Cytoplasmic and chloroplast synthesis of phycobilisome polypeptides

Trafficking of protein into the recently established photosynthetic organelles of Paulinella chromatophora

Endosymbiotic acquisition of bacteria by a protist, with subsequent evolution of the bacteria into mitochondria and plastids, had a transformative impact on eukaryotic biology. Reconstructing events that created a stable association between endosymbiont and host during the process of organellogenesis--including establishment of regulated protein import into nascent organelles--is difficult because they date back more than 1 billion years. The amoeba Paulinella chromatophora contains nascent photosynthetic organelles of more recent evolutionary origin (∼60 Mya) termed chromatophores (CRs). After the initial endosymbiotic event, the CR genome was reduced to approximately 30% of its presumed original size and more than 30 expressed genes were transferred from the CR to the amoebal nuclear genome. Three transferred genes--psaE, psaK1, and psaK2--encode subunits of photosystem I. Here we report biochemical evidence that PsaE, PsaK1, and PsaK2 are synthesized in the amoeba cytoplasm and traffic into CRs, where they assemble with CR-encoded subunits into photosystem I complexes. Additionally, our data suggest that proteins routed to CRs pass through the Golgi apparatus. Whereas genome reduction and transfer of genes from bacterial to host genome have been reported to occur in other obligate bacterial endosymbioses, this report outlines the import of proteins encoded by such transferred genes into the compartment derived from the bacterial endosymbiont. Our study showcases P. chromatophora as an exceptional model in which to study early events in organellogenesis, and suggests that protein import into bacterial endosymbionts might be a phenomenon much more widespread than currently assumed.

Isolation and characterization of a gene for a major light-harvesting polypeptide from Cyanophora paradoxa

Antibodies raised against mixtures of phycobilisome polypeptides from the eukaryotic alga Cyanidium caldarium were used in an immunological screen to detect expression of phycobiliprotein genes in an Escherichia coli library containing segments of plastid (chloroplast, cyanelle) DNA from another eukaryotic alga, Cyanophora paradoxa. The four candidate clones obtained were mapped by restriction analysis and found to be overlapping. The clone with the smallest insert (1.4 kilobases) was partially sequenced and a coding region similar to the carboxyl terminus of the phycobiliprotein subunit beta-phycocyanin was found. The coding region for the beta-phycocyanin gene in C. paradoxa has been mapped to the small single copy region on the cyanelle genome, and its orientation has been determined. A short probe unique to a conserved chromophore binding site shared by at least two phycobiliprotein subunits has now been generated from the carboxyl terminus of the beta-phycocyanin gene. This probe may be useful in identifying specific phycobiliprotein subunit genes, beta-phycocyanin, beta-phycoerythrocyanin, and possibly beta-phycoerythrin, in other eukaryotic algae and in prokaryotic cyanobacteria.

Phycobilisomes linker family in cyanobacterial genomes: divergence and evolution

Cyanobacteria are the oldest life form making important contributions to global CO2 fixation on the Earth. Phycobilisomes (PBSs) are the major light harvesting systems of most cyanobacteria species. Recent availability of the whole genome database of cyanobacteria provides us a global and further view on the complex structural PBSs. A PBSs linker family is crucial in structure and function of major light-harvesting PBSs complexes. Linker polypeptides are considered to have the same ancestor with other phycobiliproteins (PBPs), and might have been diverged and evolved under particularly selective forces together. In this paper, a total of 192 putative linkers including 167 putative PBSs-associated linker genes and 25 Ferredoxin-NADP oxidoreductase (FNR) genes were detected through whole genome analysis of all 25 cyanobacterial genomes (20 finished and 5 in draft state). We compared the PBSs linker family of cyanobacteria in terms of gene structure, chromosome location, conservation domain, and polymorphic variants, and discussed the features and functions of the PBSs linker family. Most of PBSs-associated linkers in PBSs linker family are assembled into gene clusters with PBPs. A phylogenetic analysis based on protein data demonstrates a possibility of six classes of the linker family in cyanobacteria. Emergence, divergence, and disappearance of PBSs linkers among cyanobacterial species were due to speciation, gene duplication, gene transfer, or gene loss, and acclimation to various environmental selective pressures especially light.

