Rod substructure in cyanobacterial phycobilisomes: analysis of Synechocystis 6701 mutants low in phycoerythrin

How can Phycobilisome, the unique light harvesting system in certain algae working highly efficiently: The connection in between structures and functions

Algae, which are ubiquitous in ecosystems, have evolved a variety of light-harvesting complexes to better adapt to diverse habitats. Phycobilisomes/phycobiliproteins, unique to cyanobacteria, red algae, and certain cryptomonads, compensate for the lack of chlorophyll absorption, allowing algae to capture and efficiently transfer light energy in aquatic environments. With the advancement of microscopy and spectroscopy, the structure and energy transfer processes of increasingly complex phycobilisomes have been elucidated, providing us with a vivid portrait of the dynamic adaptation of their structures to the light environment in which algae thrive: 1) Cyanobacteria living on the surface of the water use short, small phycobilisomes to absorb red-orange light and reduce the damage from blue-violet light via multiple methods; 2) Large red algae inhabiting the depths of the ocean have evolved long and dense phycobilisomes containing phycoerythrin to capture the feeble blue-green light; 3) In far-red light environments such as caves, algae use special allophycocyanin cores to optimally utilize the far-red light; 4) When the environment shifts, algae can adjust the length, composition and density of their rods to better adapt; 5) By carefully designing the position of the pigments, phycobilisomes can transfer light energy to the reaction center with nearly 100% efficiency via three energy transfer processes.

Disk-to-Disk Transfer as the Rate-Limiting Step for Energy Flow in Phycobilisomes

A broadly tunable picosecond laser source and an ultrafast streak camera were used to measure temporally and spectrally resolved emission from intact phycobilisomes and from individual phycobiliproteins as a function of excitation wavelength. Both wild-type and mutant phycobilisomes of the unicellular cyanobacterium Synechocystis 6701 were examined, as well as two biliproteins, R-phycoerythrin (240 kilodaltons, 34 bilins) and allophycocyanin (100 kilodaltons, 6 bilins). Measurements of intact phycobilisomes with known structural differences showed that the addition of an average of 1.6 phycoerythrin disks in the phycobilisome rod increased the overall energy transfer time by 30 +/- 5 picoseconds. In the isolated phycobiliproteins the onset of emission was as prompt as that of a solution of rhodamine B laser dye and was independent of excitation wavelength. This imposes an upper limit of 8 picoseconds (instrument-limited) on the transfer time from "sensitizing" to "fluorescing" chromophores in these biliproteins. These results indicate that disk-to-disk transfer is the slowest energy transfer process in phycobilisomes and, in combination with previous structural analyses, show that with respect to energy transfer the lattice of approximately 625 light-harvesting chromophores in the Synechocystis 6701 wild-type phycobilisome functions as a linear five-point array.

Cytoplasmic and chloroplast synthesis of phycobilisome polypeptides

In vivo labeling of eukaryotic phycobilisomes in the presence of inhibitors of translation on 70S and 80S ribosomes demonstrates that some of the polypeptides of this light-harvesting complex are synthesized in the cytoplasm while others are synthesized in the chloroplast. The major pigmented polypeptides, the alpha and beta subunits of the biliproteins (molecular weights between 15,000 and 20,000) and the anchor protein (molecular weight about 90,000) are translated on 70S ribosomes. This suggests that these polypeptides are made within the algal chloroplast. Because the alpha and beta subunits comprise a group of closely related polypeptides, the genes encoding these polypeptides may reside in the plastid genome as a multigene family. Other prominent phycobilisome polypeptides, including a nonpigmented polypeptide that may be involved in maintaining the structural integrity of the complex, are synthesized on cytoplasmic ribosomes. Because the synthesis of phycobilisomes appears to require the expression of genes in two subcellular compartments, this system may be an excellent model for: (i) examining interaction between nuclear and plastid genomes: (ii) elucidating the molecular processes involved in the evolution of plastid genes: (iii) clarifying the events in the synthesis and assembly of macromolecular complexes in the chloroplast.

