Regulated synthesis of phycobilisome components

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.

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.

Effects of green light on biological systems

Green light (510-565 nm) constitutes a significant portion of the visible spectrum impinging on biological systems. It plays many different roles in the biochemistry, physiology and structure of plants and animals. In only a relatively small number of responses to green light is the photoreceptor known with certainty or even provisionally and in even fewer systems has the chain of events leading from perception to response been examined experimentally. This review provides a detailed view of those biological systems shown to respond to green light, an evaluation of possible photoreceptors and a review of the known and postulated mechanisms leading to the responses.

Genes encoding both subunits of phycoerythrocyanin, a light-harvesting biliprotein from the cyanobacterium Mastigocladus laminosus

Phycocyanin (PC) and phycoerythrocyanin (PEC) are light-harvesting components of the phycobilisome (PbS) from the cyanobacterium Mastigocladus laminosus. These two biliproteins are closely related, and show a particularly high degree of sequence homology in the C-terminal part of their beta-subunits. A 198-bp gene fragment encoding this region of PC from M. laminosus was therefore used as a heterologous hybridization probe to identify the genes coding for PEC from the same organism. A 1.7-kb HindIII fragment was cloned and its sequence determined. Three open reading frames (ORFs) were found on this fragment. The gene coding for the beta-subunit of PEC (pecB) was followed downstream by the alpha-subunit encoding gene (pecA). This gene arrangement had also been found in the PC-encoding (cpc) gene pair from M. laminosus, and is conserved in cpc genes from other organisms. This finding is compatible with a model of evolution of the cpc and pec gene pairs as the product of gene duplication of an ancestral beta- and alpha-subunit-encoding pair. A third ORF starts downstream from pecA. It codes for the 34.5-kDa linker protein, which forms complexes with PEC with a 1:6 stoichiometry in the PbS. Biliprotein- and linker protein-encoding genes are frequently clustered, and this provides mechanisms for the production of the different stoichiometric amounts of these gene products required in the PbS and for coregulation by environmental factors.

DNA sequence and regulation of the gene (cbpA) encoding the 42-kilodalton cytoplasmic membrane carotenoprotein of the cyanobacterium Synechococcus sp. strain PCC 7942

The gene (cbpA) coding for a carotenoid-binding protein of the cyanobacterium Synechococcus sp. strain PCC 7942 (Anacystis nidulans R2) has been cloned and sequenced. A polyclonal antibody against the protein was used to identify immunoreactive clones from a lambda gt11 expression library of Synechococcus strain PCC 7942. The initial positive clone (lambda gtAN42) contained a 0.9-kilobase (kb) chromosomal fragment, which was used to detect a larger chromosomal fragment from a lambda EMBL3 library. The lambda EMBL3 recombinant, lambda EM109, contained an 18-kb portion of the Synechococcus strain PCC 7942 chromosome. The open reading frame of cbpA encoded 450 amino acids which give rise to a protein of 49,113 daltons. The hydrophobicity plot indicates that the protein may have a 49-residue signal sequence which is cleaved to yield a mature protein of 43,709 daltons. The protein has been localized in the cytoplasmic membrane by biochemical procedures as well as by electron microscopic immunocytochemistry. Northern (RNA) blot analysis indicates that transcription of cbpA is tightly regulated by DNA topology, light intensity, and iron concentration. Transcription is greatly induced by growth under high light intensities and repressed during growth under iron-deficient conditions. The DNA gyrase inhibitor novobiocin specifically inhibited the light-induced transcription. In Northern blots, the gene-specific probe hybridized to two size classes of RNA, with lengths of 2.0 and 6.2 kb. Since cbpA appears to be a component of the 6.2-kb transcript, it is likely part of a larger operon.