Chloroplast genome characterization in the red alga Griffithsia pacifica

It has been suggested that cyanobacteria served as the ancestors for rhodophytic algae whose chloroplasts contain chlorophyll a and phycobilins, and that a rodophyte served as the plastid source for chromophytic plants that contain chlorophylls a and c. Although organellar DNA has been used to assess phylogenetic relatedness among terrestrial plants and green algae whose chloroplasts contain chlorophylls a and b, few data are presently available on the molecular profile of plastid DNA in chromophytes or rhodophytes. In this study the chloroplast genome of the rhodophytic, filamentous alga Griffithsia pacifica has been characterized. DNA was purified from isolated chloroplasts using protease k treatment and sodium dodecyl sulfate lysis followed by density centrifugation in Hoechst-33258 dye-CsCl gradients. Single and double restriction enzyme digests demonstrate that the DNA prepared from purified chloroplasts has a genome size of about 178 kilobase pairs (kb). A restriction map of this chloroplast genome demonstrates that it is circular and, unlike the chloroplast DNA (cpDNA) in most other plants, contains only a single ribosomal DNA operon. DNA was also purified from the mitochondria that co-isolated with chloroplasts. Mitochondrial DNA consists of molecules that range in size from 27 to 350 kb based on restriction endonuclease digestion and electron microscopic analysis.

Characterization of apcC, the nuclear gene for the phycobilisome core linker polypeptide L(c)(7.8) from the glaucocystophyte alga Cyanophora paradoxa. Import of the precursor into isolated cyanelles and integration of the mature protein into intact phycobilisomes

Phycobilisomes are the complex and highly efficient light-harvesting antenna systems of cyanobacteria, glaucocystophyte algae and red algae. In the glaucocystophyte Cyanophora paradoxa, seven genes for (chromophoric) phycobilisome components are known thus far, which all reside on the cyanelle genome. Here, we report the sequence of apcC, specifying the precursor to the colorless polypeptide L(c)(7.8), the first core linker reported for a eukaryote. The precursor was efficiently imported in vitro into isolated cyanelles. Fractionation into thylakoid membranes and stroma and into intact phycobilisomes and soluble proteins, respectively, indicated a low but significant incorporation of the imported linker polypeptide into the phycobilisomes.

The origin of red algae and the evolution of chloroplasts

Chloroplast structure and genome analyses support the hypothesis that three groups of organisms originated from the primary photosynthetic endosymbiosis between a cyanobacterium and a eukaryotic host: green plants (green algae + land plants), red algae and glaucophytes (for example, Cyanophora). Although phylogenies based on several mitochondrial genes support a specific green plants/red algae relationship, the phylogenetic analysis of nucleus-encoded genes yields inconclusive, sometimes contradictory results. To address this problem, we have analysed an alternative nuclear marker, elongation factor 2, and included new red algae and protist sequences. Here we provide significant support for a sisterhood of green plants and red algae. This sisterhood is also significantly supported by a multi-gene analysis of a fusion of 13 nuclear markers (5,171 amino acids). In addition, the analysis of an alternative fusion of 6 nuclear markers (1,938 amino acids) indicates that glaucophytes may be the closest relatives to the green plants/red algae group. Thus, our study provides evidence from nuclear markers for a single primary endosymbiosis at the origin of these groups, and supports a kingdom Plantae comprising green plants, red algae and glaucophytes.

