The high light-inducible polypeptides in Synechocystis PCC6803. Expression and function in high light

Temporal dynamics of Prochlorococcus ecotypes in the Atlantic and Pacific oceans

To better understand the temporal and spatial dynamics of Prochlorococcus populations, and how these populations co-vary with the physical environment, we followed monthly changes in the abundance of five ecotypes-two high-light adapted and three low-light adapted-over a 5-year period in coordination with the Bermuda Atlantic Time Series (BATS) and Hawaii Ocean Time-series (HOT) programs. Ecotype abundance displayed weak seasonal fluctuations at HOT and strong seasonal fluctuations at BATS. Furthermore, stable 'layered' depth distributions, where different Prochlorococcus ecotypes reached maximum abundance at different depths, were maintained consistently for 5 years at HOT. Layered distributions were also observed at BATS, although winter deep mixing events disrupted these patterns each year and produced large variations in ecotype abundance. Interestingly, the layered ecotype distributions were regularly reestablished each year after deep mixing subsided at BATS. In addition, Prochlorococcus ecotypes each responded differently to the strong seasonal changes in light, temperature and mixing at BATS, resulting in a reproducible annual succession of ecotype blooms. Patterns of ecotype abundance, in combination with physiological assays of cultured isolates, confirmed that the low-light adapted eNATL could be distinguished from other low-light adapted ecotypes based on its ability to withstand temporary exposure to high-intensity light, a characteristic stress of the surface mixed layer. Finally, total Prochlorococcus and Synechococcus dynamics were compared with similar time series data collected a decade earlier at each location. The two data sets were remarkably similar-testimony to the resilience of these complex dynamic systems on decadal time scales.

Involvement of carotenoids in the synthesis and assembly of protein subunits of photosynthetic reaction centers of Synechocystis sp. PCC 6803

The crtB gene of Synechocystis sp. PCC 6803, encoding phytoene synthase, was inactivated in the Delta crtH mutant to generate a carotenoidless Delta crtH/B double mutant. Delta crtH mutant cells were used because they had better transformability than wild-type cells, most probably due to their adaptation to partial carotenoid deficiency. Cells of the Delta crtH/B mutant were light sensitive and could grow only under light-activated heterotrophic growth conditions in the presence of glucose. Carotenoid deficiency did not significantly affect the cellular content of phycobiliproteins while the chlorophyll content of the mutant cells decreased. The mutant cells exhibited no oxygen-evolving activity, suggesting the absence of photochemically active PSII complexes. This was confirmed by 2D electrophoresis of photosynthetic membrane complexes. Analyses identified only a small amount of a non-functional PSII core complex lacking CP43, while the monomeric and dimeric PSII core complexes were absent. On the other hand, carotenoid deficiency did not prevent formation of the cytochrome b(6)f complex and PSI, which predominantly accumulated in the monomeric form. Radioactive labeling revealed very limited synthesis of inner PSII antennae, CP47 and especially CP43. Thus, carotenoids are indispensable constituents of the photosynthetic apparatus, being essential not only for antioxidative protection but also for the efficient synthesis and accumulation of photosynthetic proteins and especially that of PSII antenna subunits.

The photoactive orange carotenoid protein and photoprotection in cyanobacteria

Photoprotective mechanisms have been evolved by photosynthetic organisms to cope with fluctuating high light conditions. One of these mechanisms downregulates photosynthesis by increasing thermal dissipation of the energy absorbed by the photosystem II antenna. While this process has been well studied in plants, the equivalent process in cyanobacteria was only recently discovered. In this chapter we describe the results leading to its discovery and the more recent advances in the elucidation of this mechanism. The light activation of a soluble carotenoid protein, the orange carotenoid protein (OCP), binding hydroxyechinenone, is the key inducer of this photoprotective mechanism. Light causes structural changes within both the carotenoid and the protein, leading to the conversion of an orange inactive form into a red active form. The activated red form induces an increase of energy dissipation leading to a decrease in the fluorescence of the phycobilisomes, the cyanobacterial antenna, and thus of the energy arriving to the reaction centers. The OCP, which senses light and triggers photoprotection, is a unique example of a photoactive protein containing a carotenoid as the photoresponsive chromophore.

