A shows an 5-fold higher binding affinity for PChlide and an about 6-fold higher catalytic efficiency measured as kcat/Km. Based on the reaction intermediates, which can be trapped at low temperatures the same reaction mechanism operates in both POR A and POR B. In contrast to results reported for POR enzymes from cyanobacteria, the initial light-driven step, which occurs at temperatures below 180 K already involves the full chemistry of the photoreduction and yields the reaction product, Chlide, in an enzyme-bound form. The subsequent dark reactions, which include cofactor (NADP(+)) release and cofactor (NADPH) rebinding, show different temperature dependences for POR A and POR B and suggest a higher conformational flexibility of POR B in the surrounding active center. Both the higher substrate binding affinity and well adapted enzyme dynamics are held responsible for the increased catalytic activity of POR B as compared with POR A.

Cell growth defect factor 1 is crucial for the plastid import of NADPH:protochlorophyllide oxidoreductase A in Arabidopsis thaliana

Tetrapyrroles such as chlorophyll, heme, and bacteriochlorophyll play fundamental roles in the energy absorption and transduction of all photosynthetic organisms. They are synthesized via a complex pathway taking place in chloroplasts. Chlorophyll biosynthesis in angiosperms involves 16 steps of which only one is light-requiring and driven by the NADPH:protochlorophyllide oxidoreductase (POR). Three POR isoforms have been identified in Arabidopsis thaliana--designated PORA, PORB, and PORC--that are differentially expressed in etiolated, light-exposed, and light-adapted plants. All three isoforms are encoded by nuclear genes, are synthesized as larger precursors in the cytosol (pPORs), and are imported posttranslationally into the plastid compartment. Import of the precursor to the dark-specific isoform PORA (pPORA) is protochlorophyllide (Pchlide)-dependent and due to the operation of a unique translocon complex dubbed PTC (Pchlide-dependent translocon complex) in the plastid envelope. Here, we identified a ∼30-kDa protein that participates in pPORA import. The ∼30-kDa protein is identical to the previously identified CELL GROWTH DEFECT FACTOR 1 (CDF1) in Arabidopsis that is conserved in higher plants and Synechocystis. CDF1 operates in pPORA import and stabilization and hereby acts as a chaperone for PORA protein translocation. CDF1 permits tight interactions between Pchlide synthesized in the plastid envelope and the importing PORA polypeptide chain such that no photoexcitative damage occurs through the generation of singlet oxygen operating as a cell death inducer. Together, our results identify an ancient mechanism dating back to the endosymbiotic origin of chloroplasts as a key element of Pchlide-dependent pPORA import.

Chlorophyll biosynthesis: spotlight on protochlorophyllide reduction

Photosynthetic organisms require chlorophyll or bacteriochlorophyll for their light trapping and energy transduction activities. The biosynthetic pathways of chlorophyll and bacteriochlorophyll are similar in most of their early steps, except for the reduction of protochlorophyllide (Pchlide) to chlorophyllide. Whereas angiosperms make use of a light-dependent enzyme, cyanobacteria, algae, bryophytes, pteridophytes and gymnosperms contain an additional, light-independent enzyme dubbed dark-operative Pchlide oxidoreductase (DPOR). Anoxygenic photosynthetic bacteria such as Rhodobacter capsulatus and Rhodobacter sphaeroides rely solely on DPOR. Recent atomic resolution of reductase and catalytic components of DPOR from R. sphaeroides and R. capsulatus, respectively, have revealed their similarity to nitrogenase components. In this review, we discuss the two fundamentally different mechanisms of Pchlide reduction in photosynthetic organisms.

Protein transport into chloroplasts

Most proteins in chloroplasts are encoded by the nuclear genome and synthesized as precursors with N-terminal targeting signals called transit peptides. Novel machinery has evolved to specifically import these proteins from the cytosol into chloroplasts. This machinery consists of more than a dozen components located in and around the chloroplast envelope, including a pair of GTPase receptors, a beta-barrel-type channel across the outer membrane, and an AAA(+)-type motor in the stroma. How individual components assemble into functional subcomplexes and the sequential steps of the translocation process are being mapped out. An increasing number of noncanonical import pathways, including a pathway with initial transport through the endomembrane system, is being revealed. Multiple levels of control on protein transport into chloroplasts have evolved, including the development of two receptor subfamilies, one for photosynthetic proteins and one for housekeeping proteins. The functions or expression levels of some translocon components are further adjusted according to plastid type, developmental stage, and metabolic conditions.

Chloroplast proteins without cleavable transit peptides: rare exceptions or a major constituent of the chloroplast proteome?

Most chloroplast proteins (cp proteins) are nucleus-encoded, synthesized on cytosolic ribosomes as precursor proteins containing a presequence (cTP), and post-translationally imported via the Tic/Toc complex into the organelle, where the cTP is removed. Only a few unambiguous instances of cp proteins that do not require cTPs (non-canonical cp proteins) have been reported so far. However, the survey of data from large-scale proteomic studies presented here suggests that the fraction of such proteins in the total cp proteome might be as large as approximately 30%. To explore this discrepancy, we chose a representative set of 28 putative non-canonical cp proteins, and used in vitro import and Red Fluorescent Protein (RFP)-fusion assays to determine their sub-cellular destinations. Four proteins, including embryo defective 1211, glycolate oxidase 2, protein disulfide isomerase-like protein (PDII), and a putative glutathione S-transferase, could be unambiguously assigned to the chloroplast. Several others ('potential cp proteins') were found to be imported into chloroplasts in vitro, but failed to localize to the organelle when RFP was fused to their C-terminal ends. Extrapolations suggest that the fraction of cp proteins that enter the inner compartments of the organelle, although they lack a cTP, might be as large as 11.4% of the total cp proteome. Our data also support the idea that cytosolic proteins that associate with the cp outer membrane might account for false positive cp proteins obtained in earlier studies.