Characterization of a phycoerythrin without alpha-subunits from a unicellular red alga

We describe here the spectral and biochemical properties of a novel biliprotein belonging to the phycoerythrin family, purified from the phycobilisome of a unicellular red alga, Rhodella reticulata strain R6. This biliprotein is assembled from a unique beta-type subunit, chloroplast-encoded, whose hexameric or dodecameric aggregates are stabilized by unusually large linkers (87 and 60 kDa) encoded by the nuclear genome. Although each beta-type subunit bears two phycoerythrobilins and one phycocyanobilin per chain, the linker polypeptides are non-chromophorylated. The apoprotein of the beta-subunit of the R. reticulata R6 phycoerythrin is specified by a monocistronic rpeB chloroplast gene that is split into three exons. We discuss the relationships between R6 beta-phycoerythrin and the previously published polypeptide sequences, the structural consequences due to the absence of an alpha-subunit, and its evolutionary implications.

Differential transcription of phycobiliprotein components in Rhodella violacea. Light and nitrogen effects on the 33-kilodalton phycoerythrin rod linker polypeptide, phycocyanin, and phycoerythrin transcripts

In Rhodella violacea phycoerythrin (PE) has two transcripts, a premessenger and a mature messenger (the gene contains an intron). Phycocyanin, which is plastid-encoded, and the 33-kD PE rod linker polypeptide, which is nuclear-encoded, have only one transcript. The PE premessenger had a rapid turnover; mature transcripts were stable in the light and more stable in the dark. In the presence of rifampicin, cells that shifted from dark to light exhibited an active translation of preexisting transcripts. There are indications of a modulation of the nuclear genome expression by the chloroplast; it may involve an unstable, plastid-encoded translational activator. All transcripts disappeared rapidly during nitrogen starvation. If nitrogen addition was carried out in the dark, active transcription and translation resumed as in light conditions, but ceased after 2 d. Both nitrogen and light were required for a total recovery after nitrogen starvation. Compared with the transcripts of phycobilisome components studied so far in cyanobacteria and Rhodophyceae, the mature transcripts of R. violacea are very stable when nitrogen is not limiting. The unstable PE premessenger is a good indicator of active transcription. This organism is therefore an interesting model to study the regulation of gene expression and the interactions between chloroplastic and nuclear genomes.

The phycobilisome, a light-harvesting complex responsive to environmental conditions

Photosynthetic organisms can acclimate to their environment by changing many cellular processes, including the biosynthesis of the photosynthetic apparatus. In this article we discuss the phycobilisome, the light-harvesting apparatus of cyanobacteria and red algae. Unlike most light-harvesting antenna complexes, the phycobilisome is not an integral membrane complex but is attached to the surface of the photosynthetic membranes. It is composed of both the pigmented phycobiliproteins and the nonpigmented linker polypeptides; the former are important for absorbing light energy, while the latter are important for stability and assembly of the complex. The composition of the phycobilisome is very sensitive to a number of different environmental factors. Some of the filamentous cyanobacteria can alter the composition of the phycobilisome in response to the prevalent wavelengths of light in the environment. This process, called complementary chromatic adaptation, allows these organisms to efficiently utilize available light energy to drive photosynthetic electron transport and CO2 fixation. Under conditions of macronutrient limitation, many cyanobacteria degrade their phycobilisomes in a rapid and orderly fashion. Since the phycobilisome is an abundant component of the cell, its degradation may provide a substantial amount of nitrogen to nitrogen-limited cells. Furthermore, degradation of the phycobilisome during nutrient-limited growth may prevent photodamage that would occur if the cells were to absorb light under conditions of metabolic arrest. The interplay of various environmental parameters in determining the number of phycobilisomes and their structural characteristics and the ways in which these parameters control phycobilisome biosynthesis are fertile areas for investigation.

