Genes encoding major light-harvesting polypeptides are clustered on the genome of the cyanobacterium Fremyella diplosiphon

Responding to light signals: a comprehensive update on photomorphogenesis in cyanobacteria

Cyanobacteria are ancestors of chloroplast and perform oxygen-evolving photosynthesis similar to higher plants and algae. However, an obligatory requirement of photons for their growth results in the exposure of cyanobacteria to varying light conditions. Therefore, the light environment could act as a signal to drive the developmental processes, in addition to photosynthesis, in cyanobacteria. These Gram-negative prokaryotes exhibit characteristic light-dependent developmental processes that maximize their fitness and resource utilization. The development occurring in response to radiance (photomorphogenesis) involves fine-tuning cellular physiology, morphology and metabolism. The best-studied example of cyanobacterial photomorphogenesis is chromatic acclimation (CA), which allows a selected number of cyanobacteria to tailor their light-harvesting antenna called phycobilisome (PBS). The tailoring of PBS under existing wavelengths and abundance of light gives an advantage to cyanobacteria over another photoautotroph. In this work, we will provide a comprehensive update on light-sensing, molecular signaling and signal cascades found in cyanobacteria. We also include recent developments made in other aspects of CA, such as mechanistic insights into changes in the size and shape of cells, filaments and carboxysomes.

A hybrid type of chromatic acclimation regulated by the dual green/red photosensory systems in cyanobacteria

Cyanobacteria are phototrophic bacteria that perform oxygenic photosynthesis. They use a supermolecular light-harvesting antenna complex, the phycobilisome (PBS), to capture and transfer light energy to photosynthetic reaction centers. Certain cyanobacteria alter the absorption maxima and/or overall structure of their PBSs in response to the ambient light wavelength-a process called chromatic acclimation (CA). One of the most well-known CA types is the response to green and red light, which is controlled by either the RcaEFC or CcaSR photosensory system. Here, we characterized a hybrid type of CA in the cyanobacterium Pleurocapsa sp. Pasteur Culture Collection (PCC) 7319 that uses both RcaEFC and CcaSR systems. In vivo spectroscopy suggested that strain PCC 7319 alters the relative composition of green-absorbing phycoerythrin and red-absorbing phycocyanin in the PBS. RNA sequencing and promoter motif analyses suggested that the RcaEFC system induces a gene operon for phycocyanin under red light, whereas the CcaSR system induces a rod-membrane linker gene under green light. Induction of the phycoerythrin genes under green light may be regulated through a yet unidentified photosensory system called the Cgi system. Spectroscopy analyses of the isolated PBSs suggested that hemidiscoidal and rod-shaped PBSs enriched with phycoerythrin were produced under green light, whereas only hemidiscoidal PBSs enriched with phycocyanin were produced under red light. PCC 7319 uses the RcaEFC and CcaSR systems to regulate absorption of green or red light (CA3) and the amount of rod-shaped PBSs (CA1), respectively. Cyanobacteria can thus flexibly combine diverse CA types to acclimate to different light environments.

Photoecology of the Antarctic cyanobacterium Leptolyngbya sp. BC1307 brought to light through community analysis, comparative genomics and in vitro photophysiology

Cyanobacteria are important photoautotrophs in extreme environments such as the McMurdo Dry Valleys, Antarctica. Terrestrial Antarctic cyanobacteria experience constant darkness during the winter and constant light during the summer which influences the ability of these organisms to fix carbon over the course of an annual cycle. Here, we present a unique approach combining community structure, genomic and photophysiological analyses to understand adaptation to Antarctic light regimes in the cyanobacterium Leptolyngbya sp. BC1307. We show that Leptolyngbya sp. BC1307 belongs to a clade of cyanobacteria that inhabits near-surface environments in the McMurdo Dry Valleys. Genomic analyses reveal that, unlike close relatives, Leptolyngbya sp. BC1307 lacks the genes necessary for production of the pigment phycoerythrin and is incapable of complimentary chromatic acclimation, while containing several genes responsible for known photoprotective pigments. Photophysiology experiments confirmed Leptolyngbya sp. BC1307 to be tolerant of short-term exposure to high levels of photosynthetically active radiation, while sustained exposure reduced its capacity for photoprotection. As such, Leptolyngbya sp. BC1307 likely exploits low-light microenvironments within cyanobacterial mats in the McMurdo Dry Valleys.

A proteomic approach to the analysis of the components of the phycobilisomes from two cyanobacteria with complementary chromatic adaptation: Fremyella diplosiphon UTEX B590 and Tolypothrix PCC 7601