A novel insight into the regulation of light-independent chlorophyll biosynthesis in Larix decidua and Picea abies seedlings

Light-independent chlorophyll (Chl) biosynthesis is a prerequisite for the assembly of photosynthetic pigment-protein complexes in the dark. Dark-grown Larix decidua Mill. seedlings synthesize Chl only in the early developmental stages and their Chl level rapidly declines during the subsequent development. Our analysis of the key regulatory steps in Chl biosynthesis revealed that etiolation of initially green dark-grown larch cotyledons is connected with decreasing content of glutamyl-tRNA reductase and reduced 5-aminolevulinic acid synthesizing capacity. The level of the Chl precursor protochlorophyllide also declined in the developing larch cotyledons. Although the genes chlL, chlN and chlB encoding subunits of the light-independent protochlorophyllide oxidoreductase were constitutively expressed in the larch seedlings, the accumulation of the ChlB subunit was developmentally regulated and ChlB content decreased in the fully developed cotyledons. The efficiency of chlB RNA-editing was also reduced in the mature dark-grown larch seedlings. In contrast to larch, dark-grown seedlings of Picea abies (L.) Karst. accumulate Chl throughout their whole development and show a different control of ChlB expression. Analysis of the plastid ultrastructure, photosynthetic proteins by Western blotting and photosynthetic parameters by gas exchange and Chl fluorescence measurements provide additional experimental proofs for differences between dark and light Chl biosynthesis in spruce and larch seedlings.

A pentapeptide motif related to a pigment binding site in the major light-harvesting protein of photosystem II, LHCII, governs substrate-dependent plastid import of NADPH:protochlorophyllide oxidoreductase A

NADPH:protochlorophyllide (Pchlide) oxidoreductase (POR) A is the only known example thus far of a nucleus-encoded plastid protein that is imported to its final destination in a substrate-dependent, Pchlide-regulated manner. Previous work has shown that the cytosolic PORA precursor (pPORA) does not utilize the general import site but uses a distinct translocon designated the Pchlide-dependent translocon complex. Here we demonstrate that a pentapeptide motif, threonine-threonine-serine-proline-glycine (TTSPG) in pPORA's transit peptide (transA), is involved in Pchlide-dependent transport. Deletion of this motif from the COOH-terminal end of transA abolished both Pchlide binding and protein import. Incorporation of the TTSPG motif into normally non-Pchlide-responsive transit sequences conferred the pigment binding properties onto the engineered chimeric precursors but was insufficient to render protein import substrate dependent. An additional motif was identified in the NH(2)-terminal part of transA that was needed for binding of the precursor to the Pchlide-dependent translocon complex. Point mutations of the TTSPG motif identified glycine as the Pchlide binding site. By analogy to the major light-harvesting chlorophyll a/b binding protein of photosystem II, we propose that the peptidyl carbonyl oxygen of glycine may bind directly or via a water molecule to the central Mg atom of the pigment.

Photoprotective role of NADPH:protochlorophyllide oxidoreductase A

A homology model of NADPH:protochlorophyllide (Pchlide) oxidoreductase A (POR; E.C. 1.3.33.1) of barley is developed and verified by site-directed mutagenesis. PORA is considered a globular protein consisting of nine alpha-helices and seven beta-strands. The model predicts the presence of two functionally distinctive Pchlide binding sites where the pigment is coordinated by cysteine residues. The pigment bound to the first, high-affinity Pchlide binding site is used for the formation of the photoactive state of the enzyme. The pigment bound to the second, low-affinity Pchlide binding site is involved in the PORA:PORB interaction, allowing for resonance energy transfer between the neighboring PORs in the complex. In the in vitro reconstituted light-harvesting POR:Pchlide complex (LHPP), light absorbed by PORA-bound Pchlide b is transferred to PORB-bound Pchlide a. That induces the conversion of Pchlide a to chlorophyllide (Chlide) a. This energy transfer eliminates the possibility of Pchlide b photoreduction and prevents that excited triplet states of either Pchlides a or b accumulate and provoke singlet oxygen production. Together, our results provide a photoprotective role of PORA during greening.

Regulation and evolution of chlorophyll metabolism

Chlorophylls are the most abundant tetrapyrrole molecules essential for photosynthesis in photosynthetic organisms. After many years of intensive research, most of the genes encoding the enzymes for the pathway have been identified, and recently the underlying molecular mechanisms have been elucidated. These studies revealed that the regulation of chlorophyll metabolism includes all levels of control to allow a balanced metabolic flow in response to external and endogenous factors and to ensure adaptation to varying needs of chlorophyll during plant development. Furthermore, identification of biosynthetic genes from various organisms and genetic analysis of functions of identified genes enables us to predict the evolutionary scenario of chlorophyll metabolism. In this review, based on recent findings, we discuss the regulation and evolution of chlorophyll metabolism.

Targeting of nucleus-encoded proteins to chloroplasts in plants

Most chloroplast proteins are encoded in the nucleus and synthesized on free, cytosolic ribosomes in precursor form. Each precursor has an amino-terminal extension called a transit peptide, which directs the protein through a post-translational targeting pathway and is removed upon arrival inside the organelle. This 'protein import' process is mediated by the coordinate action of two multiprotein complexes, one in each of the envelope membranes: the TOC and TIC (Translocon at the Outer/ Inner envelope membrane of Chloroplasts) machines. Many components of these complexes have been identified biochemically in pea; these include transit peptide receptors, channel proteins, and molecular chaperones. Intriguingly, the Arabidopsis genome encodes multiple, homologous genes for receptor components of the TOC complex. Careful analysis indicated that the different receptor isoforms operate in different import pathways with distinct precursor recognition specificities. These 'substrate-specific' import pathways might play a role in the differentiation of different plastid types, and/or act to prevent deleterious competition effects between abundant and nonabundant precursors. Until recently, all proteins destined for internal chloroplast compartments were thought to possess a cleavable transit peptide, and to engage the TOC/TIC machinery. New studies using proteomics and other approaches have revealed that this is far from true. Remarkably, a significant number of chloroplast proteins are transported via a pathway that involves the endoplasmic reticulum and Golgi apparatus. Other recent reports have elucidated an intriguing array of protein targeting routes leading to the envelope membranes themselves.