Genes encoding phycobilisome linker polypeptides on the plastid genome of Aglaothamnion neglectum (Rhodophyta)

The genes encoding the phycobilisome anchor protein (apcE) and rod-core linker (cpcG) are on the plastid genome of the red alga Aglaothamnion neglectum. The apcE gene product is 5' to and in the same operon as the α and β subunit genes of allophycocyanin. This arrangement is identical to the arrangement observed in many cyanobacteria. The cpcG gene product is 5' to the operon encoding the α and β subunits of phycoerythrin, but is transcribed from the opposite DNA strand. This gene arrangement is different from that observed in cyanobacteria.The amino acid sequences of the A. neglectum anchor protein and rod-core linker polypeptide, as deduced from the nucleotide sequences of the genes, are approximately 50% identical to analogous polypeptides from cyanobacteria and another eukaryotic alga Cyanophora paradoxa. The conserved nature of these proteins suggests that the structure of the core and the rod-core interface are very similar in phycobilisomes of cyanobacteria and eukaryotic red algae.Environmental factors such as nutrient availability and light intensity can significantly affect the levels of mRNAs encoding the anchor protein and the rod-core linker polypeptide. Most of these changes are similar to the environmentally-controlled changes in the levels of phycobiliprotein transcripts of A. neglectum (Apt and Grossman 1992b). However, unlike the mRNAs encoding other phycobilisome components, the apcE transcript remains high during growth under conditions of nutrient deprivation.

Organization, expression and nucleotide sequence of the operon encoding R-phycoerythrin alpha and beta subunits from the red alga Polysiphonia boldii

The characterization of the operon encoding the alpha and beta subunits of rhodophytan (R)-phycoerythrin (PE) from the macrophytic red alga Polysiphonia boldii is reported. This plastid-encoded operon was cloned, its nucleotide sequence determined, and its expression characterized by northern and primer extension analyses. The arrangement and expression of the PE alpha and beta genes, named rpeA and rpeB, are similar to those of the cyanobacterial (C)-PE genes: rpeB is located 5' of rpeA, with an intergenic region of 64 nucleotides. The two genes are transcribed on a 1.25 kb dicistronic transcript, and each coding region is preceded by a prokaryotic ribosome binding site consensus sequence. Transcription is initiated 95 nucleotides upstream of the initiating methionine codon of rpeB. The promoter region resembles that of prokaryotic genes, with an AT-rich -10 sequence. A direct pentanucleotide repeat (5'-TGTTA-3') was found in the -35 region. This pentanucleotide is present upstream of all PE operons that have been characterized thus far. An extensive inverted repeat is present 3' of rpeA; inverted repeats are found downstream of all PE operons sequenced to date, although the sequence is not conserved. The deduced amino acid sequences from these genes provide complete sequences for an R-PE. Of the amino acid residues 85% are identical to those of bangeophycean (B)-PE from the unicellular red alga Porphyridium cruentum. Conserved residues include cysteines at the bilin attachment sites of C- and B-PEs, aspartates at positions postulated to interact with bilin chromophores, and an apparent consensus sequence for N-methylation of an asparagine residue in C-PEs.

Characterization and transcript analysis of the major phycobiliprotein subunit genes from Aglaothamnion neglectum (Rhodophyta)

The genes encoding the alpha and beta subunits of allophycocyanin, phycocyanin and phycoerythrin from the red alga Aglaothamnion neglectum were isolated and characterized. While the operons containing the different phycobiliprotein genes are dispersed on the plastid genome, the genes encoding the alpha and beta subunits for each phycobiliprotein are contiguous. The beta subunit gene is 5' for both the phycocyanin and phycoerythrin operons, while the alpha subunit gene is 5' for the allophycocyanin operon. The amino acid sequences of A. neglectum phycobiliproteins, as deduced from the nucleotide sequences of the genes, are 65-85% identical to analogous proteins from other red algae and cyanobacteria. The conserved nature of the plastid-encoded red algal and cyanobacterial phycobiliprotein genes supports the proposed origin of red algal plastids from cyanobacterial endosymbionts. Many environmental factors effect phycobilisome biosynthesis. The effect of both nutrient availability and light quantity on the level of A. neglectum phycobiliprotein subunits and the mRNA species encoding those subunits is described.