Tolypothrix PCC 7601 and Fremyella diplosiphon UTEX B590 can produce two alternative phycobilisome (PBS) rods. PE-PBSs with one phycocyanin (PC) disk and multiple phycoerythrin (PE) disks are found in cells grown under green light (GL). PC-PBSs with only PC disks are obtained from cells grown under red light (RL). In this manuscript, we show the localization of the linker proteins and ferredoxin-NADP(+) oxidoreductase (FNR) in the PC-PBS and of PE-PBS rods using visible spectroscopy and mass spectrometry. PE-PBSs with different [PE]/[PC] ratios and PC-PBSs with different [PC]/[AP] (AP, allophycocyanin) ratios were isolated. CpeC was the primary rod linker protein found in the PBSs with a [PE]/[PC] ratio of 1.1, which indicates that this is the rod linker at the interphase PC-PE. CpeC and CpeD were identified in the PBSs with a [PE]/[PC] ratio of 1.6, which indicates that CpcD is the linker between the first and the second PE hexamers. Finally, CpeC, CpeD, and CpeE were found in the PBSs with a [PE]/[PC] ratio of 2.9, indicating the position of CpeE between the second and third PE moieties. CpcI2 was identified in the two PC-PBSs obtained from cells grown under RL, which indicates that CpcI2 is the linker between the first and second PC hexamers. CpcH2 was identified only in the PC-PBSs from Tolypothrix with a high [PC]/[AP] ratio of 1.92, which indicates that CpcH2 is the linker between the second and third PC hexamers. The PC-PBSs contained the rod cap protein L(R)(10) (CpcD), but this protein was absent in the PE-PBSs. PE-PBSs (lacking L(R)(10)) incorporated exogenous rFNR in a stoichiometry of up to five FNRs per PBS. A maximum of two FNRs per PBS were found in PC-PBSs (with L(R)(10)). These observations support the hypothesis that FNR binds at the distal ends of the PBS rods in the vacant site of CpcD L(R)(10). Finally, the molecular mass of the core membrane linker (L(CM)) was determined to be 102 kDa from a mass spectrometry analysis.

Emerging perspectives on the mechanisms, regulation, and distribution of light color acclimation in cyanobacteria

Chromatic acclimation (CA) provides many cyanobacteria with the ability to tailor the properties of their light-harvesting antennae to the spectral distribution of ambient light. CA was originally discovered as a result of its dramatic cellular phenotype in red and green light. However, discoveries over the past decade have revealed that many pairs of light colors, ranging from blue to infrared, can trigger CA responses. The capacity to undergo CA is widespread geographically, occurs in most habitats around the world, and is found within all major cyanobacterial groups. In addition, many other cellular activities have been found to be under CA control, resulting in distinct physiological and morphological states for cells under different light-color conditions. Several types of CA appear to be the result of convergent evolution, where different strategies are used to achieve the final goal of optimizing light-harvesting antenna composition to maximize photon capture. The regulation of CA has been found to occur primarily at the level of RNA abundance. The CA-regulatory pathways uncovered thus far are two-component systems that use phytochrome-class photoreceptors with sensor-kinase domains to control response regulators that function as transcription factors. However, there is also at least one CA-regulatory pathway that operates at the post-transcriptional level. It is becoming increasingly clear that large numbers of cyanobacterial species have the capacity to acclimate to a wide variety of light colors through the use of a range of different CA processes.

Temporal responses of wild-type pigmentation and RcaE-deficient strains of Fremyella diplosiphon during light transitions

A temporal study was conducted to evaluate the dynamic complementary chromatic adaptation (CCA) response of two Fremyella diplosiphon strains-wild-type pigmentation strain SF33 and an RcaE-deficient (ΔrcaE) strain, which lacks the photosensor that regulates CCA. SF33 and ΔrcaE cultures were monitored for 15 days after transition of green-light (GL) acclimated cultures to red light (RL) and vice versa. SF33 showed similar growth irrespective of the external light quality; however, a ΔrcaE strain grew slower than SF33 under both RL and GL. Chlorophyll a (chla) content increased in both strains over time and was not much different under RL and GL indicating that chla biosynthesis is not affected significantly by light quality or RcaE function. Phycoerythrin is the sole pigment to absorb GL, whereas several pigments, i.e., allophycocyanin, phycocyanin and chla, function under RL to drive photosynthesis. SF33 compensates for this by synthesizing a higher percentage of PE under GL. The final pigment distribution in the ΔrcaE mutant was found to be more different from SF33 under GL than under RL indicating that RcaE is needed for a transitional response to RL and RL-dependent repression of PE accumulation, yet RcaE is virtually critical for both transitioning to and a full adaptation to GL.

Functional characterization of a cyanobacterial OmpR/PhoB class transcription factor binding site controlling light color responses

Complementary chromatic acclimation (CCA) allows many cyanobacteria to change the composition of their light-harvesting antennae for maximal absorption of different wavelengths of light. In the freshwater species Fremyella diplosiphon, this process is controlled by the ratio of red to green light and allows the differential regulation of two subsets of genes in the genome. This response to ambient light color is controlled in part by a two-component system that includes a phytochrome class photoreceptor and a response regulator with an OmpR/PhoB class DNA binding domain called RcaC. During growth in red light, RcaC is able to simultaneously activate expression of red light-induced genes and repress expression of green light-induced genes through binding to the L box promoter element. Here we investigate how the L box functions as both an activator and a repressor under the same physiological conditions by analyzing the effects of changing the position, orientation, and sequence of the L box. We demonstrate that changes in the local sequences surrounding the L box affect the strength of its activity and that the activating and repressing functions of the L box are orientation dependent. Also, the spacing between the L box and the transcription start site is critical for it to work as an activator, while its repressing role during light regulation requires additional upstream and downstream DNA sequence elements. The latter result suggests that the repressing function of RcaC requires it to operate in association with multiple additional DNA binding proteins, at least one of which is functioning as an activator.