Arabidopsis nuclear-encoded plastid transit peptides contain multiple sequence subgroups with distinctive chloroplast-targeting sequence motifs

The N-terminal transit peptides of nuclear-encoded plastid proteins are necessary and sufficient for their import into plastids, but the information encoded by these transit peptides remains elusive, as they have a high sequence diversity and lack consensus sequences or common sequence motifs. Here, we investigated the sequence information contained in transit peptides. Hierarchical clustering on transit peptides of 208 plastid proteins showed that the transit peptide sequences are grouped to multiple sequence subgroups. We selected representative proteins from seven of these multiple subgroups and confirmed that their transit peptide sequences are highly dissimilar. Protein import experiments revealed that each protein contained transit peptide-specific sequence motifs critical for protein import into chloroplasts. Bioinformatics analysis identified sequence motifs that were conserved among members of the identified subgroups. The sequence motifs identified by the two independent approaches were nearly identical or significantly overlapped. Furthermore, the accuracy of predicting a chloroplast protein was greatly increased by grouping the transit peptides into multiple sequence subgroups. Based on these data, we propose that the transit peptides are composed of multiple sequence subgroups that contain distinctive sequence motifs for chloroplast targeting.

Recent overview of the Mg branch of the tetrapyrrole biosynthesis leading to chlorophylls

In plants, chlorophylls (chlorophyll a and chlorophyll b) are the most abundant tetrapyrrole molecules and are essential for photosynthesis. The first committed step of chlorophyll biosynthesis is the insertion of Mg(2+) into protoporphyrin IX, and thus subsequent steps of the biosynthesis are called the Mg branch. As the Mg branch in higher plants is complex, it was not until the last decade--after many years of intensive research--that most of the genes encoding the enzymes for the pathway were identified. Biochemical and molecular genetic analyses have certainly modified the classic metabolic map of tetrapyrrole biosynthesis, and only recently have the molecular mechanisms of regulatory pathways governing chlorophyll metabolism been elucidated. As a result, novel functions of tetrapyrroles and biosynthetic enzymes have been proposed. In this review, I summarize the recent findings on enzymes involved in the Mg branch, mainly in higher plants.

A substrate-independent, 14:3:3 protein-mediated plastid import pathway of NADPH:protochlorophyllide oxidoreductase A

Plastids are semiautonomous organelles that contain only limited coding information in their own DNA. Because most of their genome was transferred to the nucleus after their endosymbiotic origin, plastids must import the major part of their protein constituents from the cytosol. The exact role of cytosolic targeting factors in the regulation of plastid protein import has not been determined. Here, we report that the nucleus-encoded NADPH:protochlorophyllide (Pchlide) oxidoreductase A plastid precursor (pPORA) can use two different plastid import pathways that differ by the requirements for cytosolic 14:3:3 proteins and Hsp70. pPORA synthesized in a wheat germ lysate segregated into different precursor fractions. While import of free pPORA and only Hsp70-complexed pPORA was Pchlide-dependent and involved the previously identified Pchlide-dependent translocon, 14:3:3 protein- and Hsp70-complexed pPORA was transported into Pchlide-free chloroplasts through the Toc75-containing standard translocon at the outer chloroplast membrane/translocon at the inner chloroplast membrane machinery. A 14:3:3 protein binding site was identified in the mature region of the (35)S-pPORA, which governed 14:3:3 protein- and Hsp70-mediated, Pchlide-independent plastid import. Collectively, our results reveal that the import of pPORA into the plastids is tightly regulated and involves different cytosolic targeting factors and plastid envelope translocon complexes.

A plant porphyria related to defects in plastid import of protochlorophyllide oxidoreductase A

The plastid envelope of higher plant chloroplasts is a focal point of plant metabolism. It is involved in numerous pathways, including tetrapyrrole biosynthesis and protein translocation. Chloroplasts need to import a large number of proteins from the cytosol because most are encoded in the nucleus. Here we report that a loss-of-function mutation in the outer plastid envelope 16-kDa protein (oep16) gene causes a conditional seedling lethal phenotype related to defects in import and assembly of NADPH:protochlorophyllide (Pchlide) oxidoreductase A. In the isolated knockout mutant of Arabidopsis thaliana, excess Pchlide accumulated in the dark operated as photosensitizer and provoked cell death during greening. Our results highlight the essential role of the substrate-dependent plastid import pathway of precursor Pchlide oxidoreductase A for seedling survival and the avoidance of developmentally programmed porphyria in higher plants.

Gene duplication, exon gain and neofunctionalization of OEP16-related genes in land plants

OEP16, a channel protein of the outer membrane of chloroplasts, has been implicated in amino acid transport and in the substrate-dependent import of protochlorophyllide oxidoreductase A. Two major clades of OEP16-related sequences were identified in land plants (OEP16-L and OEP16-S), which arose by a gene duplication event predating the divergence of seed plants and bryophytes. Remarkably, in angiosperms, OEP16-S genes evolved by gaining an additional exon that extends an interhelical loop domain in the pore-forming region of the protein. We analysed the sequence, structure and expression of the corresponding Arabidopsis genes (atOEP16-S and atOEP16-L) and demonstrated that following duplication, both genes diverged in terms of expression patterns and coding sequence. AtOEP16-S, which contains multiple G-box ABA-responsive elements (ABREs) in the promoter region, is regulated by ABI3 and ABI5 and is strongly expressed during the maturation phase in seeds and pollen grains, both desiccation-tolerant tissues. In contrast, atOEP-L, which lacks promoter ABREs, is expressed predominantly in leaves, is induced strongly by low-temperature stress and shows weak induction in response to osmotic stress, salicylic acid and exogenous ABA. Our results indicate that gene duplication, exon gain and regulatory sequence evolution each played a role in the divergence of OEP16 homologues in plants.