Characterization of the genes encoding phycoerythrin in the red alga Rhodella violacea: evidence for a splitting of the rpeB gene by an intron

The phycobilisome of the eukaryotic unicellular red alga Rhodella violacea presents in some respects an organization that is intermediate between those of the homologous counterparts found in cyanobacteria (the putative chloroplast progenitor) and more advanced, pluricellular red algae. This suggests evolutionary relationships that we investigated at the genome level. The present work describes the sequences of two rhodophytan phycobilisome genes, rpeA and rpeB. These chloroplast genes encode the alpha and beta subunits of phycoerythrin, the major component of the light-harvesting antennae and one of the most abundant cellular proteins in these algae. The amino acid sequences deduced from both rpeA and rpeB present strong homologies with those previously reported for phycoerythrin subunits of cyanobacteria, rhodophyta, and cryptomonads. The main difference with the corresponding cyanobacterial genes was the unexpected occurrence of an intervening sequence that split rpeB into two exons. This intervening sequence presents characteristics of group II introns but lacks several structural domains. Transcriptional analyses showed that the two rpe genes are cotranscribed and that the major RNA species detected corresponds to a mature mRNA lacking the intron. As the phycobiliproteins form a group of closely related polypeptides in cyanobacteria and rhodophyta, the molecular events affecting the corresponding genes, such as the rpeB intron, may be a clue to elucidate some aspects of the molecular processes involved in the evolution of plastid genes.

Localization and quantitation of chloroplast enzymes and light-harvesting components using immunocytochemical methods

Seven chloroplast proteins were localized in Porphyridium cruentum (ATCC 50161) by immunolabeling with colloidal gold on electron microscope sections of log phase cells grown under red, green, and white light. Ribulose bisphosphate carboxylase labeling occurred almost exclusively in the pyrenoid. The major apoproteins of photosystem I (56-64 kD) occurred mostly over the stromal thylakoid region and also appeared over the thylakoids passing through the pyrenoid. Labeling for photosystem II core components (D2 and a 45 kD Chl-binding protein), for phycobilisomes (allophycocyanin, and a 91 kD L(cm) linker) and for ATP synthase (beta subunit) were predominantly present in the thylakoid region but not in the pyrenoid region of the chloroplast. Red light cells had increased labeling per thylakoid length for polypeptides of photosystem II and of phycobilisomes, while photosystem I density decreased, compared to white light cells. Conversely, green light cells had a decreased density of photosystem II and phycobilisome polypeptides, while photosystem I density changed little compared with white light cells. A comparison of the immunogold labeling results with data from spectroscopic methods and from rocket immunoelectrophoresis indicates that it can provide a quantitative measure of the relative amounts of protein components as well as their localization in specific organellar compartments.

Studies on Cyanidium caldarium Phycobiliprotein Pigment Mutants

Phycobiliprotein biosynthesis was investigated in four strains of the unicellular rhodophyte, Cyandium caldarium, with different pigment phenotypes. All strains were incapable of synthesizing phycobiliproteins when grown in the dark. Western blotting experiments showed that dark-grown cells of the wild-type and mutant GGB synthesized the alpha and beta subunit polypeptides of allophyocyanin and phycocyanin after exposure to light for 24 hours, whereas cells of mutant IIIC and GGBY did not. Similarly, light promoted the appearance of allophycocyanin and phycocyanin mRNAs in the wild-type and GGB but not in IIIC and GGBY. However, Southern blots of restricted genomic DNA from the wild type, IIIC, GGBY, and GGB, all hybridized with heterologous phycobiliprotein gene probes and revealed that all four strains contained identical Pst, EcoRI, and Dral restriction fragments containing allophycocyanin and phycocyanin genes. Cells of the wild type and GGB incubated in the dark with the heme precursor. delta-aminolevulinate, synthesized allophycocyanin and phycocyanin apoproteins providing strong evidence for the role of a tetrapyrrole in regulation of phycobiliprotein gene expression. However, cells of IIIC and GGBY incubated in the dark with delta-aminolevulinate did not contain detectable quantities of allophycocyanin or phycocyanin apoproteins. The possible role of a tetrapyrrole in phycobiliprotein gene expression and basis for the genetic lesion in mutants IIIC and GGBY is discussed.