De novo sequence analysis and intact mass measurements for characterization of phycocyanin subunit isoforms from the blue-green alga Aphanizomenon flos-aquae

In this work, partial characterization of the primary structure of phycocyanin from the cyanobacterium Aphanizomenon flos-aquae (AFA) was achieved by mass spectrometry de novo sequencing with the aid of chemical derivatization. Combining N-terminal sulfonation of tryptic peptides by 4-sulfophenyl isothiocyanate (SPITC) and MALDI-TOF/TOF analyses, facilitated the acquisition of sequence information for AFA phycocyanin subunits. In fact, SPITC-derivatized peptides underwent facile fragmentation, predominantly resulting in y-series ions in the MS/MS spectra and often exhibiting uninterrupted sequences of 20 or more amino acid residues. This strategy allowed us to carry out peptide fragment fingerprinting and de novo sequencing of several peptides belonging to both alpha- and beta-phycocyanin polypeptides, obtaining a sequence coverage of 67% and 75%, respectively. The presence of different isoforms of phycocyanin subunits was also revealed; subsequently Intact Mass Measurements (IMMs) by both MALDI- and ESI-MS supported the detection of these protein isoforms. Finally, we discuss the evolutionary importance of phycocyanin isoforms in cyanobacteria, suggesting the possible use of the phycocyanin operon for a correct taxonomic identity of this species.

Abundance changes of the response regulator RcaC require specific aspartate and histidine residues and are necessary for normal light color responsiveness

RcaC is a large, complex response regulator that controls transcriptional responses to changes in ambient light color in the cyanobacterium Fremyella diplosiphon. The regulation of RcaC activity has been shown previously to require aspartate 51 and histidine 316, which appear to be phosphorylation sites that control the DNA binding activity of RcaC. All available data suggest that during growth in red light, RcaC is phosphorylated and has relatively high DNA binding activity, while during growth in green light RcaC is not phosphorylated and has less DNA binding activity. RcaC has also been found to be approximately sixfold more abundant in red light than in green light. Here we demonstrate that the light-controlled abundance changes of RcaC are necessary, but not sufficient, to direct normal light color responses. RcaC abundance changes are regulated at both the RNA and protein levels. The RcaC protein is significantly less stable in green light than in red light, suggesting that the abundance of this response regulator is controlled at least in part by light color-dependent proteolysis. We provide evidence that the regulation of RcaC abundance does not depend on any RcaC-controlled process but rather depends on the presence of the aspartate 51 and histidine 316 residues that have previously been shown to control the activity of this protein. We propose that the combination of RcaC abundance changes and modification of RcaC by phosphorylation may be necessary to provide the dynamic range required for transcriptional control of RcaC-regulated genes.

Inverse transcriptional activities during complementary chromatic adaptation are controlled by the response regulator RcaC binding to red and green light-responsive promoters

Complementary chromatic adaptation (CCA) provides cyanobacteria with the ability to shift between red and blue-green phenotypes that are optimized for absorption of different wavelengths of light. Controlled by the ratio of green to red light, this process results from differential expression of two groups of operons, many of which encode proteins involved in photosynthetic light harvesting antennae biogenesis. In the freshwater species Fremyella diplosiphon, the inverse regulation of these two classes is complex and occurs through different mechanisms. It also involves a two-component pathway that includes a phytochrome-class photoreceptor and the response regulator RcaC. Here we uncover the mechanism through which this system controls CCA by demonstrating that RcaC binds to the L Box within promoters of both classes of light-regulated operons. We provide functional evidence that complementary regulation of these operons occurs by RcaC's simultaneous activation and repression of transcription in red light. We identify rcaC and L Boxes in the genome of a marine cyanobacterium capable of CCA, suggesting widespread use of this control system. These results provide important insights into the long-standing enigma of CCA regulation and complete the first description of an entire two-component system controlled by a phytochrome-class photoreceptor.

A light regulated OmpR-class promoter element co-ordinates light-harvesting protein and chromophore biosynthetic enzyme gene expression

Co-ordination of chromophore and apoprotein biosynthesis is required during photosynthetic light-harvesting antennae production, such as occurs during complementary chromatic adaptation (CCA). This response to ambient light colour changes is controlled by a phytochrome-class photoreceptor and involves changes in the synthesis of cyanobacterial light-harvesting antennae. During growth in red light, CCA activates cpc2 transcription, an operon that encodes the light-harvesting protein phycocyanin. In order to function, this apoprotein must have covalently attached phycocyanobilin chromophores, which are synthesized by PcyA. We show that pcyA is also transcriptionally activated by CCA during red light growth and is not regulated via feedback that senses cpc2 RNA levels. The pcyA and cpc2 promoters contain a common regulatory element, a direct repeat typical of OmpR-class transcription factor binding sites, at similar positions relative to their red light-controlled transcription start sites. Deletion of this element from the pcyA promoter eliminated CCA-regulated transcription, and insertion of the element into a non-light responsive promoter conferred CCA regulation. We conclude that this element is necessary and sufficient to confer CCA transcriptional regulation and that it co-ordinates phycocyanin and phycocyanobilin biosynthesis in red light.

Responding to color: the regulation of complementary chromatic adaptation

The acclimation of photosynthetic organisms to changes in light color is ubiquitous and may be best illustrated by the colorful process of complementary chromatic adaptation (CCA). During CCA, cyanobacterial cells change from brick red to bright blue green, depending on their light color environment. The apparent simplicity of this spectacular, photoreversible event belies the complexity of the cellular response to changes in light color. Recent results have shown that the regulation of CCA is also complex and involves at least three pathways. One is controlled by a phytochrome-class photoreceptor that is responsive to green and red light and a complex two-component signal transduction pathway, whereas another is based on sensing redox state. Studies of CCA are uncovering the strategies used by photosynthetic organisms during light acclimation and the means by which they regulate these responses.