A role for chlorophyllide a oxygenase in the regulated import and stabilization of light-harvesting chlorophyll a/b proteins

The Arabidopsis CAO gene encodes a 52-kDa protein with predicted localization in the plastid compartment. Here, we report that CAO is an intrinsic Rieske iron-sulfur protein of the plastid-envelope inner and thylakoid membranes. Activity measurements revealed that CAO catalyzes chlorophyllide a to chlorophyllide b conversion in vitro and that the enzyme was only slightly active with protochlorophyllide a, the nonreduced precursor of chlorophyllide a. Protein import and organelle fractionation studies identified CAO to be distinct from Ptc52 in the substrate-dependent transport pathway of NADPH:protochlorophyllide oxidoreductase A but instead to be part of a separate translocon complex. This complex was involved in the regulated import and stabilization of the chlorophyllide b-binding light-harvesting proteins Lhcb1 (LHCII) and Lhcb4 (CP29) in chloroplasts. Together, our results provide insights into the plastid subcompartmentalization and evolution of chlorophyll precursor biosynthesis in relation to protein import in higher plants.

In vitro-mutagenesis of NADPH:protochlorophyllide oxidoreductase B: two distinctive protochlorophyllide binding sites participate in enzyme catalysis and assembly

NADPH:protochlorophyllide oxidoreductase (POR) B is a key enzyme for the light-induced greening of etiolated angiosperm plants. It is nucleus-encoded, imported into the plastids posttranslationally, and assembled into larger light-harvesting POR:protochlorophyllide complexes termed LHPP (Reinbothe et al., Nature 397:80-84, 1999). An in vitro-mutagenesis approach was taken to study the role of the evolutionarily conserved Cys residues in pigment binding. Four Cys residues are present in the PORB of which two, Cys276 and Cys303, established distinct pigment binding sites, as shown by biochemical tests, protein import studies, and in vitro-reconstitution experiments. While Cys276 constituted the Pchlide binding site in the active site of the enzyme, Cys303 established a second, low affinity pigment binding site that was involved in the assembly and stabilization of imported PORB enzyme inside etioplasts.

Multiplicity of different cell- and organ-specific import routes for the NADPH-protochlorophyllide oxidoreductases A and B in plastids of Arabidopsis seedlings

The NADPH-dependent protochlorophyllide (Pchlide) oxidoreductase (POR) is a photoenzyme that requires light for its catalytic activity and uses Pchlide itself as a photoreceptor. In Arabidopsis there are three PORs denoted PORA, PORB and PORC. The PORA and PORB genes are strongly expressed early in seedling development. In contrast to PORB the import of PORA into plastids of cotyledons is substrate-dependent and organ-specific. These differences in the import reactions between PORA and PORB most likely are due to different import mechanisms that are responsible for the uptake of these proteins. The two major core constituents of the translocon of the outer plastid envelope, Toc159 and Toc34, have been implicated in the binding and recognition of precursors of nuclear-encoded plastid proteins. Their involvement in conferring substrate dependency and organ specificity of PORA import was analyzed in intact Arabidopsis seedlings of wild type and the three mutants ppi3, ppi1 and ppi2 that are deficient in atToc34, atToc33, a closely related isoform of atToc34, and atToc159. Whereas none of these three Toc constituents is required for maintaining the organ specificity and substrate dependency of PORA import, atToc33 is indispensable for the import of PORB in cotyledons and true leaves suggesting that in these parts of the plant translocation of PORA and PORB occurs via two distinct import pathways. The analysis of PORA and PORB import into plastids of intact seedlings revealed an unexpected multiplicity of import routes that differed by their substrate, cell, tissue and organ specificities. This versatility of pathways for protein targeting to plastids suggests that in intact seedlings not only the constituents of the core complex of import channels but also other factors are involved in mediating the import of nuclear-encoded plastid proteins.

A role of Toc33 in the protochlorophyllide-dependent plastid import pathway of NADPH:protochlorophyllide oxidoreductase (POR) A

NADPH:protochlorophyllide oxidoreductase (POR) A is a key enzyme of chlorophyll biosynthesis in angiosperms. It is nucleus-encoded, synthesized as a larger precursor in the cytosol and imported into the plastids in a substrate-dependent manner. Plastid envelope membrane proteins, called protochlorophyllide-dependent translocon proteins, Ptcs, have been identified that interact with pPORA during import. Among them are a 16-kDa ortholog of the previously characterized outer envelope protein Oep16 (named Ptc16) and a 33-kDa protein (Ptc33) related to the GTP-binding proteins Toc33 and Toc34 of Arabidopsis. In the present work, we studied the interactions and roles of Ptc16 and Ptc33 during pPORA import. Radiolabeled Ptc16/Oep16 was synthesized from a corresponding cDNA and imported into isolated Arabidopsis plastids. Crosslinking experiments revealed that import of 35S-Oep16/Ptc16 is stimulated by GTP. 35S-Oep16/Ptc16 forms larger complexes with Toc33 but not Toc34. Plastids of the ppi1 mutant of Arabidopsis lacking Toc33, were unable to import pPORA in darkness but imported the small subunit precursor of ribulose-1,5-bisphosphate carboxylase/oxygenase (pSSU), precursor ferredoxin (pFd) as well as pPORB which is a close relative of pPORA. In white light, partial suppressions of pSSU, pFd and pPORB import were observed. Our results unveil a hitherto unrecognized role of Toc33 in pPORA import and suggest photooxidative membrane damage, induced by excess Pchlide accumulating in ppi1 chloroplasts because of the lack of pPORA import, to be the cause of the general drop of protein import.