Changes in polypeptide composition of Synechocystis sp. strain 6308 phycobilisomes induced by nitrogen starvation

Phycobilisomes isolated from actively growing Synechocystis sp. strain 6308 (ATCC 27150) consist of 12 polypeptides ranging in molecular mass from 11.5 to 95 kilodaltons. The phycobilisome anchor and linker polypeptides are glycosylated. Nitrogen starvation causes the progressive loss of phycocyanin and allophycocyanin subunits with molecular masses between 16 and 20 kilodaltons and of two linker polypeptides with molecular masses of 27 and 33 kilodaltons. Nitrogen starvation also leads to enrichment of four additional polypeptides with molecular masses of 46, 53, 57, and 61 kilodaltons and a transient enrichment of 35- and 41-kilodalton polypeptides in isolated phycobilisomes. The 57-kilodalton additional polypeptide was identified by immunoblotting as the large subunit of ribulosebisphosphate carboxylase/oxygenase. Proteins with the same molecular weights as the additional polypeptides were also coisolated with the 12 phycobilisome polypeptides in the supernatant of nitrogen-replete Synechocystis thylakoid membranes extracted in high-ionic-strength buffer and washed with deionized water. These observations suggest that the additional polypeptides in phycobilisomes from nitrogen-starved cells may be soluble or loosely bound membrane proteins which associate with phycobilisomes. The composition and degree of association of phycobilisomes with soluble and adjacent membrane polypeptides appear to be highly dynamic and specifically regulated by nitrogen availability. Possible mechanisms for variation in the strength of association between phycobilisomes and other polypeptides are suggested.

Localization of phycoerythrin at the lumenal surface of the thylakoid membrane in Rhodomonas lens

The thylakoids of cryptomonads are unique in that their lumens are filled with an electron-dense substance postulated to be phycobiliprotein. In this study, we used an antiserum against phycoerythrin (PE) 545 of Rhodomonas lens (gift of R. MacColl, New York State Department of Health, Albany, NY) and protein A-gold immunoelectron microscopy to localize this light-harvesting protein in cryptomonad cells. In sections of whole cells of R. lens labeled with anti-PE 545, the gold particles were not uniformly distributed over the dense thylakoid lumens as expected, but instead were preferentially localized either over or adjacent to the thylakoid membranes. A similar pattern of labeling was observed in cell sections labeled with two different antisera against PE 566 from Cryptomonas ovata. To determine whether PE is localized on the outer or inner side of the membrane, chloroplast fragments were isolated from cells fixed in dilute glutaraldehyde and labeled in vitro with anti-PE 545 followed by protein A-small gold. These thylakoid preparations were then fixed in glutaraldehyde followed by osmium tetroxide, embedded in Spurr, and sections were labeled with anti-PE 545 followed by protein A-large gold. Small gold particles were found only at the broken edges of the thylakoids, associated with the dense material on the lumenal surface of the membrane, whereas large gold particles were distributed along the entire length of the thylakoid membrane. We conclude that PE is located inside the thylakoids of R. lens in close association with the lumenal surface of the thylakoid membrane.

Characterization of phycobiliprotein and linker polypeptide genes in Fremyella diplosiphon and their regulated expression during complementary chromatic adaptation

Phycobilisomes, comprised of both chromophoric (phycobiliproteins) and non-chromophoric (linker polypeptides) proteins, are light-harvesting complexes present in the prokaryotic cyanobacteria and the eukaryotic red algae. Many cyanobacteria exhibit complementary chromatic adaptation, a process which enables these organisms to optimize absorption of prevalent wavelengths of light by altering the composition of the phycobilisome. To examine the mechanisms involved in adjusting the levels of phycobilisome components during complementary chromatic adaptation, we have isolated and sequenced genes encoding phycobiliprotein and linker polypeptides in the cyanobacterium Fremyella diplosiphon, analyzed their transcriptional characteristics (transcript sizes and abundance when F. diplosiphon is grown in different light qualities) and mapped transcript initiation and termination sites. Our results demonstrate that genes encoding phycobilisome components are often cotranscribed as polycistronic messenger RNAs. Light quality regulates the composition of the phycobilisome by causing changes in the abundance of transcripts encoding specific components, suggesting that regulation is at the level of transcription (although not eliminating the possibility of changes in mRNA stability). The work presented here sets the foundation for analyzing the evolution of the different phycobilisome components and exploring signal transduction from photoperception to activation of specific genes using in vivo and in vitro genetic technology.

