Structure, composition, and distribution of plastid nucleoids in Narcissus pseudonarcissus

Spatial Features and Functional Implications of Plant 3D Genome Organization

The advent of high-throughput sequencing-based methods for chromatin conformation, accessibility, and immunoprecipitation assays has been a turning point in 3D genomics. Altogether, these new tools have been pushing upward the interpretation of pioneer cytogenetic evidence for a higher order in chromatin packing. Here, we review the latest development in our understanding of plant spatial genome structures and different levels of organization and discuss their functional implications. Then, we spotlight the complexity of organellar (i.e., mitochondria and plastids) genomes and discuss their 3D packing into nucleoids. Finally, we propose unaddressed research axes to investigate functional links between chromatin-like dynamics and transcriptional regulation within organellar nucleoids.

Multifunctionality of plastid nucleoids as revealed by proteome analyses

Protocols aimed at the isolation of nucleoids and transcriptionally active chromosomes (TACs) from plastids of higher plants have been established already decades ago, but only recent improvements in the mass spectrometry methods enabled detailed proteomic characterization of their components. Here we present a comprehensive analysis of the protein compositions obtained from two proteomic studies of TAC fractions isolated from Arabidopsis/mustard and spinach chloroplasts, respectively, as well as nucleoid fractions from Arabidopsis, maize and pea. Interestingly, different approaches as well as the use of diverse starting materials resulted in the detection of varying protein catalogues with a number of shared proteins. Possible reasons for the discrepancies between the protein repertoires and for missing out some of the nucleoid proteins that have been identified previously by other means than mass spectrometry as well as the repeated identification of "unexpected" proteins indicating potential links between DNA/RNA-associated nucleoid core functions and energy metabolism as well as biosynthetic activities of plastids will be discussed. In accordance with the nucleoid association of proteins involved in key functions of plastids including photosynthesis, the phenotypes of mutants lacking one or the other plastid nucleoid-associated protein (ptNAP) show the importance of nucleoid proteins for overall plant development and growth. This article is part of a Special Issue entitled: Plant Proteomics--a bridge between fundamental processes and crop production, edited by Dr. Hans-Peter Mock.

New insights into plastid nucleoid structure and functionality

Investigations over many decades have revealed that nucleoids of higher plant plastids are highly dynamic with regard to their number, their structural organization and protein composition. Membrane attachment and environmental cues seem to determine the activity and functionality of the nucleoids and point to a highly regulated structure-function relationship. The heterogeneous composition and the many functions that are seemingly associated with the plastid nucleoids could be related to the high number of chromosomes per plastid. Recent proteomic studies have brought novel nucleoid-associated proteins into the spotlight and indicated that plastid nucleoids are an evolutionary hybrid possessing prokaryotic nucleoid features and eukaryotic (nuclear) chromatin components, several of which are dually targeted to the nucleus and chloroplasts. Future studies need to unravel if and how plastid-nucleus communication depends on nucleoid structure and plastid gene expression.

Proteomic Characterization of A Triton-Insoluble Fraction from Chloroplasts Defines A Novel Group of Proteins Associated with Macromolecular Structures

Proteomic analysis of a Triton X-100 insoluble, 30,000 x g pellet from purified pea chloroplasts resulted in the identification of 179 nonredundant proteins. This chloroplast fraction was mostly depleted of chloroplast membranes since only 23% and 9% of the identified proteins were also observed in envelope and thylakoid membranes, respectively. One of the most abundant proteins in this fraction was sulfite reductase, a dual function protein previously shown to act as a plastid DNA condensing protein. Approximately 35 other proteins known (or predicted) to be associated with high-density protein-nucleic acid particles (nucleoids) were also identified including a family of DNA gyrases, as well as proteins involved in plastid transcription and translation. Although nucleoids appeared to be the predominant component of 30k x g Triton-insoluble chloroplast preparations, multi-enzyme protein complexes were also present including each subunit to the pyruvate dehydrogenase and acetyl-CoA carboxylase multi-enzyme complexes, as well as a proposed assembly of the first three enzymes of the Calvin cycle. Approximately 18% of the proteins identified were annonated as unknown or hypothetical proteins and another 20% contained "putative" or "like" in the identifier tag. This is the first proteomic characterization of a membrane-depleted, high-density fraction from plastids and demonstrates the utility of this simple procedure to isolate intact macromolecular structures from purified organelles for analysis of protein-protein and protein-nucleic acid interactions.