Genomic DNA microarray analysis: identification of new genes regulated by light color in the cyanobacterium Fremyella diplosiphon

Many cyanobacteria use complementary chromatic adaptation to efficiently utilize energy from both green and red regions of the light spectrum during photosynthesis. Although previous studies have shown that acclimation to changing light wavelengths involves many physiological responses, research to date has focused primarily on the expression and regulation of genes that encode proteins of the major photosynthetic light-harvesting antennae, the phycobilisomes. We have used two-dimensional gel electrophoresis and genomic DNA microarrays to expand our understanding of the physiology of acclimation to light color in the cyanobacterium Fremyella diplosiphon. We found that the levels of nearly 80 proteins are altered in cells growing in green versus red light and have cloned and positively identified 17 genes not previously known to be regulated by light color in any species. Among these are homologs of genes present in many bacteria that encode well-studied proteins lacking clearly defined functions, such as tspO, which encodes a tryptophan-rich sensory protein, and homologs of genes encoding proteins of clearly defined function in many species, such as nblA and chlL, encoding phycobilisome degradation and chlorophyll biosynthesis proteins, respectively. Our results suggest novel roles for several of these gene products and highly specialized, unique uses for others.

CotB is essential for complete activation of green light-induced genes during complementary chromatic adaptation in Fremyella diplosiphon

The dramatic modifications of photosynthetic light harvesting antennae called phycobilisomes that occur during complementary chromatic adaptation in cyanobacteria are controlled by two separate photosensory systems. The first system involves the signal transduction components RcaE, RcaF and RcaC, which appear to make up a complex multistep phosphorelay. This system controls the light responsive expression of the cpcB2A2H2I2D2, cpeBA and cpeCDE operons, which encode phycobilisome proteins. The second system, which is not yet characterized, acts in concert with the first but only regulates the light responses of cpeBA and cpeCDE. We have generated and characterized a new mutant class, named the Tan mutants. In at least one member of this class, light-regulated RNA accumulation patterns are altered for cpeBA and cpeCDE, but not for cpcB2A2H2I2D2. Thus this mutant contains a lesion that may impair the operation of the second system. We demonstrate that several Tan mutants are the result of improper expression of the gene cotB. CotB has limited similarity to lyase class proteins, particularly those related to NblB, which is required for degradation of phycobilisomes in other cyanobacteria. Possible roles of CotB in the biogenesis of phycobilisomes are discussed.

Lesions in phycoerythrin chromophore biosynthesis in Fremyella diplosiphon reveal coordinated light regulation of apoprotein and pigment biosynthetic enzyme gene expression

We have characterized the regulation of the expression of the pebAB operon, which encodes the enzymes required for phycoerythrobilin synthesis in the filamentous cyanobacterium Fremyella diplosiphon. The expression of the pebAB operon was found to be regulated during complementary chromatic adaptation, the system that controls the light responsiveness of genes that encode several light-harvesting proteins in F. diplosiphon. Our analyses of pebA mutants demonstrated that although the levels of phycoerythrin and its associated linker proteins decreased in the absence of phycoerythrobilin, there was no significant modulation of the expression of pebAB and the genes that encode phycoerythrin. Instead, regulation of the expression of these genes is coordinated at the level of RNA accumulation by the recently discovered activator CpeR.

A molecular understanding of complementary chromatic adaptation

Photosynthetic activity and the composition of the photosynthetic apparatus are strongly regulated by environmental conditions. Some visually dramatic changes in pigmentation of cyanobacterial cells that occur during changing nutrient and light conditions reflect marked alterations in components of the major light-harvesting complex in these organisms, the phycobilisome. As noted well over 100 years ago, the pigment composition of some cyanobacteria is very sensitive to ambient wavelengths of light; this sensitivity reflects molecular changes in polypeptide constituents of the phycobilisome. The levels of different pigmented polypeptides or phycobiliproteins that become associated with the phycobilisome are adjusted to optimize absorption of excitation energy present in the environment. This process, called complementary chromatic adaptation, is controlled by a bilin-binding photoreceptor related to phytochrome of vascular plants; however, many other regulatory elements also play a role in chromatic adaptation. My perspectives and biases on the history and significance of this process are presented in this essay.

In vivo and in vitro characterization of the light-regulated cpcB2A2 promoter of Fremyella diplosiphon

When exposed to different spectral qualities of light, many cyanobacteria dramatically alter their phycobilisome rod composition in a process termed complementary chromatic adaptation. In the cyanobacterium Fremyella diplosiphon, this response is associated with differential expression of the cpcB2A2, cpeBA, and cpeCDE operons, which code for the phycobiliproteins phycocyanin and phycoerythrin and the phycoerythrin linker polypeptides, respectively. To define components of the signal transduction pathway involved in light-regulated expression of genes encoding phycobilisome polypeptides, we have used in vivo and in vitro techniques to identify cis-acting sequences and trans-acting factors necessary for the regulation of the red-light-inducible cpcB2A2 operon. Deletion of the cpcB2A2 upstream sequences to -76 bp with respect to the transcription start site had no effect on red-light induction of a cpcB2A2-beta-glucuronidase (GUS) chimeric gene, while deletion to -37 bp abolished GUS expression. Furthermore, a fragment of the cpcB2A2 gene from -76 to +25 bp linked to the untranslated leader of cpcB1A1 (a constitutively expressed operon encoding phycocyanin) is sufficient to drive high-level GUS expression in red light. Therefore, the sequence between positions -76 and -37 is necessary for the expression of cpcB2A2, and the region extending from -76 to +25 is sufficient for red-light induction of the operon. Attempts were made to correlate the in vivo data with protein binding in the region upstream of the transcription start site of cpcB2A2. Using in vitro analysis, we detected two protein-binding sites in the cpcB2A2 promoter which were localized to positions -162 to -122 and -37 to +25. Proteins from both red- and green-light-grown cells interacted with the former site, while only proteins present in extracts from red-light-grown cells interacted with the latter site. The data from both the in vivo and in vitro analyses suggest that while two regions upstream of the cpcB2A2 transcription initiation site specifically bind proteins, only the binding site bordering the transcription start site is important for complementary chromatic adaptation.