Choreography of the transcriptome, photophysiology, and cell cycle of a minimal photoautotroph, prochlorococcus

The marine cyanobacterium Prochlorococcus MED4 has the smallest genome and cell size of all known photosynthetic organisms. Like all phototrophs at temperate latitudes, it experiences predictable daily variation in available light energy which leads to temporal regulation and partitioning of key cellular processes. To better understand the tempo and choreography of this minimal phototroph, we studied the entire transcriptome of the cell over a simulated daily light-dark cycle, and placed it in the context of diagnostic physiological and cell cycle parameters. All cells in the culture progressed through their cell cycles in synchrony, thus ensuring that our measurements reflected the behavior of individual cells. Ninety percent of the annotated genes were expressed, and 80% had cyclic expression over the diel cycle. For most genes, expression peaked near sunrise or sunset, although more subtle phasing of gene expression was also evident. Periodicities of the transcripts of genes involved in physiological processes such as in cell cycle progression, photosynthesis, and phosphorus metabolism tracked the timing of these activities relative to the light-dark cycle. Furthermore, the transitions between photosynthesis during the day and catabolic consumption of energy reserves at night— metabolic processes that share some of the same enzymes — appear to be tightly choreographed at the level of RNA expression. In-depth investigation of these patterns identified potential regulatory proteins involved in balancing these opposing pathways. Finally, while this analysis has not helped resolve how a cell with so little regulatory capacity, and a ‘deficient’ circadian mechanism, aligns its cell cycle and metabolism so tightly to a light-dark cycle, it does provide us with a valuable framework upon which to build when the Prochlorococcus proteome and metabolome become available.

Horizontal gene transfer and mobile genetic elements in marine systems

The pool of mobile genetic elements (MGE) in microbial communities consists of viruses, plasmids, and associated elements (insertion sequences, transposons, and integrons) that are either self-transmissible or use mobile plasmids and viruses as vehicles for their dissemination. This mobilome facilitates the horizontal transfer of genes that promote the evolution and adaptation of microbial communities. Efforts to characterize MGEs from microbial populations resident in a variety of ecological habitats have revealed a surprisingly novel and seemingly untapped biodiversity. To better understand the impact of horizontal gene transfer (HGT), as well as the agents that promote HGT in marine ecosystems and to determine whether or not environmental parameters can effect the composition and structure of the mobilome in marine microbial communities, information on the distribution, diversity, and ecological traits of the marine mobilome is presented. In this chapter we discuss recent insights gained from different methodological approaches used to characterize the biodiversity and ecology of MGE in marine environments and their contributions to HGT. In addition, we present case studies that highlight specific HGT examples in coastal, open-ocean, and deep-sea marine ecosystems.

Auxiliary proteins involved in the assembly and sustenance of photosystem II

Chloroplast proteins that regulate the biogenesis, performance and acclimation of the photosynthetic protein complexes are currently under intense research. Dozens, possibly even hundreds, of such proteins in the stroma, thylakoid membrane and the lumen assist the biogenesis and constant repair of the water splitting photosystem (PS) II complex. During the repair cycle, assistance is required at several levels including the degradation of photodamaged D1 protein, de novo synthesis, membrane insertion, folding of the nascent protein chains and the reassembly of released protein subunits and different co-factors into PSII in order to guarantee the maintenance of the PSII function. Here we review the present knowledge of the auxiliary proteins, which have been reported to be involved in the biogenesis and maintenance of PSII.

The small CAB-like proteins of Synechocystis sp. PCC 6803 bind chlorophyll. In vitro pigment reconstitution studies on one-helix light-harvesting-like proteins

The large family of light-harvesting-like proteins contains members with one to four membrane spanning helices with significant homology to the chlorophyll a/b-binding antenna proteins of plants. From structural as well as evolutionary perspective, it is likely that the members of this family bind chlorophylls and carotenoids. However, undisputable evidence is still lacking. The cyanobacterial small CAB-like proteins (SCPs) are one-helix proteins with compelling similarity to the first and third transmembrane helix of LHCII (LHCIIb) including the chlorophyll-binding motifs. They have been proposed to act as chlorophyll-carrier proteins. Here, we analyze the in vivo absorption spectra of single scp deletion mutants in Synechocystis sp. PCC 6803 and compare the in vitro pigment binding ability of the SCP pairs ScpC/D and ScpB/E with the one of LHCII and a synthetic peptide containing the chlorophyll-binding motif (Eggink LL, Hoober JK (2000) J Biol Chem 275:9087-9090). We demonstrate that deletion of scpB alters the pigmentation in the cyanobacterial cell. Furthermore, we are able to show that chlorophylls and carotenoids interact in vitro with the pairs of ScpC/D and ScpB/E, demonstrated by fluorescence resonance energy transfer and circular dichroism.