Organelle Nuclei in Higher Plants: Structure, Composition, Function, and Evolution

Plant cells have two distinct types of energy-converting organelles: plastids and mitochondria. These organelles have their own DNAs and are regarded as descendants of endosymbiotic prokaryotes. The organelle DNAs associate with various proteins to form compact DNA-protein complexes, which are referred to as organelle nuclei or nucleoids. Various functions of organelle genomes, such as DNA replication and transcription, are performed within these compact structures. Fluorescence microscopy using the DNA-specific fluorochrome 4',6-diamidino-2-phenylindole has played a pivotal role in establishing the concept of "organelle nuclei." This fluorochrome has also facilitated the isolation of morphologically intact organelle nuclei, which is indispensable for understanding their structure and composition. Moreover, development of an in vitro transcription?DNA synthesis system using isolated organelle nuclei has provided us with a means of measuring and analyzing the function of organelle nuclei. In addition to these morphological and biochemical approaches, genomics has also had a great impact on our ability to investigate the components of organelle nuclei. These analyses have revealed that organelle nuclei are not a vestige of the bacterial counterpart, but rather are a complex system established through extensive interaction between organelle and cell nuclear genomes during evolution. Extensive diversion or exchange during evolution is predicted to have occurred for several important structural proteins, such as major DNA-compacting proteins, and functional proteins, such as RNA and DNA polymerases, resulting in complex mechanisms to control the function of organelle genomes. Thus, organelle nuclei represent the most dynamic front of interaction between the three genomes (cell nuclear, plastid, and mitochondrial) constituting eukaryotic plant cells.

Plastid division: its origins and evolution

Photosynthetic eukaryotes have evolved plastid division mechanisms since acquisition of plastids through endosymbiosis. The emerging evolutionary origin of the plastid division mechanism is remarkably complex. The constituents of the division apparatus of plastids may have complex origins. The one constituent is the plastid FtsZ ring taken over from the cyanobacteria-like ancestral endosymbionts. The second is the doublet of concentric plastid dividing rings (or triplet in red algae), possibly acquired by ancestral host eukaryotes following the primary endosymbiotic event. Placement of the division apparatus at the correct division site may involve a system analogous to the bacterial Min system. Plastid nucleoid partitioning may be mediated by binding to envelope or thylakoid membranes. Multiple copies of plastid DNA and symmetrical distribution of the nucleoids in the plastids may permit faithful transmission to daughter plastids via equal binary plastid divisions. Cyanelles retain peptidoglycan wall and cyanelle division occurs through septum formation such as bacterial cell division. Cyanelle division involves the cyanelle ring analogous to the inner stromal plastid-dividing (PD) ring. According to the prevailing hypothesis that primary endosymbiosis occurred only once, cyanelle division may represent an intermediate stage between cyanobacterial division and the well-known plastid division among extant plants. With the secondary plastids, which are surrounded by three or four membranes, the PD ring also participates in division of the inner two "true" plastid envelope membranes, and the third and the outermost membranes divide by unknown mechanisms.

Organization, Developmental Dynamics, and Evolution of Plastid Nucleoids

The plastid is a semiautonomous organelle essential in photosynthesis and other metabolic activities of plants and algae. Plastid DNA is organized into the nucleoid with various proteins and RNA, and the nucleoid is subject to dynamic changes during the development of plant cells. Characterization of the major DNA-binding proteins of nucleoids revealed essential differences in the two lineages of photosynthetic eukaryotes, namely nucleoids of green plants contain sulfite reductase as a major DNA-binding protein that represses the genomic activity, whereas the prokaryotic DNA-binding protein HU is abundant in plastid nucleoids of the rhodophyte lineage. In addition, current knowledge on DNA-binding proteins, as well as the replication and transcription systems of plastids, is reviewed from comparative and evolutionary points of view. A revised hypothesis on the discontinuous evolution of plastid genomic machinery is presented: despite the cyanobacterial origin of plastids, the genomic machinery of the plastid genome is fundamentally different from its counterpart in cyanobacteria.