A novel role of water-soluble chlorophyll proteins in the transitory storage of chorophyllide

All chlorophyll (Chl)-binding proteins involved in photosynthesis of higher plants are hydrophobic membrane proteins integrated into the thylakoids. However, a different category of Chl-binding proteins, the so-called water-soluble Chl proteins (WSCPs), was found in members of the Brassicaceae, Polygonaceae, Chenopodiaceae, and Amaranthaceae families. WSCPs from different plant species bind Chl a and Chl b in different ratios. Some members of the WSCP family are induced after drought and heat stress as well as leaf detachment. It has been proposed that this group of proteins might have a physiological function in the Chl degradation pathway. We demonstrate here that a protein that shared sequence homology to WSCPs accumulated in etiolated barley (Hordeum vulgare) seedlings exposed to light for 2 h. The novel 22-kD protein was attached to the outer envelope of barley etiochloroplasts, and import of the 27-kD precursor was light dependent and induced after feeding the isolated plastids the tetrapyrrole precursor 5-aminolevulinic acid. HPLC analyses and spectroscopic pigment measurements of acetone-extracted pigments showed that the 22-kD protein is complexed with chlorophyllide. We propose a novel role of WSCPs as pigment carriers operating during light-induced chloroplast development.

Identification of plastid envelope proteins required for import of protochlorophyllide oxidoreductase A into the chloroplast of barley

Chloroplasts synthesize an abundance of different tetrapyrrole compounds. Among them are chlorophyll and its precursor protochlorophyllide (Pchlide), which accumulate in light- and dark-grown plants, respectively. Pchlide is converted to chlorophyllide by virtue of the NADPH:Pchlide oxidoreductase (POR), which, in angiosperms, is the only known light-dependent enzyme of the chlorophyll biosynthetic pathway. In etiolated barley plants, two closely related POR proteins exist termed PORA and PORB, which are nuclear gene products. Here we identified plastid envelope proteins that interact with the cytosolic PORA precursor (pPORA) during its posttranslational chloroplast import. We demonstrate that pPORA interacts with several previously unreported components. Among them is a Pchlide a oxygenase, which provides Pchlide b as import substrate for pPORA, and a tyrosine aminotransferase thought to be involved in the synthesis of photoprotective vitamin E. Two other constituents were found to be orthologs of the GTP-binding proteins Toc33/34 and of the outer plastid envelope protein Oep16.

The outer plastid envelope protein Oep16: role as precursor translocase in import of protochlorophyllide oxidoreductase A

A 16-kDa plastid envelope protein was identified by chemical crosslinking that interacts with the precursor of NADPH:protochlorophyllide oxdidoreductase A (pPORA) during its posttranslational import into isolated barley chloroplasts. Protein purification and subsequent protein sequencing showed that the 16-kDa protein is an ortholog of a previously identified outer plastid envelope protein, Oep16. A protein of identical size was present in barley etioplasts and interacted with pPORA. Similar 16-kDa protein-dependent crosslink products of pPORA were detected in wheat, pea, and Arabidopsis chloroplasts. Database analyses revealed that the 16-kDa protein belongs to a family of preprotein and amino acid transporters found in free-living bacteria and endosymbiotic mitochondria and chloroplasts. Antibodies raised against the 16-kDa protein inhibited import of pPORA, highlighting its role in protein import.

Novel Insights into the Enzymology, Regulation and Physiological Functions of Light-dependent Protochlorophyllide Oxidoreductase in Angiosperms

The reduction of protochlorophyllide (Pchlide) is a key regulatory step in the biosynthesis of chlorophyll in phototrophic organisms. Two distinct enzymes catalyze this reduction; a light-dependent NADPH:protochlorophyllide oxidoreductase (POR) and light-independent Pchlide reductase (DPOR). Both enzymes are widely distributed among phototrophic organisms with the exception that only POR is found in angiosperms and only DPOR in anoxygenic photosynthetic bacteria. Consequently, angiosperms become etiolated in the absence of light, since the reduction of Pchlide in angiosperms is solely dependent on POR. In eukaryotic phototrophs, POR is a nuclear-encoded single polypeptide and post-translationally imported into plastids. POR possesses unique features, its light-dependent catalytic activity, accumulation in plastids of dark-grown angiosperms (etioplasts) via binding to its substrate, Pchlide, and cofactor, NADPH, resulting in the formation of prolamellar bodies (PLBs), and rapid degradation after catalysis under subsequent illumination. During the last decade, considerable progress has been made in the study of the gene organization, catalytic mechanism, membrane association, regulation of the gene expression, and physiological function of POR. In this review, we provide a brief overview of DPOR and then summarize the current state of knowledge on the biochemistry and molecular biology of POR mainly in angiosperms. The physiological and evolutional implications of POR are also discussed.

Substrate-dependent and organ-specific chloroplast protein import in planta

The NADPH-dependent protochlorophyllide (Pchlide) oxidoreductase (POR) is unique because it is a photoenzyme that requires light for its catalytic activity and uses Pchlide itself as a photoreceptor. In Arabidopsis, there are three structurally related PORs, denoted PORA, PORB, and PORC. The import of one of them, PORA, into plastids of cotyledons is substrate dependent. This substrate dependence is demonstrated in intact seedlings of wild-type Arabidopsis and two mutants, xantha2, which is devoid of Pchlide, and flu, which upon redarkening rapidly accumulates Pchlide. In true leaves, PORA uptake does not require the presence of Pchlide. The organ specificity of the substrate-dependent import of PORA reveals a means of controlling plastid protein translocation that is closely associated with a key step in plant development, the light-dependent transformation of cotyledons from a storage organ to a photosynthetically active leaf.