Isolation and characterization of light-regulated phycobilisome linker polypeptide genes and their transcription as a polycistronic mRNA

Several cyanobacteria adjust both the phycobiliprotein and linker protein composition of the phycobilisome, a light-harvesting complex in cyanobacteria and some eucaryotic algae, to maximize absorption of prevalent wavelengths of light. This process is called complementary chromatic adaptation. We sequenced the amino terminus of a linker polypeptide which is associated with phycocyanin and accumulates to high levels during growth of the cyanobacterium Fremyella diplosiphon in red light. A mixed oligonucleotide encoding a region of this amino terminus was synthesized and used to identify a fragment of F. diplosiphon genomic DNA encoding the linker polypeptide. This linker gene was located between two other linker genes and contiguous to the red-light-induced phycocyanin gene set. Sequences of all three linker genes are presented. These genes were transcribed together onto a large polycistronic mRNA which also encoded the red-light-induced phycocyanin subunits. The relationship of this transcript to the biogenesis of the phycobilisome when F. diplosiphon is grown under different conditions of illumination is discussed.

Polypeptides of a Light-Harvesting Complex of the Diatom Phaeodactylum tricornutum Are Synthesized in the Cytoplasm of the Cell as Precursors

A light-harvesting fucoxanthin-chlorophyll a/c-protein complex has been isolated from the diatom Phaeodactylum tricornutum by detergent extraction of thylakoid membranes coupled with sucrose density gradient centrifugation. The isolated complex was devoid of photochemical activity and displayed spectral characteristics consistent with light harvesting function. It has three major polypeptides of apparent molecular weights 18,000, 19,000, and 19,500 as determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Using protein synthesis inhibitors, these polypeptides were shown to be synthesized on 80S cytoplasmic ribosomes. Antibodies raised to a mixture of the 19,000 and 19,500 dalton components of the complex were used to demonstrate structural similarity among the three polypeptide components. Immunoprecipitation from primary translation products synthesized in a reticulocyte lysate system primed with P. tricornutum poly(A) RNA, indicates that the polypeptide components are synthesized as precursors 3,000 to 5,000 daltons larger than the mature polypeptides.

Biogenesis and light regulation of the major light harvesting chlorophyll-protein of diatoms

The apoprotein of the major light harvesting pigment-protein complex from the diatom Phaeodactylum tricornutum (UTEX 646) is composed of two similar polypeptides of 17.5 and 18.0 kilodaltons (kD). The in vivo synthesis of these polypeptides is inhibited by the 80s protein synthesis inhibitor cycloheximide, but not by the 70s ribosome inhibitor chloramphenicol. When total poly(A)(+) RNA was used in in vitro protein synthesis, a number of polypeptides were synthesized with a dominant product at 22 kD. When the polypeptides were immunoprecipitated with monospecific antibodies to the 17.5 and 18.0 polypeptides, a single protein zone of 22 kD was detected. Immunoprecipitation with preimmune serum failed to precipitate detectable levels of protein at any relative molecular weight (M(r)). These findings indicate that the two apoprotein polypeptides of the diatom light harvesting pigment-protein are translated from polyadenylated message on cytoplasmic ribosomes as either a single or two (or more) similar M(r) precursor proteins. These findings also suggest that this protein is encoded in the nucleus.Photosynthetic light adaptation features of P. tricornutum UTEX 646 indicate that it responds to low light by increasing cell size and numbers of photosystem I and II reaction centers per cell, but does not change photosynthetic rate per cell or photosynthetic unit sizes significantly. When low light cells are exposed to higher photon flux densities, the in vivo incorporation of label into the apoprotein of the light harvesting complex decreases. In contrast, high light grown cells show rapid (<3 hour) increases in apoprotein synthesis when exposed to low light levels. This is the first demonstration of a specific role of photon flux density in regulating the synthesis of a major light harvesting pigment-protein during photosynthetic light adaptation.