The multisubunit acetyl-CoA carboxylase is strongly associated with the chloroplast envelope through non-ionic interactions to the carboxyltransferase subunits

The committed step for de novo fatty acid biosynthesis is the carboxylation of acetyl-CoA catalyzed by acetyl-CoA carboxylase (ACCase). Plastidial ACCase from most plants is a multisubunit complex composed of multiple copies of four different polypeptides, biotin carboxyl carrier protein (BCCP), biotin carboxylase (BC), and carboxyltransferase (alpha-CT and beta-CT). Immunoblot analyses revealed these four proteins were mostly (69% of total) associated with a 17,000 g insoluble fraction from lysed pea chloroplasts. Under the same conditions only 8% of ribulose-1,5-bisphosphate carboxylase was associated with this insoluble fraction. BCCP and biotin carboxylase BC subunits freely dissociated from 17 kg insoluble fractions under high ionic strength conditions, whereas alpha-CT and beta-CT subunits remained tightly associated. Both CT subunits were highly enriched in envelope versus stroma and thylakoid preparations whereas BC and BCCP subunits were predominantly stromal-localized due to partial dissociation. Rapid solubilization of intact chloroplasts with Triton X-100 followed by centrifugation at 30 kg resulted in a pellet that was up to 8-fold enriched in ACCase activity and 21-fold enriched in BC activity. Triton-insoluble 30 kg pellets were reduced in lipid and chlorophyll content but enriched in chloroplast DNA due to the isolation of nucleoid particles. However, ACCase was not directly associated with nucleoids since enzymatic digestion of DNA or RNA had no effect on the association with Triton-insoluble matter. The amount of Triton-insoluble ACCase was similar in chloroplasts isolated from dark- or light-adapted leaves suggesting transitory starch granules were also not involved in this association. It is proposed that ACCase is associated with envelope membranes through interactions with an unidentified integral membrane protein.

Was the evolution of plastid genetic machinery discontinuous?

The plastid nucleoid consists of plastid DNA and various, mostly uncharacterized, DNA-binding proteins. The plastid DNA undoubtedly originated from an ancestral cyanobacterial genome, but the origin of the nucleoid proteins appears complex. Initial biochemical analysis of these proteins, as well as comparative genome informatics, suggest that proteins of eukaryotic origin replaced most of the original prokaryotic proteins during the evolution of plastids in the lineage of green plants.

The 70-kDa major DNA-compacting protein of the chloroplast nucleoid is sulfite reductase

The chloroplast nucleoid is a complex of chloroplast DNA and various, mostly uncharacterized proteins. An abundant 70-kDa protein of the isolated nucleoids of pea chloroplasts was identified as sulfite reductase by N-terminal sequence analysis as well as immunoblot analysis, spectrophotometry and enzyme activity analysis. Recombinant maize sulfite reductase was indeed able to compact chloroplast DNA and to form nucleoid-like particles in vitro. The role of sulfite reductase in the structural organization of the nucleoid is discussed.

Temporal and spatial coordination of cells with their plastid component

Careful coordination of cell multiplication with plastid multiplication and partition at cytokinesis is required to maintain the universal presence of plastids in the major photosynthetic lines of evolution. However, no cell cycle control points are known that might underlie this coordination. We review common properties, and their variants, of plastids and plastid DNA in germline, multiplying, and mature cells of phyla capable of photosynthesis. These suggest a basic level of control dictated perhaps by the same mechanisms that coordinate cell size with the nuclear ploidy level. No protein synthesis within the plastid appears to be necessary for this system to operate successfully at the level that maintains the presence of plastids in cells. A second, and superimposed, level of controls dictates expansion of the plastid in both size and number in response to signals associated with differentiation and with the environment. We also compare the germane properties of plastids with those of mitochondria. With the advent of genomics and new cell and molecular techniques, the players in these control mechanisms should now be identifiable.

Do plastid envelope membranes play a role in the expression of the plastid genome?