Interference of an apcA insertion with complementary chromatic adaptation in the diazotrophic Synechocystis sp. strain BO 8402

Complementary chromatic adaptation was studied in two unicellular diazotrophic Synechocystis-type cyanobacteria, strains BO 8402 and BO 9201. Strain BO 8402 was isolated from Lake Constance as a mutant lacking phycobilisomes due to an insertion sequence element in the gene apcA, encoding alpha-allophycocyanin. Strain BO 9201 recovered the ability to assemble functional phycobilisomes after a spontaneous excision of the insertion sequence element in apcA. Simultaneously, the strain became able to perform group II complementary chromatic adaptation by regulating the synthesis of phycoerythrin. The two strains had identical phycoerythrin operons, cpeBA, and similar-sized transcripts were formed upon induction by green light. However, in strain BO 8402 the cpeBA transcript level was approx. 20-fold lower than in strain BO 9201. Because strain BO 8402 cannot synthesize allophycocyanin and phycocyanin is sequestered in paracrystalline inclusion bodies, non-assembled phycoerythrin may accumulate inside the cells. It was examined whether non-assembled phycoerythrin or other effects caused by the absence of phycobilisomes, such as a permanently oxidized redox status of the photosynthetic electron transport chain or a distorted ratio of C and N assimilation mediated the repression of cpeBA transcription in strain BO 8402. No such links could be established. We therefore concluded that in these diazotrophic Synechocystis-type cyanobacteria the green light-induced transcription of the cpe operon directly required a functional apc operon.

Effect of nutrients and aeration on O2 evolution and photosynthetic pigments of Anabaena torulosa during akinete differentiation

The addition of a nitrogen (nitrate) and carbon sources (acetate, citrate and fructose) and phosphate deficiency (nitrate medium deficient in phosphate) under unaerated conditions induced akinete differentiation in Anabaena torulosa. Aerated cultures of this organism in these nutrients did not differentiate akinetes. Oxygen evolution by aerated cultures was higher when compared to unaerated cultures, which concurred with high chlorophyll content of aerated cultures. Nitrate nitrogen supported high phycocyanin content in unaerated cultures; phycocyanin and allophycocyanin contents were low under aerated conditions. The contents of phycocyanin, allophycocyanin, phycoerythrin and carotenoids gradually decreased at the mature akinete phase. Under aerated conditions, chlorophyll content rose and the content of all the pigments increased with the growth rate of the organism.

Cyanobacterial phycobilisomes

Cyanobacterial phycobilisomes harvest light and cause energy migration usually toward photosystem II reaction centers. Energy transfer from phycobilisomes directly to photosystem I may occur under certain light conditions. The phycobilisomes are highly organized complexes of various biliproteins and linker polypeptides. Phycobilisomes are composed of rods and a core. The biliproteins have their bilins (chromophores) arranged to produce rapid and directional energy migration through the phycobilisomes and to chlorophyll a in the thylakoid membrane. The modulation of the energy levels of the four chemically different bilins by a variety of influences produces more efficient light harvesting and energy migration. Acclimation of cyanobacterial phycobilisomes to growth light by complementary chromatic adaptation is a complex process that changes the ratio of phycocyanin to phycoerythrin in rods of certain phycobilisomes to improve light harvesting in changing habitats. The linkers govern the assembly of the biliproteins into phycobilisomes, and, even if colorless, in certain cases they have been shown to improve the energy migration process. The Lcm polypeptide has several functions, including the linker function of determining the organization of the phycobilisome cores. Details of how linkers perform their tasks are still topics of interest. The transfer of excitation energy from bilin to bilin is considered, particularly for monomers and trimers of C-phycocyanin, phycoerythrocyanin, and allophycocyanin. Phycobilisomes are one of the ways cyanobacteria thrive in varying and sometimes extreme habitats. Various biliprotein properties perhaps not related to photosynthesis are considered: the photoreversibility of phycoviolobilin, biophysical studies, and biliproteins in evolution. Copyright 1998 Academic Press.