Changes in gene expression during nitrogen starvation in Anabaena variabilis ATCC 29413

When the filamentous, nitrogen-fixing cyanobacterium Anabaena variabilis ATCC 29413 was subjected to nitrogen starvation under aerobic conditions, a complex series of events was initiated which resulted in heterocyst formation and derepression of the ability to fix dinitrogen. Using DNA-RNA hybridization techniques, we monitored the expression of several genes during nitrogen starvation and correlated changes in the mRNA levels with changes in enzyme activity, protein levels, and morphology. Nitrogenase mRNA was first observed after about 8.5 h of nitrogen starvation, as was nitrogenase activity. Late proheterocysts were present at that time. The level of nitrogenase mRNA increased for 5 to 6 h and then leveled off. Phycocyanin and allophycocyanin mRNA levels decreased rapidly within 1 h of nitrogen starvation; the levels increased later, as nitrogen starvation was alleviated, first by protein breakdown and then by nitrogen fixation. The average half-life of A. variabilis mRNA was determined by pulse-labeling techniques to be 16 to 18 min. Hybridization analysis showed that cpc and apc mRNAs also had half-lives of 16 to 18 min; the half-lives were not significantly different under nitrogen starvation conditions. Our results support the idea that the changes induced by nitrogen starvation are primarily the result of transcriptional regulation.

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.

Photoregulation of gene expression in the filamentous cyanobacterium Calothrix sp. PCC 7601: light-harvesting complexes and cell differentiation

Light plays a major role in many physiological processes in cyanobacteria. In Calothrix sp. PCC 7601, these include the biosynthesis of the components of the light-harvesting antenna (phycobilisomes) and the differentiation of the vegetative trichomes into hormogonia (short chains of smaller cells). In order to study the molecular basis for the photoregulation of gene expression, physiological studies have been coupled with the characterization of genes involved either in the formation of phycobilisomes or in the synthesis of gas vesicles, which are only present at the hormogonial stage.In each system, a number of genes have been isolated and sequenced. This demonstrated the existence of multigene families, as well as of gene products which have not yet been identified biochemically. Further studies have also established the occurrence of both transcriptional and post-transcriptional regulation. The transcription of genes encoding components of the phycobilisome rods is light-wavelength dependent, while translation of the phycocyanin genes may require the synthesis of another gene product irrespective of the light regime. In this report, we propose two hypothetical models which might be part of the complex regulatory mechanisms involved in the formation of functional phycobilisomes. On the other hand, transcription of genes involved in the gas vesicles formation (gvp genes) is turned on during hormogonia differentiation, while that of phycobiliprotein genes is simultaneously turned off. In addition, and antisense RNA which might modulate the translation of the gvp mRNAs is synthezised.

Regulation of cyanobacterial pigment-protein composition and organization by environmental factors

The coordinate expression of stress-specific genes is a common response of all organisms to altered environmental conditions. In cyanobacteria, the physiological consequences of stress are often reflected in both the ultrastructure of the cell and in photosynthesis-related properties. This review will focus on the alterations in cyanobacterial pigment-protein organization which occur under different growth conditions, and how several molecular genetic aproaches are being used in this laboratory to investigate the regulatory mechanisms underlying these alterations. We will discuss in detail the response to iron starvation, and present a testable hypothesis for the mechanism of thylakoid reorganization mediated by this response.

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.

Structure and regulation of genes encoding phycocyanin and allophycocyanin from Anabaena variabilis ATCC 29413

Gene clones encoding phycocyanin and allophycocyanin were isolated from an Anabaena variabilis ATCC 29413-Charon 30 library by using the phycocyanin (cpc) genes of Agmenellum quadruplicatum and the allophycocyanin (apc) genes of Cyanophora paradoxa as heterologous probes. The A. variabilis cpcA and cpcB genes occur together in the genome, as do the apcA and apcB genes; the two sets of genes are not closely linked, however. The cpc and apc genes appear to be present in only one copy per genome. DNA-RNA hybridization analysis showed that expression of the cpc and apc genes is greatly decreased during nitrogen starvation; within 1 h no cpc or apc mRNA could be detected. The source of nitrogen for growth did not influence expression of the genes; vegetative cells from nitrogen-fixing and ammonia-grown cultures had approximately the same levels of cpc and apc mRNAs. Heterocysts had less than 5% as much cpc mRNA as vegetative cells from nitrogen-fixing cultures. Northern hybridization (RNA blot) analysis showed that the cpc genes are transcribed to give an abundant 1.4-kilobase (kb) RNA as well as two less prominent 3.8- and 2.6-kb species. The apc genes gave rise to two transcripts, a 1.4-kb predominant RNA and a minor 1.75-kb form.

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.