Functional analysis of isoforms of NADPH: protochlorophyllide oxidoreductase (POR), PORB and PORC, in Arabidopsis thaliana

NADPH:protochlorophyllide oxidoreductase (POR) catalyzes the light-dependent reduction of protochlorophyllide. To elucidate the physiological function of three differentially regulated POR isoforms (PORA, PORB and PORC) in Arabidopsis thaliana, we isolated T-DNA tagged null mutants of porB and porC. The mature seedlings of the mutants had normal photosynthetic competencies, showing that PORB and PORC are interchangeable and functionally redundant in developed plants. In etiolated seedlings, only porB showed a reduction in the photoactive protochlorophyllide and the size of prolamellar bodies (PLBs), indicating that PORB, as well as PORA, functioned in PLB assembly and photoactive protochlorophyllide formation in etiolated seedlings. When illuminated, the etiolated porB seedling was able to green to a similar extent as the wild type, whereas the greening was significantly reduced under low light conditions. During greening, high light irradiation increased the level of PORC protein, and the greening of porC was repressed under high light conditions. The porB, but not porC, etiolated seedling was more sensitive to the far-red block of greening than the wild type, which is caused by depletion of endogenous POR proteins resulting in photo-oxidative damage. These results suggest that, at the onset of greening, PLBs are important for efficient capture of light energy for photoconversion under various light conditions, and PORC, which is induced by high light irradiation, contributes to photoprotection during greening of the etiolated seedlings.

Toc, tic, and chloroplast protein import

The vast majority of chloroplast proteins are synthesized in precursor form on cytosolic ribosomes. Chloroplast precursor proteins have cleavable, N-terminal targeting signals called transit peptides. Transit peptides direct precursor proteins to the chloroplast in an organelle-specific way. They can be phosphorylated by a cytosolic protein kinase, and this leads to the formation of a cytosolic guidance complex. The guidance complex--comprising precursor, hsp70 and 14-3-3 proteins, as well as several unidentified components--docks at the outer envelope membrane. Translocation of precursor proteins across the envelope is achieved by the joint action of molecular machines called Toc (translocon at the outer envelope membrane of chloroplasts) and Tic (translocon at the inner envelope membrane of chloroplasts), respectively. The action of the Toc/Tic apparatus requires the hydrolysis of ATP and GTP at different levels, indicating energetic requirements and regulatory properties of the import process. The main subunits of the Toc and Tic complexes have been identified and characterized in vivo, in organello and in vitro. Phylogenetic evidence suggests that several translocon subunits are of cyanobacterial origin, indicating that today's import machinery was built around a prokaryotic core.

Identification of two differentially regulated isoforms of protochlorophyllide oxidoreductase (POR) from tobacco revealed a wide variety of light- and development-dependent regulations of POR gene expression among angiosperms

NADPH-protochlorophyllide oxidoreductase (POR) catalyzes the light-dependent reduction of protochlorophyllide a in the chlorophyll biosynthetic pathway. Here, we identified two distinct POR cDNAs from tobacco. Both POR isoforms are encoded by a respective single copy gene in tobacco genome. The overall deduced amino acid sequences of two tobacco cDNAs, designated here POR1 and POR2, displayed significant identities ( approximately 75%), but showed different patterns of light and developmental regulation. In contrast to the previously isolated POR isoforms of Arabidopsis thaliana and barley, the expression of both tobacco POR isoforms were not negatively regulated by light and persisted in matured green tissues. Furthermore, the expression of both genes appeared to be regulated by a diurnal regulation. These results show a wide variety of light- and development-dependent regulations of POR gene expression among angiosperms. Furthermore, phylogenetic analysis including tobacco revealed that POR gene family is differentially represented by angiosperms, most of which is probably caused by independent gene duplication in individual plant. Present results imply a modification of the previous concept that chlorophyll biosynthesis and chloroplast differentiation in angiosperms are ubiquitously controlled by unique functions of two POR isoforms.

Toc, Tic, and chloroplast protein import

The vast majority of chloroplast proteins are synthesized in precursor form on cytosolic ribosomes. Chloroplast precursor proteins have cleavable, N-terminal targeting signals called transit peptides. Transit peptides direct precursor proteins to the chloroplast in an organelle-specific way. They can be phosphorylated by a cytosolic protein kinase, and this leads to the formation of a cytosolic guidance complex. The guidance complex--comprising precursor, hsp70 and 14-3-3 proteins, as well as several unidentified components--docks at the outer envelope membrane. Translocation of precursor proteins across the envelope is achieved by the joint action of molecular machines called Toc (translocon at the outer envelope membrane of chloroplasts) and Tic (translocon at the inner envelope membrane of chloroplasts), respectively. The action of the Toc/Tic apparatus requires the hydrolysis of ATP and GTP at different levels, indicating energetic requirements and regulatory properties of the import process. The main subunits of the Toc and Tic complexes have been identified and characterized in vivo, in organello and in vitro. Phylogenetic evidence suggests that several translocon subunits are of cyanobacterial origin, indicating that today's import machinery was built around a prokaryotic core.