Subunit interactions and protein stability in the cyanobacterial light-harvesting proteins

Strain 4R is a phycocyanin-minus mutant of the unicellular cyanobacterium Synechocystis sp. strain 6803. Although it lacks the light-harvesting protein phycocyanin, 4R has normal levels of phycocyanin (cpc) transcripts. Sequence analysis of the cpcB gene encoding the phycocyanin beta subunit shows an insertion mutation in 4R that causes early termination of translation. Other work has shown that the phycocyanin alpha subunit and the linker proteins encoded on the cpc transcripts are all functional in 4R, yet the defective phycocyanin beta subunit results in the complete absence of the alpha subunit and the linkers. Phycocyanin-minus mutants were constructed in a wild-type background by interruption of cpcB and cpcA with an antibiotic resistance gene and were compared with the 4R strain. Immunoblot analysis of the mutants demonstrated that interruption of one subunit was accompanied by a complete absence of the unassembled partner subunit. Phycocyanin assembly begins with the formation of the alpha beta heterodimer (the monomer) and continues through higher-order trimeric and hexameric aggregates that associate with linker proteins to form the phycobilisome rods. The results in this paper indicate that monomer formation is a critical stage in the biliprotein assembly pathway and that unassembled subunits are subject to stringent controls that prevent their appearance in vivo.

Heterologous assembly and rescue of stranded phycocyanin subunits by expression of a foreign cpcBA operon in Synechocystis sp. strain 6803

Light harvesting in cyanobacteria is performed by the biliproteins, which are organized into membrane-associated complexes called phycobilisomes. Most phycobilisomes have a core substructure that is composed of the allophycocyanin biliproteins and is energetically linked to chlorophyll in the photosynthetic membrane. Rod substructures are attached to the phycobilisome cores and contain phycocyanin and sometimes phycoerythrin. The different biliproteins have discrete absorbance and fluorescence maxima that overlap in an energy transfer pathway that terminates with chlorophyll. A phycocyanin-minus mutant in the cyanobacterium Synechocystis sp. strain 6803 (strain 4R) has been shown to have a nonsense mutation in the cpcB gene encoding the phycocyanin beta subunit. We have expressed a foreign phycocyanin operon from Synechocystis sp. strain 6701 in the 4R strain and complemented the phycocyanin-minus phenotype. Complementation occurs because the foreign phycocyanin alpha and beta subunits assemble with endogenous phycobilisome components. The phycocyanin alpha subunit that is normally absent in the 4R strain can be rescued by heterologous assembly as well. Expression of the Synechocystis sp. strain 6701 cpcBA operon in the wild-type Synechocystis sp. strain 6803 was also examined and showed that the foreign phycocyanin can compete with the endogenous protein for assembly into phycobilisomes.

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.

Characterization of the light-regulated operon encoding the phycoerythrin-associated linker proteins from the cyanobacterium Fremyella diplosiphon

Many biological processes in photosynthetic organisms can be regulated by light quantity or light quality or both. A unique example of the effect of specific wavelengths of light on the composition of the photosynthetic apparatus occurs in cyanobacteria that undergo complementary chromatic adaptation. These organisms alter the composition of their light-harvesting organelle, the phycobilisome, and exhibit distinct morphological features as a function of the wavelength of incident light. Fremyella diplosiphon, a filamentous cyanobacterium, responds to green light by activating transcription of the cpeBA operon, which encodes the pigmented light-harvesting component phycoerythrin. We have isolated and determined the complete nucleotide sequence of another operon, cpeCD, that encodes the linker proteins associated with phycoerythrin hexamers in the phycobilisome. The cpeCD operon is activated in green light and expressed as two major transcripts with the same 5' start site but differing 3' ends. Analysis of the kinetics of transcript accumulation in cultures of F. diplosiphon shifted from red light to green light and vice versa shows that the cpeBA and cpeCD operons are regulated coordinately. A common 17-base-pair sequence is found upstream of the transcription start sites of both operons. A comparison of the predicted amino acid sequences of the phycoerythrin-associated linker proteins CpeC and CpeD with sequences of other previously characterized rod linker proteins shows 49 invariant residues, most of which are in the amino-terminal half of the proteins.

Structure and light-regulated expression of phycoerythrin genes in wild-type and phycobilisome assembly mutants of Synechocystis sp. strain PCC 6701