Phycobilisome structure and function

Phycobilisomes are aggregates of light-harvesting proteins attached to the stroma side of the thylakoid membranes of the cyanobacteria (blue-green algae) and red algae. The water-soluble phycobiliproteins, of which there are three major groups, tetrapyrrole chromophores covalently bound to apoprotein. Several additional protiens are found within the phycobilisome and serve to link the phycobiliproteins to each other in an ordered fashion and also to attach the phycobilisome to the thylakoid membrane. Excitation energy absorbed by phycoerythrin is transferred through phycocyanin to allophycocyanin with an efficiency approximating 100%. This pathway of excitation energy transfer, directly confirmed by time-resolved spectroscopic measurements, has been incorporated into models describing the ultrastructure of the phycobilisome. The model for the most typical type of phycobilisome describes an allophycocyanin-containing core composed of three cylinders arranged so that their longitudinal axes are parallel and their ends form a triangle. Attached to this core are six rod structures which contain phycocyanin proximal to the core and phycoerythrin distal to the core. The axes of these rods are perpendicular to the longitudinal axis of the core. This arrangement ensures a very efficient transfer of energy. The association of phycoerythrin and phycocyanin within the rods and the attachment of the rods to the core and the core to the thylakoid require the presence of several 'linker' polypeptides. It is recently possible to assemble functionally and structurally intact phycobilisomes in vitro from separated components as well as to reassociate phycobilisomes with stripped thylakoids. Understanding of the biosynthesis and in vivo assembly of phycobilisomes will be greatly aided by the current advances in molecular genetics, as exemplified by recent identification of several genes encoding phycobilisome components.Combined ultrastructural, biochemical and biophysical approaches to the study of cyanobacterial and red algal cells and isolated phycobilisome-thylakoid fractions are leading to a clearer understanding of the phycobilisome-thylakoid structural interactions, energy transfer to the reaction centers and regulation of excitation energy distribution. However, compared to our current knowledge concerning the structural and functional organization of the isolated phycobilisome, this research area is relatively unexplored.

Gene map for the Cyanophora paradoxa cyanelle genome

The genes for the following proteins were localized by hybridization analysis on the cyanelle genome of Cyanophora paradoxa: the alpha and beta subunits of phycocyanin (cpcA and cpcB); the alpha and beta subunits of allophycocyanin (apcA and apcB); the large and small subunits of ribulose-1,5-bisphosphate carboxylase (rbcL and rbcS); the two putative chlorophyll alpha-binding apoproteins of the photosystem I-P700 complex (psaA and psaB); four apoproteins believed to be components of the photosystem II core complex (psbA, psbB, psbC, and psbD); the two apoprotein subunits of cytochrome b-559 which is also found in the core complex of photosystem II (psbE and psbF); three subunits of the ATP synthase complex (atpA and atpBE); and the cytochrome f apoprotein (petA). Eighty-five percent of the genome was cloned as BamHI, BglII, or PstI fragments. These cloned fragments were used to construct a physical map of the cyanelle genome and to localize more precisely some of the genes listed above. The genes for phycocyanin and allophycocyanin were not clustered and were separated by about 25 kilobases. Although the rbcL gene was adjacent to the atpBE genes and the psbC and psbD genes were adjacent, the arrangement of other genes encoding various polypeptide subunits of protein complexes involved in photosynthetic functions was dissimilar to that observed for known chloroplast genomes. These results are consistent with the independent development of this cyanelle from a cyanobacterial endosymbiont.