A phosphorylated DNA-binding protein is specific for the red-light signal during complementary chromatic adaptation in cyanobacteria

Complementary chromatic adaptation is a mechanism by which some cyanobacteria that are able to synthesize phycoerythrin can adapt their pigment (phycobiliprotein) content to the incident wavelengths of the light. In Calothrix sp. PCC 7601 it concerns phycoerythrin (cpe operon), synthesized under green light, and phycocyanin-2 (cpc2 operon), expressed under red light, and involves transcriptional controls. With cell-free extracts from Calothrix sp. PCC 7601 grown under various light regimes, a protein designated RcaD was found by gel retardation experiments to specifically bind to the cpc2 promoter region and to be present only in red-light-grown cells. This protein was partially purified and its binding activity was shown to be sensitive to an alkaline phosphatase treatment. RcaD can protect two regions of the cpc2 promoter sequence against degradation by DNase I. Because its activity is detected only under the conditions required for cpc2 expression, we propose that RcaD is a positive effector of transcription.

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.

Organization and transcription of the genes encoding two differentially expressed phycocyanins in the cyanobacterium Pseudanabaena sp. PCC 7409

The cpc1 and cpc2 operons of the group III chromatically adapting cyanobacterium Pseudanabaena sp. PCC 7409 were isolated and their nucleotide sequences determined. The cpc1 operon consists of the genes cpcB1A1EF and gives rise to an abundant 1400-nucleotide transcript encoding the cpcB1A1 genes and two low-abundance transcripts of 1000 nucleotides and 1100 nucleotides encoding the cpcF gene. Two extremely low-abundance transcripts of approximately 2900 nucleotides and 4800 nucleotides possibly encode cpcB1A1E and cpcB1A1EF, respectively. All transcripts were present in cultures grown in either red or green light. The transcription start of the cpcB1A1 mRNA was mapped to a position 238 bp 5' to the cpcB1 translation start. The cloned fragment containing the cpcB2A2 genes was found to contain only a portion of the cpc2 operon and consisted of the cpcB2A2 genes and the 5' portion of the linker gene cpcH2. On the basis of biochemical evidence, as well as sequence data from other cpc operons, it is probable that the complete Pseudanabaena sp. PCC 7409 cpc2 operon consists of the genes cpcB2A2H2I2D2. This operon is almost exclusively transcribed in cells grown in red light and gives rise to an abundant mRNA 1400 nucleotides in length that encodes the cpcB2A2 genes. A second transcript of 2400 nucleotides encodes the cpcB2A2H2 genes. A third transcript of 3800 nucleotides encodes the cpcB2A2H2 genes and probably the cpcI2 and cpcD2 genes as well. Transcription of the cpc2 mRNAs inititates 219 bp 5' to the cpcB2 translation start. The promoter region of the Pseudanabaena sp. PCC 7409 cpc1 operon contains the sequence 5' ttGTATaa 3' that is also found to occur within 20 bp of the transcription initiation sites of a number of other constitutively expressed cpc promoters. A high level of sequence similarity also occurs between the red-light-inducible cpc2 promoters of Pseudanabaena sp. PCC 7409 and Calothrix sp. PCC 7601.

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 the phycobilisome rod substructure are clustered on the Anabaena chromosome: characterization of the phycoerythrocyanin operon

The phycoerythrocyanin (pec) operon, cloned from Anabaena sp. strain PCC 7120, encodes four genes, pecBACE, located upstream of the C-phycocyanin (cpc) operon. This pec-cpc cluster includes all the genes for the structural components of the phycobilisome rod. Oligonucleotide probes based on the amino-terminal sequence of the phycoerythrocyanin beta subunit were used to clone an 8.0-kbp EcoRI fragment which was determined, by sequencing, to partially overlap the previously cloned cpc operon. A 5.0-kbp EcoRI-ClaI fragment corresponding to the region upstream of the cpc operon was subsequently subcloned and sequenced. Five open reading frames whose polarity of transcription is parallel to that of the cpc genes were identified. pecB and pecA encode the beta and alpha subunits of phycoerythrocyanin, respectively. pecC encodes the phycoerythrocyanin-associated linker polypeptide LR34.5,PEC. The identities of these genes are confirmed by agreement with amino-terminal sequences determined from purified phycobilisome components. A gene homologous to cpcE, found downstream of pecC, has been designated pecE. The cpcE gene product is involved in the attachment of the phycocyanobilin chromophore to the alpha subunit of phycocyanin. Three transcripts were observed by Northern (RNA) analyses. The most abundant of these transcripts, 1.35 kbp, corresponds to the beta and alpha subunit genes, whereas the less-abundant transcripts, 2.3 and 3.1 kbp, correspond to pecBAC and pecBACE, respectively. Phycoerythrocyanin is strongly induced in cells cultured under low light. In parallel, all three transcripts were present at much higher levels in cells cultured under low light.