Phycoerythrin is a major pigmented component of the phycobilisome, a cyanobacterial light-harvesting complex. It contains bilin-type chromophores that absorb and transfer light energy to chlorophyll protein complexes of the photosynthetic membranes. In many cyanobacteria, phycoerythrin expression is regulated by light wavelength in a response known as chromatic adaptation. Green light-grown cells contain higher levels of this biliprotein than do cells grown in red light. The phycoerythrin gene set from the unicellular cyanobacterium Synechocystis sp. strain PCC 6701 was cloned and sequenced, and the 5' end of the phycoerythrin mRNA was localized. The amino acid sequences of the phycoerythrin subunits from Synechocystis strain 6701 and Fremyella diplosiphon were 90% identical. As observed in F. diplosiphon, the Synechocystis strain 6701 phycoerythrin transcript accumulated to high levels in green light-grown cells and low levels in red light-grown cells. Similar nucleotide sequences, which might control gene expression, occurred upstream of the transcription initiation sites of the phycoerythrin genes in both organisms. While the phycoerythrin structure and light-regulated transcript accumulation were similar in Synechocystis strain 6701 and F. diplosiphon, the steady-state levels of phycoerythrin subunits during growth in red light were quite different for the two organisms. This observation suggests that control of phycoerythrin levels in Synechocystis strain 6701 is complex and may involve posttranscriptional processes. We also characterized the phycoerythrin genes and mRNA levels in two phycobilisome assembly mutants, UV16-40 and UV16.

Spectroscopic studies of cyanobacterial phycobilisomes lacking core polypeptides

Synechococcus sp. PCC 7002 (Agmenellum quadruplicatum PR6) genes encoding two highly conserved phycobilisome core polypeptides, a small linker polypeptide (LC8, apcC) and the allophycocyanin-B alpha-subunit (alpha APB, apcD), respectively, were interrupted by insertion of restriction fragments carrying the neomycin phosphotransferase gene of Tn5. The interrupted genes were used to transform Synechococcus sp. PCC 7002 to kanamycin resistance. The apcC- mutant assembled phycobilisomes lacking the LC8 polypeptide and the apcD- mutant assembled phycobilisomes lacking alpha APB. No other differences between the compositions of the mutant and wild-type phycobilisomes were detected. The apcC- strain grew about 25% more slowly than the wild-type, and its phycobilisomes dissociated more rapidly in 0.33 M Na/K-PO4 (pH 8.0) or in 0.75 M Na/K-PO4 at pH 8.0, at 40 degrees C, than did those of the wild-type. The phycobilisomes of this mutant were indistinguishable from those of the wild-type with respect to absorption and circular dichroism spectra, as well as time-resolved fluorescence emission. Steady-state emission spectra indicate a small decrease in long wavelength (680 nm) emission from the apcC- phycobilisomes and a complementary increase in shorter wavelength (665 nm) emission, relative to wild-type phycobilisomes. Strain apcD- phycobilisomes appear to be functionally indistinguishable from those of the wild-type, in spite of the absence of the two alpha APB subunits which bear terminal acceptor bilins. The only spectroscopic difference was seen in the steady-state fluorescence emission, for which the emission of the mutant was about 15% higher than that of the wild-type and was slightly blue-shifted. A phenotype has yet to be found for the apcD- mutation.

Mutations that affect structure and assembly of light-harvesting proteins in the cyanobacterium Synechocystis sp. strain 6701

The unicellular cyanobacterium Synechocystis sp. strain 6701 was mutagenized with UV irradiation and screened for pigment changes that indicated genetic lesions involving the light-harvesting proteins of the phycobilisome. A previous examination of the pigment mutant UV16 showed an assembly defect in the phycocyanin component of the phycobilisome. Mutagenesis of UV16 produced an additional double mutant, UV16-40, with decreased phycoerythrin content. Phycocyanin and phycoerythrin were isolated from UV16-40 and compared with normal biliproteins. The results suggested that the UV16 mutation affected the alpha subunit of phycocyanin, while the phycoerythrin beta subunit from UV16-40 had lost one of its three chromophores. Characterization of the unassembled phycobilisome components in these mutants suggests that these strains will be useful for probing in vivo the regulated expression and assembly of phycobilisomes.