The high light-inducible polypeptides stabilize trimeric photosystem I complex under high light conditions in Synechocystis PCC 6803

The high light-inducible polypeptides (HLIPs) are critical for survival under high light (HL) conditions in Synechocystis PCC 6803. In this article, we determined the localization of all four HLIPs in thylakoid protein complexes and examined effects of hli gene deletion on the photosynthetic protein complexes. The HliA and HliB proteins were found to be associated with trimeric photosystem I (PSI) complexes and the Slr1128 protein, whereas HliC was associated with PsaL and TMP14. The HliD was associated with partially dissociated PSI complexes. The PSI activities of the hli mutants were 3- to 4-fold lower than that of the wild type. The hli single mutants lost more than 30% of the PSI trimers after they were incubated in intermediate HL for 12 h. The reduction of PSI trimers were further augmented in these cells by the increase of light intensity. The quadruple hli deletion mutant contained less than one-half of PSI trimers following 12-h incubation in intermediate HL. It lost essentially all of the PSI trimers upon exposure to HL for 12 h. Furthermore, a mutant lacking both PSI trimers and Slr1128 showed growth defects similar to that of the quadruple hli deletion mutant under different light conditions. These results suggest that the HLIPs stabilize PSI trimers, interact with Slr1128, and protect cells under HL conditions.

Association of small CAB-like proteins (SCPs) of Synechocystis sp. PCC 6803 with Photosystem II

The cyanobacterial small CAB-like proteins (SCPs) are one-helix proteins with compelling similarity to the first and third transmembrane helix of proteins belonging to the CAB family of light-harvesting complex proteins in plants. The SCP proteins are transiently expressed at high light intensity and other stress conditions but their exact function remains largely unknown. Recently we showed association of ScpD with light-stressed, monomeric Photosystem II in Synechocystis sp. PCC 6803 (Yao et al. J Biol Chem 282:267-276, 2007). Here we show that ScpB associates with Photosystem II at normal growth conditions. Moreover, upon introduction of a construct into Synechocystis so that ScpB is expressed continuously under normal growth conditions, ScpE was detected under non-stressed conditions as well, and was copurified with tagged ScpB and Photosystem II. We also report on a one-helix protein, Slr1544, that is somewhat similar to the SCPs and whose gene is cotranscribed with that of ScpD; Slr1544 is another member of the extended light-harvesting-like (Lil) protein family, and we propose to name it LilA.

A novel two domain-fusion protein in cyanobacteria with similarity to the CAB/ELIP/HLIP superfamily: evolutionary implications and regulation

Vascular plants contain abundant, light-harvesting complexes in the thylakoid membrane that are non-covalently associated with chlorophylls and carotenoids. These light-harvesting chlorophyll a/b binding (LHC) proteins are members of an extended CAB/ELIP/HLIP superfamily of distantly related polypeptides, which have between one and four transmembrane helices (TMH). This superfamily includes the single TMH, high-light-inducible proteins (Hlips), found in cyanobacteria that are induced by various stress conditions, including high light, and are considered ancestral to the LHC proteins. The roles of, and evolutionary relationships between, these superfamily members are of particular interest, since they function in both light harvesting and photoprotection and may have evolved through tandem gene duplication and fusion events. We have investigated the Hlips (hli gene family) in the thermophilic unicellular cyanobacterium Synechococcus OS-B'. The five hli genes present on the genome of Synechococcus OS-B' are relatively similar, but transcript analyses indicate that there are different patterns of transcript accumulation when the cells are exposed to various growth conditions, suggesting that different Hlips may have specific functions. Hlip5 has an additional TMH at the N-terminus as a result of a novel fusion event. This additional TMH is very similar to a conserved hypothetical, single membrane-spanning polypeptide present in most cyanobacteria. The evolutionary significance of these results is discussed.