Three C-phycoerythrin-associated linker polypeptides in the phycobilisome of green-light-grown Calothrix sp. PCC 7601 (cyanobacteria)

Microanalyses by SDS-PAGE and microsequencing demonstrate that, under green-light conditions, 3 C-phycoerythrin associated rod-linker polypeptides with different N-terminal amino acid sequences are present in phycobilisomes (PBS) from Calothrix sp. 7601 cells. Two of these polypeptides, corresponding to SDS-PAGE bands at 36 and 37 kDa, could be assigned, respectively, to the cpeC and cpcD genes found on a separate cpeCD-operon in Calothrix sp. 7601 (Federspiel, N.A. and Grossman, A.R. (1990) J. Bacteriol, 172, 4072-4081). The third C-PE rod-linker polypeptide, LR,2PE,33, requires, therefore, a third gene with the suggested locus designation 'cpeE'. A C-PE (alpha beta)6-LR,2PE,33 complex containing this third rod-linker polypeptide could be isolated from phycobilisomes and characterized. PBS from both green- and red-light cells of Calothrix contain a single, unique LRC28 rod-core linker polypeptide which is not altered during chromatic adaptation.

Molecular cloning and transcriptional analysis of the cpeBA operon of the cyanobacterium Pseudanabaena species PCC7409

The cpeBA operon of the Group III chromatically adapting cyanobacterium Pseudanabaena species PCC 7409 was cloned, sequenced and characterized. The cpeBA genes are transcribed in green-light-grown cells as an abundant 1400-nucleotide mRNA which initiates 69 nucleotides upstream from the cpeB translation start. Extensive sequence identity, extending 70 nucleotides 5' to the transcription start, occurs among cpeBA promoters of Group II and III chromatic adapters. Cell extracts of green-light-grown Calothrix species PCC 7601 contain an activity which specifically binds a restriction fragment containing the Pseudanabanea species PCC 7409 cpeBA promoter. Green-light-dependent cpeBA transcription in Group II and III chromatically adapting cyanobacteria is suggested to be similarly controlled by a transcriptional activator.

Effect of Light Quality on Phycobilisome Components of the Cyanobacterium Spirulina platensis

Phycobilisomes from the nonchromatic adapting cyanobacterium Spirulina platensis are composed of a central core containing allophycocyanin and rods with phycocyanin and linker polypeptides in a regular array. Room temperature absorption spectra of phycobilisomes from this organism indicated the presence of phycocyanin and allophycocyanin. However, low temperature absorption spectra showed the association of a phycobiliviolin type of chromophore within phycobilisomes. This chromophore had an absorption maximum at 590 nanometers when phycobilisomes were suspended in 0.75 molar K-phosphate buffer (pH 7.0). Purified phycocyanin from this cyanobacterium was found to consist of three subparticles and the phycobiliviolin type of chromophore was associated with the lowest density subparticle. Circular dichroism spectra of phycocyanin subparticles also indicated the association of this chromophore with the lowest density subparticle. Absorption spectral analysis of alpha and beta subunits of phycocyanin showed that phycobiliviolin type of chromophore was attached to the alpha subunit, but not the beta subunit. Effect of light quality showed that green light enhanced the synthesis of this chromophore as analyzed from the room temperature absorption spectra of phycocyanin subparticles and subunits, while red or white light did not have any effect. Low temperature absorption spectra of phycobilisomes isolated from green, red, and white light conditions also indicated the enhancement of phycobiliviolin type of chromophore under green light.

Genes for phycocyanin subunits in Synechocystis sp. strain PCC 6701 and assembly mutant UV16

The cyanobacterial phycobilisome is a large protein complex located on the photosynthetic membrane. It harvests light energy and transfers it to chlorophyll for use in photosynthesis. Phycobilisome assembly mutants in the unicellular cyanobacterium Synechocystis sp. strain 6701 have been characterized. One such mutant, UV16, contains a defect in the assembly of the biliprotein phycocyanin. We report the cloning and sequencing of the phycocyanin genes from wild-type Synechocystis strain 6701 and demonstrate an alteration in the gene for the phycocyanin alpha subunit in UV16. Possible consequences of the lesion on phycobilisome assembly were assessed from its position in the phycocyanin tertiary and quaternary structures. The UV16 phenotype is complex and includes a reduced level of phycocyanin relative to that in the wild type. To determine whether the lower phycocyanin content results from lower transcript levels, a fragment of cpcBA was used as a probe for quantitating phycocyanin mRNA. Both the wild type and UV16 contained two phycocyanin transcripts of approximately 1.4 and 1.5 kilobases that were equal in abundance and that did not vary with light quality during cell growth. Equal levels of these transcripts in the wild type and UV16 suggest that the lower phycocyanin content in the mutant may be due to posttranscriptional events. The 5' ends of the two phycocyanin mRNAs were mapped at 100 and 223 base pairs upstream of the cpcB initiation codon. Homologous regions upstream of the putative transcription initiation sites may be important for maintaining high levels of transcription from the Synechocystis strain 6701 phycocyanin gene set.