Association of the NADPH:protochlorophyllide oxidoreductase (POR) with isolated etioplast inner membranes from wheat

Membrane association of NADPH:protochlorophyllide oxidoreductase (POR, EC: 1.6.99.1) with isolated prolamellar bodies (PLBs) and prothylakoids (PTs) from wheat etioplasts was investigated. In vitro-expressed radiolabelled POR, with or without transit peptide, was used to characterize membrane association conditions. Proper association of POR with PLBs and PTs did not require the presequence, whereas NADPH and hydrolysable ATP were vital for the process. After treating the membranes with thermolysin, sodium hydroxide or carbonate, a firm attachment of the POR protein to the membrane was found. Although the PLBs and PTs differ significantly in their relative amount of POR in vivo, no major differences in POR association capacity could be observed between the two membrane systems when exogenous NADPH was added. Experiments run with only an endogenous NADPH source almost abolished association of POR with both PLBs and PTs. In addition, POR protein carrying a mutation in the putative nucleotide-binding site (ALA06) was unable to bind to the inner membranes in the presence of NADPH, which further demonstrates that the co-factor is essential for proper membrane association. POR protein carrying a mutation in the substrate-binding site (ALA24) showed less binding to the membranes as compared to the wild type. The results presented here introduce studies of a novel area of protein-membrane interaction, namely the association of proteins with a paracrystalline membrane structure, the PLB.

Maize non-photosynthetic ferredoxin precursor is mis-sorted to the intermembrane space of chloroplasts in the presence of light

Preprotein translocation across the outer and inner envelope membranes of chloroplasts is an energy-dependent process requiring ATP hydrolysis. Several precursor proteins analyzed so far have been found to be imported into isolated chloroplasts equally well in the dark in the presence of ATP as in the light where ATP is supplied by photophosphorylation in the chloroplasts themselves. We demonstrate here that precursors of two maize (Zea mays L. cv Golden Cross Bantam) ferredoxin isoproteins, pFdI and pFdIII, show distinct characteristics of import into maize chloroplasts. pFdI, a photosynthetic ferredoxin precursor, was efficiently imported into the stroma of isolated maize chloroplasts both in the light and in the dark. In contrast pFdIII, a non-photosynthetic ferredoxin precursor, was mostly mis-sorted to the intermembrane space of chloroplastic envelopes as an unprocessed precursor form in the light but was efficiently imported into the stroma and processed to its mature form in the dark. The mis-sorted pFdIII, which accumulated in the intermembrane space in the light, could not undergo subsequent import into the stroma in the dark, even in the presence of ATP. However, when the mis-sorted pFdIII was recovered and used for a separate import reaction, pFdIII was capable of import into the chloroplasts in the dark. pFNRII, a ferredoxin-NADP+ reductase isoprotein precursor, showed import characteristics similar to those of pFdIII. Moreover, pFdIII exhibited similar import characteristics with chloroplasts isolated from wheat (Pennisetum americanum) and pea (Pisum sativum cv Alaska). These findings suggest that the translocation of precursor proteins across the envelope membranes of chloroplasts may involve substrate-dependent light-regulated mechanisms.

NADPH:Protochlorophyllide oxidoreductase uses the general import route into chloroplasts

Chloroplast differentiation in angiosperm plants depends on the light-dependent conversion of protochlorophyllide to chlorophyllide by NADPH:protochlorophyllide oxidoreductase (PORA; EC 1.6.99.1), a nuclearly encoded protein. The protein import of the precursor form of PORA into plastids was shown previously to strictly depend on the presence of its substrate protochlorophyllide. PORA seemed to follow a novel, posttranslationally regulated import route. Here we demonstrate that the precursor of PORA from barley is imported into isolated barley plastids independently of protochlorophyllide. PORA as well as PORB import is competed for by the precursor of the small subunit of Rubisco. The data demonstrate that the PORA precursor uses the general import pathway into plastids. Furthermore, en route into chloroplasts the pea POR precursor can be cross-linked to the protein import channel in the outer envelope Toc75 from pea.

Photoactive protochlorophyllide regeneration in cotyledons and leaves from higher plants

Chlorophyll accumulation during greening implies the continuous transformation of photoactive protochlorophyllide (Pchlide) to chlorophyllide. Since this reaction is a light-dependent step, the study of regeneration of photoactive Pchlide under a continuous illumination is difficult. Therefore this process is best studied on etiolated plants during a period of darkness following the initial photoreduction of photoactive Pchlide. In this study, the regeneration process has been studied using spinach cotyledons, as well as barley and bean leaves, illuminated by a single saturating flash. The regeneration was characterized using 77 K fluorescence emission and excitation spectra and high-performance liquid chromatography. The fluorescence data indicated that the same spectral forms of photoactive Pchlide are regenerated by different pathways: (1) photoactive Pchlide regeneration starts immediately after the photoreduction through the formation of a nonphotoactive Pchlide form, emitting fluorescence at approximately 651 nm. This form is similar to the large aggregate of photoactive Pchlide present before the illumination, but it contains oxidized form of nicotinamide adenine dinucleotide phosphate, instead of the reduced form (NADPH), in the ternary complexes; and (2) after the dislocation of the large aggregates of chlorophyllide-light-dependent NADPH:Pchlide a photooxidoreductase-NADPH ternary complexes, the regeneration occurs at the expense of the several nonphotoactive Pchlide spectral forms present before the illumination.

A second, substrate-dependent site of protein import into chloroplasts

Chloroplasts must import a large number of proteins from the cytosol. It generally is assumed that this import proceeds for all stromal and thylakoid proteins in an identical manner and is caused by the operation of two distinctive protein import machineries in the outer and inner plastid envelope, which form the general import site. Here we show that there is a second site of protein translocation into chloroplasts of barley, tobacco, Arabidopsis thaliana, and five other tested monocotyledonous and dicotyledonous plant species. This import site is specific for the cytosolic precursor of the NADPH:protochlorophyllide (Pchlide) oxidoreductase A, pPORA. It couples Pchlide synthesis to pPORA import and thereby reduces the actual level of free Pchlide, which, because of its photodynamic properties, would be destructive to the plastids. Consequently, photoprotection is conferred onto the plant.