Asymmetrical core structure in phycobilisomes of the cyanobacterium Synechocystis 6701

The light-harvesting complex of cyanobacteria and red algae, the phycobilisome, has two structural domains, the core and the rods. Both contain biliproteins and linker peptides. The core contains the site of attachment to the thylakoid membrane and the energy transfer link between the phycobilisome and chlorophyll. There are also six rod-binding sites in the membrane-distal periphery of the core. The structure of phycobilisomes in the cyanobacterium Synechococcus 6301 was studied by Glazer, who proposed a model for the internal organization of the bicylindrical core. In the construction of that model, it was necessary to make arbitrary decisions between two possible locations for one of the trimeric protein complexes within a core cylinder and between two possible orientations of the basal core cylinders relative to one another. We isolated the tricylindrical cores from an ultraviolet-light-induced mutant of the cyanobacterium Synechocystis 6701 and obtained, by partial dissociation, a unique core substructure that maintained some contacts between the two basal cylinders. From its structure and spectral properties, we conclude that this particle is a central core substructure that resulted from dissociation of the two layers of peripheral trimers in the intact core. The compositions of this particle and the dissociated trimers were inconsistent with the proposed location of one of the trimers in the 6301 core model, but supported the placement of that trimer in the alternative position within the basal core cylinder. Rod-binding sites within the central core substructure were studied by partial dissociation of the short-rod phycobilisomes from another mutant of 6701. This dissociation generated particles that were interpreted as being central core substructures with the two basal rods attached. The appearance of these particles in the electron microscope suggested that both basal rods would be localized towards the same side of the intact core. Such an asymmetrical arrangement of basal rods is supported by previously published edge-views of intact cores with basal rods from strain 6701. These observations suggest a parallel arrangement of the basal cylinders with respect to each other, creating an asymmetrical core. A phycobilisome model was constructed that incorporated core asymmetry. This model predicts the energy transfer pathways from the basal and upper rods to specific trimers in the core.

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.

Role of the Colorless Polypeptides in Phycobilisome Assembly in Nostoc sp

We have identified the function of the ;extra' polypeptides involved in phycobilisome assembly in Nostoc sp. These phycobilisomes, as those of other cyanobacteria, are composed of an allophycocyanin core, phycoerythrin- and phycocyanin-containing rods, and five additional polypeptides of 95, 34.5, 34, 32, and 29 kilodaltons. The 95 kilodalton polypeptide anchors the phycobilisome to the thylakoid membrane (Rusckowski, Zilinskas 1982 Plant Physiol 70: 1055-1059); the 29 kilodalton polypeptide attaches the phycoerythrin- and phycocyanin-containing rods to the allophycocyanin core (Glick, Zilinskas 1982 Plant Physiol 69: 991-997). Two populations of rods can exist simultaneously or separately in phycobilisomes, depending upon illumination conditions. In white light, only one type of rod with phycoerythrin and phycocyanin in a 2:1 molar ratio is synthesized. Associated with this rod are the 29, 32, and 34 kilodalton colorless polypeptides; the 32 kilodalton polypeptide links the two phycoerythrin hexamers, and the 34 kilodalton polypeptide attaches a phycoerythrin hexamer to a phycocyanin hexamer. The second rod, containing predominantly phycocyanin, and the 34.5 and 29 kilodalton polypeptides, is synthesized by redlight-adapted cells; the 34.5 kilodalton polypeptide links two phycocyanin hexamers. These assignments are based on isolation of rods, dissociation of these rods into their component biliproteins, and analysis of colorless polypeptide composition, followed by investigation of complexes formed or not formed upon their recombination.

Core substructure in cyanobacterial phycobilisomes

The tricylindrical core of Synechocystis 6701 phycobilisomes is made up of four types of allophycocyanin-containing complexes: A, (alpha AP beta AP)3; B, (alpha AP beta AP)3 .10K; C, (alpha APB1 alpha AP2 beta AP3).10K; D, (alpha AP beta AP)2.18.5K.99K; where AP is allophycocyanin, APB is allophycocyanin B, and 10K, 18.5K, and 99K are polypeptides of 10,000, 18,500, and 99,000 daltons, respectively. The 18.5K polypeptide is a hitherto unrecognized biliprotein subunit with a single phycocyanobilin prosthetic group. The tricylindrical core is made up of 12 subcomplexes in the molar ratio of A:B:C:D: of 4:4:2:2. Complexes C and D act as terminal energy acceptors. From these results and previous analysis of the bicylindrical core of Synechococcus 6301 phycobilisomes [14,15] it is proposed that the two cylinders of the Synechocystis 6701 core, proximal to the thylakoid membrane, each have the composition ABCD, and that the distal cylinder has the composition A2B2.