Role of Protein Synthesis in Regulation of Phycobiliprotein mRNA Abundance by Light Quality in Fremyella diplosiphon

If green light-acclimated Fremyella diplosiphon cultures are transferred to red light, the transcription from the inducible phycocyanin gene set increases at least 30-fold within 60 minutes. This effect is inhibited completely by the protein synthesis inhibitors chloramphenicol and spectinomycin. Application of chloramphenicol 30 minutes after transfer of cultures to inductive red light prevents further phycocyanin mRNA accumulation within 10 minutes. If red light-acclimated cells are transferred to green light, the phycocyanin transcript level declines by about 70% within 1 hour. Most of the green light-dependent decline results from the rapid cessation of transcription from the PC gene set. Chloramphenicol slows the decline to some extent by decreasing the rate of mRNA degradation in a light-independent manner. The accumulation of phycoerythrin mRNA after transfer of red light-acclimated cells to green light is also inhibited by chloramphenicol. However, there is no red light-dependent mechanism that rapidly halts phycoerythin mRNA synthesis after transfer of cultures from green to red light. Therefore, at least three light-dependent processes are involved in regulating phycobiliproteingene expression: chloramphenicol-sensitive processes required for the activation of phycocyanin and phycoerythrin gene sets and a chloramphenicol-insensitive process which blocks phycocyanin mRNA synthesis after transfer of cells from red to green light.

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.

Molecular characterization of phycobilisome regulatory mutants of Fremyella diplosiphon

Three classes of pigment mutants were generated in Fremyella diplosiphon in the course of electroporation experiments. The red mutant class had high levels of phycoerythrin in both red and green light and no inducible phycocyanin in red light. Thus, this mutant behaved as if it were always in green light, regardless of light conditions. Blue mutants exhibited normal phycoerythrin photoregulation, whereas the inducible phycocyanin was present at high levels in both red- and green-light-grown cells. Furthermore, the absolute amount of allophycocyanin was increased threefold in comparison with our wild-type strain. Green mutants lost the capacity to accumulate phycoerythrin in green light but showed normal photoregulation of phycocyanin. Analyses of transcript abundance in these mutants demonstrated that changes in the levels of the different phycobilisome components correlated with changes in the levels of mRNAs encoding those components. The characterization of these mutants supports hypotheses previously discussed concerning molecular mechanisms involved in the regulation of the phycobiliprotein gene sets during chromatic adaptation.

Changes in Accumulation and Synthesis of Transcripts Encoding Phycobilisome Components during Acclimation of Fremyella diplosiphon to Different Light Qualities

We have used gene-specific DNA fragments as hybridization probes to quantitate the levels of transcripts encoding several phycobilisome polypeptides in the cyanobacterium Fremyella diplosiphon in response to changes in the light environment. While the levels of transcripts encoding allophycocyanin, the core linker polypeptide, and the constitutive phycocyanin subunits are similar in F. diplosiphon grown either in red or green light, the levels of other transcripts change dramatically. Transcripts encoding the inducible phycocyanin subunits are barely detected in green light-grown cells and very abundant in red light-grown cells, while the level of phycoerythrin mRNA is approximately 10-fold more in green than red light-grown cells. Quantitation of the phycoerythrin and inducible phycocyanin transcripts after transfer of cultures from green to red light and red to green light demonstrate that both increase rapidly upon exposure of cells to inductive illumination. The decrease in the phycoerythrin mRNA level in red light is much slower than the decline in the levels of the inducible phycocyanin transcripts in green light. Since the half-lives of the inducible phycocyanin and phycoerythrin transcripts do not change when F. diplosiphon is exposed to red or green illumination, the steady state levels of these mRNAs are primarily controlled by the rate of transcription. Therefore, the high level of phycoerythrin mRNA maintained for several hours after cultures are transferred from green to red illumination must result from continued transcription of the phycoerythrin gene set. Differences in expression from the phycoerythrin and inducible phycocyanin gene sets in response to light quality are discussed in terms of possible mechanisms involved in their regulation.

Genes encoding core components of the phycobilisome in the cyanobacterium Calothrix sp. strain PCC 7601: occurrence of a multigene family

The phycobilisome is the major light-harvesting complex of cyanobacteria. It is composed of a central core from which six rods radiate. Allphycocyanin, an alpha beta oligomer (alpha AP and beta AP), is the main component of the core which also contains three other phycobiliproteins (alpha APB, beta 18.3, and L92CM) and a small linker polypeptide (L7.8C). By heterologous DNA hybridization, two EcoRI DNA fragments of 3.5 and 3.7 kilobases have been cloned from the chromatically adapting cyanobacterium Calothrix sp. strain PCC 7601. Nucleotide sequence determination has allowed the identification of five apc genes: apcA1 (alpha AP1), apcA2 (alpha AP2), apcB1 (beta AP1), apcC (L7.8C), and apcE (L92CM). Four of these genes are adjacent on the chromosome and form the apcEA1B1C gene cluster. In contrast, no genes have been found close to the apcA2 gene which is carried by the 3.5-kilobase EcoRI fragment. Transcriptional analysis and 5'-end-mapping experiments were performed. The results obtained demonstrate that the apcEA1B1C gene cluster forms an operon from which segmented transcripts originate, whereas the apcA2 gene behaves as a monocistronic unit. Qualitatively, the same transcripts were identified regardless of the light wavelengths received during cell growth. The deduced amino acid sequences of the apc gene products are very similar to their known homologs of either cyanobacterial or eucaryotic origin. It was interesting, however, that in the apcA1 and apcA2 genes, whose products correspond to alpha-type allophycocyanin subunits, nucleotide sequences were more conserved (67%) than were the deduced amino acid sequences (59%).