NADPH-protochlorophyllide oxidoreductase in cucumber is encoded by a single gene and its expression is transcriptionally enhanced by illumination

NADPH-protochlorophyllide oxidoreductase (POR) catalyzes the photoreduction of protochlorophyllide a in the chlorophyll biosynthetic pathway. Recently, two POR genes, the expressions of which were differently regulated by light, were isolated in barley and Arabidopsis, and it is suggested that in angiosperms, the chlorophyll synthesis is controlled by the differential action of the two distinct POR isozymes. Meanwhile, we have isolated POR cDNA from cucumber, and reported that its expression was enhanced by illumination. In this study, we analyzed the gene expression and organization of cucumber POR in more detail. Northern blot analysis with 5'- or 3'-noncoding gene-specific and full length probes and RNase protection assay revealed that cucumber POR mRNA is transcribed from a single gene. Run-on assay revealed that the transcript level of isolated nuclei from illuminated cotyledons was 6-fold that of dark control, indicating that the light-enhanced expression of cucumber POR is caused by transcriptional activation. Using degenerated primers and the genomic DNA of cucumber as template, PCR showed the presence of only one type of fragment encoding cucumber POR. The PCR product was hybridized with the genomic Southern blot from cucumber and only one band was detected under low stringency conditions. From these results, we conclude that there is a single POR gene in cucumber and PORs are organized by different gene families among higher plants, and in cucumber the single POR may play the roles of two POR genes, which are present in other higher plants such as Arabidopsis and barley.

Localization of NADPH-protochlorophyllide reductase in plastids of barley at different greening stages

The localization of protochorophyllide (Pchlide) and of NADPH-protochlorophyllide oxidoreductase (POR, EC 1.6.99.1) within (etio)chloroplasts has been investigated at selected stages of greening of barley seedlings. Pchlide pigment and POR protein contents were evaluated in different plastid membrane fractions by fluorescence spectroscopy and immunoblot analysis using a monospecific polyclonal antibody raised against the purified enzyme. Fluorescence analysis showed the presence of Pchlide in both the envelope and thylakoid membranes. During greening, the Pchlide content, expressed on a total protein basis, decreased in thylakoid membranes, whereas it increased in the envelope membranes. POR proteins were detected mainly in thylakoid membranes at early greening stages. In contrast, the weak amount of POR proteins was associated more specifically with envelope membranes of mature chloroplasts. Whatever the greening stage, thylakoid-bound Pchlide and POR proteins were more abundant in the thylakoid regions which remained unsolubilized after mild Triton treatment used as standard procedure to prepare PS II particles. This suggests the preferential association of Pchlide and POR to the appressed regions of thylakoids.

Pigment-free NADPH:protochlorophyllide oxidoreductase from Avena sativa L. Purification and substrate specificity

The enzyme NADPH:protochlorophyllide oxidoreductase (POR) is the key enzyme for light-dependent chlorophyll biosynthesis. It accumulates in dark-grown plants as the ternary enzyme-substrate complex POR-protochlorophyllide a-NADPH. Here, we describe a simple procedure for purification of pigment-free POR from etioplasts of Avena sativa seedlings. The procedure implies differential solubilization with n-octyl-beta-D-glucoside and one chromatographic step with DEAE-cellulose. We show, using pigment and protein analysis, that etioplasts contain a one-to-one complex of POR and protochlorophyllide a. The preparation of 13 analogues of protochlorophyllide a is described. The analogues differ in the side chains of the macrocycle and in part contain zinc instead of the central magnesium. Six analogues with different side chains at rings A or B are active substrates, seven analogues with different side chains at rings D or E are not accepted as substrates by POR. The kinetics of the light-dependent reaction reveals three groups of substrate analogues with a fast, medium and slow reaction. To evaluate the kinetic data, the molar extinction coefficients in the reaction buffer had to be determined. At concentrations above 2 mole substrate/mole enzyme, inhibition was found for protochlorophyllide a and for the analogues.

Differential expression of genes encoding the light-dependent and light-independent enzymes for protochlorophyllide reduction during development in loblolly pine

The expression patterns of the two distinct subfamilies of genes (designated porA and porB) encoding the light-dependent NADPH:protochlorophyllide oxidoreductases (PORs) in loblolly pine (Pinus taeda L.) were examined. Transcripts arising from both gene subfamilies were shown to be present at high levels in the cotyledons of dark-grown pine seedlings and to a lesser extent in their stems. Exposure of dark-grown seedlings to light resulted in increased levels of both porA and porB transcripts, as well as increased levels of mRNAs encoding other photosynthesis-related gene products, suggesting that they are under a common mode of regulation. Relative levels of the porA and porB transcripts were similar in seedling cotyledons and primary needles of two-month-old pine trees, whereas only porB transcripts were present at a significant level in mature secondary needles of two-year-old trees. Immunoblot analysis showed that the 37 kDa PORA protein was most abundant in dark-grown tissues, decreased dramatically upon exposure to light, but could still be detected at low levels in light-grown seedlings. In comparison, levels of the 38 kDa PORB protein were not significantly changed upon transfer of dark-grown tissues to light. While both PORA and PORB were detected in cotyledons and primary needles, only PORB could be detected in mature needles. Transcripts derived from the three plastid genes, chlL, chlN, and chlB, encoding subunits of the light-independent protochlorophyllide reductase were detected in the cotyledons and stems of dark-grown seedlings, and in mature needles. The highest levels of chlL, chlN, and chlB transcripts were detected within the top one-third of the stem and decreased gradually towards the stem/root transition zone. Correspondingly, the highest levels of light-independent chlorophyll formation took place near the top of the hypocotyl. A similar pattern of expression was observed for other photosynthesis-related gene products, including porA and porB. Our results suggest that many aspects of the light-dependent, tissue-specific and developmental regulation of POR expression first described in angiosperms were already established in the less evolutionarily advanced gymnosperms. However, unlike angiosperms, light is not the dominant regulatory factor controlling porA expression in these species.