Growth and Chromatic Adaptation of Nostoc sp. Strain MAC and the Pigment Mutant R-MAC

A spontaneous, stable, pigmentation mutant of Nostoc sp. strain MAC was isolated. Under various growth conditions, this mutant, R-MAC, had similar phycoerythrin contents (relative to allophycocyanin) but significantly lower phycocyanin contents (relative to allophycocyanin) than the parent strain. In saturating white light, the mutant grew more slowly than the parent strain. In nonsaturating red light, MAC grew with a shorter generation time than the mutant; however, R-MAC grew more quickly in nonsaturating green light.When the parental and mutant strains were grown in green light, the phycoerythrin contents, relative to allophycocyanin, were significantly higher than the phycoerythrin contents of cells grown in red light. For both strains, the relative phycocyanin contents were only slightly higher for cells grown in red light than for cells grown in green light. These changes characterize both MAC and R-MAC as belonging to chromatic adaptation group II: phycoerythrin synthesis alone photocontrolled.A comparative analysis of the phycobilisomes, isolated from cultures of MAC and R-MAC grown in both red and green light, was performed by polyacrylamide gel electrophoresis in the presence of 8.0 molar urea or sodium dodecyl sulfate. Consistent with the assignment of MAC and R-MAC to chromatic adaptation group II, no evidence for the synthesis of red light-inducible phycocyanin subunits was found in either strain. Phycobilisomes isolated from MAC and R-MAC contained linker polypeptides with relative molecular masses of 95, 34.5, 34, 32, and 29 kilodaltons. When grown in red light, phycobilisomes of the mutant R-MAC appeared to contain a slightly higher amount of the 32-kilodalton linker polypeptide than did the phycobilisomes isolated from the parental strain under the same conditions. The 34.5-kilodalton linker polypeptide was totally absent from phycobilisomes isolated from cells of either MAC or R-MAC grown in green light.

Rod substructure in cyanobacterial phycobilisomes: phycoerythrin assembly in synechocystis 6701 phycobilisomes

Synechocystis 6701 phycobilisomes consist of a core of three cylindrical elements in an equilateral array from which extend in a fanlike manner six rods, each made up of three to four stacked disks. Previous studies (see Gingrich, J. C., L. K. Blaha, and A. N. Glazer, 1982. J. Cell Biol. 92:261-268) have shown that the rods consist of four disk-shaped complexes of biliproteins with "linker" polypeptides of 27-, 33.5-, 31.5-, and 30.5-kdaltons, listed in order starting with the disk proximal to the core: phycocyanin (alpha beta)6-27 kdalton, phycocyanin (alpha beta)6-33.5 kdalton, phycoerythrin (alpha beta)6-31.5 kdalton, phycoerythrin (alpha beta)6-30.5 kdalton, where alpha beta is the monomer of the biliprotein. Phycoerythrin complexes of the 31.5- and 30.5-kdalton polypeptides were isolated in low salt. In 0.05 M K-phosphate-1 mM EDTA at pH 7.0, these complexes had the average composition (alpha beta)2-31.5 and (alpha beta)-30.5 kdalton polypeptide, respectively. Peptide mapping of purified 31.5- and 30.5-kdalton polypeptides showed that they differed significantly in primary structure. In 0.65 M Na-K-phosphate at pH 8, these phycoerythrin complexes formed rods of stacked disks of composition (alpha beta)6-31.5 or (alpha beta)6-30.5 kdaltons. For the (alpha beta)-30.5 kdalton complex, the yield of rod assemblies was variable and the self-association of free phycoerythrin to smaller aggregates was an important competing reaction. Complementation experiments were performed with incomplete phycobilisomes from Synechocystis 6701 mutant strain CM25. These phycobilisomes are totally lacking in phycoerythrin and the 31.5- and 30.5-kdalton polypeptides, but have no other apparent structural defects. In high phosphate at pH 8, the phycoerythrin-31.5-kdalton complex formed disk assemblies at the end of the rod substructures of CM25 phycobilisomes whereas no interaction with the phycoerythrin-30.5 kdalton complex was detected. In mixtures of both the phycoerythrin-31.5 and -30.5 kdalton complexes with CM25 phycobilisomes, both complexes were incorporated at the distal ends of the rod substructures. The efficiency of energy transfer from the added phycoerythrin in complemented phycobilisomes was approximately 96%. The results show that the ordered assembly of phycoerythrin complexes seen in phycobilisomes is reproduced in the in vitro assembly process.