Photoreversibility of the Effect of Red and Green Light Pulses on the Accumulation in Darkness of mRNAs Coding for Phycocyanin and Phycoerythrin in Fremyella diplosiphon

DNA fragments encoding a red light-inducible phycocyanin gene and a green light-inducible phycoerythrin gene have been used to investigate the effect of red and green pulses on the accumulation of phycocyanin and phycoerythrin mRNA in subsequent darkness. A red pulse promotes phycocyanin and suppresses phycoerythrin mRNA accumulation while a green pulse has an opposite effect on both transcript levels. The effect of a saturating light pulse is canceled by a subsequently given pulse of the other light quality. For a given mRNA, the positive and negative effects require the same fluence for saturation, whereas to saturate the phycoerythrin mRNA response requires at least twice as much light as to saturate the phycocyanin mRNA response. Calculations of the apparent extinction coefficients for the pigments mediating the light-regulated mRNA increase and decrease are of the order of 2 x 10(4) for phycocyanin mRNA and less than 10(4) for phycoerythrin mRNA. The data are consistent with the hypothesis that the light-induced increase and decrease of a particular phycobiliprotein mRNA is controlled by a single red/green photoreversible photosystem, but that phycoerythrin and phycocyanin mRNA levels are either controlled by two distinct photoreversible systems or that marked differences occur in the chain of events leading from photoperception to gene activation. These system(s) differ from most phytochrome systems in several ways: First, they remain fully on or off depending upon the light quality of the terminal irradiation. Second, they can be completely reversed by light of the appropriate wavelength after several hours of darkness without diminution of the effectiveness of the reversing light pulse. These two features argue against the existence of dark reversion or dark destruction of the biologically active moiety. Third, signal transduction is rapid-measurable mRNA changes occur even during a 10 minute irradiation.

Light Regulation of Peridinin-Chlorophyll a-Protein (PCP) Complexes in the Dinoflagellate, Glenodinium sp. : Use of Anti-Pcp Antibodies to Detect Pcp Gene Products in Cells Grown in Different Light Conditions

As a step toward developing the tools needed to study the molecular bases of light regulation of gene expression in dinoflagellates, light-harvesting peridinin-chlorophyll a-protein (PCP) complexes from Glenodinium sp. were purified and used to generate anti-PCP antibodies. Affinity purified anti-PCP antibodies were isolated from the crude anti-PCP antiserum resulting in improved specificity of immune reactions. The affinity purified anti-PCP antibodies were shown to react strongly and specifically with all major isoforms of PCP complexes in Glenodinium sp. cells, and were used to assess qualitative changes in the levels of PCP gene products in cells grown under different light conditions. Western blot analysis revealed a two- to three-fold increase in detectable PCP apoprotein in low light compared to high light grown cells. In vitro translation reactions supplied with total RNA from high and low light grown Glenodinium sp. cultures also showed an approximate twofold increase in translatable PCP mRNAs in low light grown cells as determined by immunoprecipitation of the primary translation products with affinity purified anti-PCP antibodies. In addition, PCP apoproteins appear to be encoded as larger pre-proteins, since the major immunoprecipitated products from in vitro translation are 23 and 22 kilodaltons, while mature PCP apoproteins are 15.5 kilodaltons. The parallel increases in PCP apoprotein and translatable PCP mRNAs indicate that light regulation of PCP complexes occurs at the level of PCP mRNA abundance.

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.

Transcriptional organization of the phycocyanin subunit gene clusters of the cyanobacterium Anacystis nidulans UTEX 625

The phycocyanin subunit gene cluster is duplicated on the chromosome of the cyanobacterium Anacystis nidulans UTEX 625. The two gene clusters cpcB1A1 (left) and cpcB2A2 (right) are separated by about 2,500 base pairs, and in each cluster the beta-subunit gene is located upstream from the alpha-subunit gene. Filter hybridizations with phycocyanin-specific probes to total RNA detected at least two major transcripts that were 1,300 to 1,400 nucleotides long. Besides these major mRNA species, two minor transcripts of 3,400 and 3,700 nucleotides covering one of the gene clusters and the region between the clusters were found. No additional minor transcripts were found in the intergenic region between the two phycocyanin gene clusters. The lengths of the major mRNAs indicated that the beta- and alpha-subunit genes were cotranscribed. No apparent homologies were found when the DNA sequences located upstream from the proposed ribosome-binding site of the two phycocyanin beta-subunit genes were compared. Northern hybridizations with gene cluster-specific probes from the regions 5' of the beta-subunit genes, as well as S1 nuclease mapping and mRNA primer extension experiments, showed that both gene clusters were transcribed. The minor transcripts were found to initiate upstream from the left gene cluster. Two mRNA 5' ends were mapped upstream from the cpcB1A1 gene cluster, while only one 5' end was mapped in front of the cpcB2A2 gene cluster. All transcripts were present in RNA preparations from cultures grown under high levels of white light as well as under low levels of red light. The level of phycocyanin-specific mRNA, measured as part of the total RNA, was lower under low levels of red light compared with that under high levels of white light. Conserved sequence motifs were found when the promoter region of the cpcB1A1 gene cluster and promoter regions from other cyanobacterial photosynthesis genes were compared. The DNA sequences covering the proposed transcriptional attenuators and transcriptional stop signals contained several potential hairpin structures. One potential hairpin structure was located immediately downstream of the left phycocyanin gene cluster and was concluded to limit the level of transcription for the minor transcripts initiating upstream of the cpcB1A1 gene cluster.

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.