A unique biochemical machinery is present within the two envelope membranes surrounding plastids (Joyard et al., Plant Physiol. 118 (1998) 715-723) that reflects the stage of development of the plastid and the specific metabolic requirements of the various tissues. Envelope membranes are the site for the synthesis and metabolism of specific lipids. They are also the site of transport of metabolites, proteins and information between plastids and surrounding cellular compartments. For instance, a complex machinery for the import of nuclear-encoded plastid proteins is rapidly being elucidated. The functional studies of plastid envelope membranes result in the characterization of an increasing number of envelope proteins with unexpected functions. For instance, recent experiments have demonstrated that envelope membranes bind specifically to plastid genetic systems, the nucleoids surrounded by plastid ribosomes. At early stages of plastid differentiation, the inner envelope membrane contains a unique protein (named PEND protein) that binds specifically to plastid DNA. This tight connection suggests that the PEND protein is at least involved in partitioning the plastid DNA to daughter plastids during division. The PEND protein can also provide a physical support for replication and transcription. In addition, factors involved in the control of plastid protein synthesis can become associated to envelope membranes. This was shown for a protein homologous to the E. coli ribosome recycling factor and for the stabilizing factors of some specific chloroplast mRNAs encoding thylakoid membrane proteins. In fact, the envelope membranes together with the plastid DNA are the two essential constituents of plastids that confer identity to plastids and their interactions are becoming uncovered through molecular as well as cytological studies. In this review, we will focus on these recent observations (which are consistent with the endosymbiotic origin of plastids) and we discuss possible roles for the plastid envelope in the expression of plastid genome.

The recombinant product of the Chryptomonas phi plastid gene hlpA is an architectural HU-like protein that promotes the assembly of complex nucleoprotein structures

The HlpA protein which is encoded by the hlpA gene in the plastid genome of the cryptomonad alga Chryptomonas phi is structurally related to the non-sequence-specific DNA-binding and DNA-bending HU family of chromatin-associated proteins. The expression of the HlpA protein complements the mutant phenotype of Bacillus subtilis cells impaired in the Hbsu protein (B. subtilis HU), as measured by the resistance of the cells to methylmethane sulphonate. To analyse the interactions of HlpA with DNA, we expressed the protein in Escherichia coli and purified it to homogeneity. HlpA interacts preferentially with four-way junction DNA or DNA minicircles, when compared with linear DNA, recognising DNA structure. HlpA and E. coli HU display comparable affinities for all types of DNA tested; however, HlpA exhibits a stronger tendency to self-associate in the presence of DNA. Accordingly, HlpA oligomerises more readily than HU in protein crosslinking experiments. In the presence of topoisomerase I, HlpA constrains negative superhelical turns in closed circular plasmid DNA. The HlpA protein mediates the joining of distant recombination sites into a complex nucleoprotein structure, as judged by beta-mediated site-specific recombination. The results presented provide evidence that HlpA is a functional plastid equivalent of nuclear and mitochondrial HMG1-like proteins and bacterial HU proteins.

Biochemistry and molecular biology of chromoplast development

Plant cells contain a unique class of organelles, designated the plastids, which distinguish them from animal cells. According to the largely accepted endosymbiotic theory of evolution, plastids are descendants of prokaryotes. This process requires several adaptative changes which involve the maintenance and the expression of part of the plastid genome, as well as the integration of the plastid activity to the cellular metabolism. This is illustrated by the diversity of plastids encountered in plant cells. For instance, in tissues undergoing color changes, i.e., flowers and fruits, the chromoplasts produce and accumulate excess carotenoids. In this paper we attempt to review the basic aspects of chromoplast development.

Preferential mitochondrial and plastid DNA synthesis before multiple cell divisions in Nicotiana tabacum

Organelle DNA synthesis in root meristem and cultured cell line BY-2, both derived from Nicotiana tabacum cv. Bright Yellow 2, was examined by immunofluorescence microscopy of Technovit sections with antibody against 5- bromodeoxyuridine (BrdU) and co-fluorescent staining with 4′,6-diamidino-2-phenylindole (DAPI) and quantitative Southern hybridization. In the root meristem, the mitochondrial DNAs (mtDNAs) were synthesized in a specific region near to the quiescent center, where a low frequency of DNA synthesis of cell nuclei was observed. The mitochondrial nuclei (nucleoids) changed morphologically from long ellipsoids with a high frequency of DNA synthesis, in the region just above the quiescent center, to granules with a low frequency of DNA synthesis, as cell distance from the quiescent center increased. Similar patterns were observed in the cultured tobacco cell line (BY-2), in which large amounts of preferential synthesis of DNA of both mitochondria and plastids occurred prior to cell nuclear DNA synthesis just after stationary phase cells were transferred to fresh medium. Granular mitochondria which vigorously synthesized mtDNA were observed in both lag phase and logarithmic growth phase cells. However, long, ellipsoidal mitochondria which showed a low frequency of mtDNA synthesis were observed in stationary phase cells. Morphological changes of plastids were more conspicuous than those of mitochondria. After the medium was renewed, spherical plastids became extremely elongated and string-like, for 24 h, but were divided into small pieces after the third day. Vigorous synthesis of plastid DNA (ptDNA) occurred during this period of plastids elongation.

Structural organization and transcription of plant mitochondrial and chloroplast genomes

Experimental evidence is presented showing that the plant mitochondrial and chloroplast genomes are multipartite and, that besides a large circular genomic DNA, they contain subgenomic minicircular and plasmid-like molecules. It is demonstrated that plant mitochondrial and chloroplast DNAs are packaged into deoxynucleoprotein fibrils comprising nucleosome-like and nucleomere-like globules; the fibrils form loops and rosette-like structures with central proteinaceous components. A similar structure is characteristic of the subgenomic DNAs. The basic proteins involved in the formation of nucleosome-like globules are quite different from the nuclear histones, indeed the basic proteins from plant mitochondria and chloroplasts are also distinct. Some of the basic proteins share common antigens with the E. coli HU protein. The genetic code for the mitochondrial and chloroplast genes is universal. The only codon now thought to be different from the universal in the mitochondrial genome is corrected during post-transcriptional mRNA editing. There are two hexanucleotides in the promoters of the chloroplast genes homologous to the sequences in -10 and -35 regions of the prokaryotic genes promoters requisite for transcription. Promoter sequences of the plant mitochondria genes responsible for transcription regulation were not identified. Immunoelectronmicroscopic evidence suggest that mitochondrial and chloroplast RNA polymerases have antigens in common with the beta-subunit of E. coli RNA polymerase. It is shown that the mitochondrial genes are intensely transcribed in the dark and repressed by illumination. Electron microscopy demonstrated that about 70% of plant mitochondria contain numerous RNA polymerase molecules in the dark, but this percentage falls to 10-15% after light exposure.

Absence of ribosomes in Capsicum chromoplasts

Ribosome development was followed by electron microscopy and gel electrophoresis of ribosomal (r)RNAs in the plastids of fully expanded fruits of Capsicum annuum L. during ripening. Chloroplasts from young Capsicum leaves were used as a structural and electrophoretic standard. Four stages were distinguished on the basis of colour changes during fruit ripening. Chloroplasts of the green fruit had a lower content of 16S and 23S rRNAs than leaf chloroplasts. They contained only a few ribosomes, some more discrete "ribosomal particles", and the contrast of ribosomal structures was faint. From the outset of ripening, most of the ribosomal structures in the plastid stroma disappeared. A continuous decrease in plastid rRNAs occurred during ripening. Fully differentiated chromoplasts of the red fruit did not contain rRNAs or ribosomes. Throughout plastid development, DNA nucleoids were evident and there was only a small decrease in the DNA peak on electrophoretograms. The loss of ribosomes during the chloroplast-to-chromoplast conversion in Capsicum fruit is discussed in relation to the variations in pigments and enzymic systems in both plastid types.

Characterization and properties of the spinach chloroplast transcriptionally active chromosome isolated at high ionic strength

The transcriptionally active chromosome (TAC) of spinach (Spinacia oleracea L.) chloroplasts has been isolated at a high ionic strength, with low mechanical shearing, by glycerol gradient centrifugation. The properties of the TAC differ from those previously reported for the TAC isolated either from Euglena chloroplasts or from spinach using a low-ionic-strength solubilization medium and gel filtration. The high-salt-isolated TAC is homogenous in density but not in size and contains fewer weakly bound proteins than its lowsalt-isolated homologue. In vitro, it promotes elongation of the RNA chains previously initiated in vivo. Transcription is not limited to the ribosomal DNA. The transcriptional pattern is not strongly affected by the high-salt preparation. Ribonuclease pretreatment of the TAC, prior to the in-vitro transcription, leads to a more than tenfold increase of the transcription activity. These properties are discussed in relation to the structure of the spinach chloroplast chromosome.