Small Cab-like proteins retard degradation of photosystem II-associated chlorophyll in Synechocystis sp. PCC 6803: kinetic analysis of pigment labeling with 15N and 13C

Isotope (Na(15)NO(3), ((15)NH(4))SO(4) or [(13)C]glucose) labeling was used to analyze chlorophyll synthesis and degradation rates in a set of Synechocystis mutants that lacked single or multiple small Cab-like proteins (SCPs), as well as photosystem I or II. When all five small Cab-like proteins were inactivated in the wild-type background, chlorophyll stability was not affected unless the scpABCDE(-) strain was grown at a moderately high light intensity of 100-300 micromol photons m(-2) s(-1). However, the half-life time of chlorophyll was 5-fold shorter in the photosystem I-less/scpABCDE(-) strain than in the photosystem I-less strain even when grown at low light intensity (~3 micromol photons m(-2) s(-1)) (32 +/- 5 and 161 +/- 25 h, respectively). In other photosystem I-less mutants that lacked one to four of the scp genes the chlorophyll lifetime was in between these two values, with the chlorophyll lifetime generally decreasing with an increasing number of inactivated scps. In contrast, the chlorophyll biosynthesis rate was only marginally affected by inactivation of scps except when all five scp genes were deleted. Small Cab-like protein deficiency did not significantly affect photoinhibition or turnover of photosystem II-associated beta-carotene. It is concluded that SCPs do not alter the stability of functional photosystem II complexes but retard the degradation of photosystem II-associated chlorophyll, consistent with the proposed involvement of SCPs in photosystem II re-assembly or/and repair processes by temporarily binding chlorophyll while photosystem II protein components are being replaced.

Cadmium triggers an integrated reprogramming of the metabolism of Synechocystis PCC6803, under the control of the Slr1738 regulator

Background

Cadmium is a persistent pollutant that threatens most biological organisms, including cyanobacteria that support a large part of the biosphere. Using a multifaceted approach, we have investigated the global responses to Cd and other relevant stresses (H₂O₂ and Fe) in the model cyanobacterium Synechocystis PCC6803.

Results

We found that cells respond to the Cd stress in a two main temporal phases process. In the "early" phase cells mainly limit Cd entry through the negative and positive regulation of numerous genes operating in metal uptake and export, respectively. As time proceeds, the number of responsive genes increases. In this "massive" phase, Cd downregulates most genes operating in (i) photosynthesis (PS) that normally provides ATP and NADPH; (ii) assimilation of carbon, nitrogen and sulfur that requires ATP and NAD(P)H; and (iii) translation machinery, a major consumer of ATP and nutrients. Simultaneously, many genes are upregulated, such as those involved in Fe acquisition, stress tolerance, and protein degradation (crucial to nutrients recycling). The most striking common effect of Cd and H₂O₂ is the disturbance of both light tolerance and Fe homeostasis, which appeared to be interdependent. Our results indicate that cells challenged with H₂O₂ or Cd use different strategies for the same purpose of supplying Fe atoms to Fe-requiring metalloenzymes and the SUF machinery, which synthesizes or repairs Fe-S centers. Cd-stressed cells preferentially breakdown their Fe-rich PS machinery, whereas H₂O₂-challenged cells preferentially accelerate the intake of Fe atoms from the medium.

Conclusion

We view the responses to Cd as an integrated "Yin Yang" reprogramming of the whole metabolism, we found to be controlled by the Slr1738 regulator. As the Yin process, the ATP- and nutrients-sparing downregulation of anabolism limits the poisoning incorporation of Cd into metalloenzymes. As the compensatory Yang process, the PS breakdown liberates nutrient assimilates for the synthesis of Cd-tolerance proteins, among which we found the Slr0946 arsenate reductase enzyme.

Code and context: Prochlorococcus as a model for cross-scale biology

Prochlorococcus is a simple cyanobacterium that is abundant throughout large regions of the oceans, and has become a useful model for studying the nature and regulation of biological diversity across all scales of complexity. Recent work has revealed that environmental factors such as light, nutrients and predation influence diversity in different ways, changing our image of the structure and dynamics of the global Prochlorococcus population. Advances in metagenomics, transcription profiling and global ecosystem modeling promise to deliver an even greater understanding of this system and further demonstrate the power of cross-scale systems biology.

The PsbP-like protein (sll1418) of Synechocystis sp. PCC 6803 stabilises the donor side of Photosystem II

The PsbP-like protein of the cyanobacterium Synechocystis sp. PCC 6803 is a peripheral component of Photosystem II, located at the lumenal side of the thylakoid membrane. Removal of this protein leads to decreased competitive potential of a PsbP-like deletion mutant when grown in a mixture with wild-type cells. Flash-induced oxygen evolution traces of the mutant show a higher probability of misses, correlated with increased amplitudes of the S-states decay in the dark. Thermoluminescence emission traces demonstrate a changed charge recombination pattern in the mutant, the S(3)Q(B)(-) couple becoming the major species instead of the S(2)Q(B)(-). Our data suggest a possible role of the PsbP-like protein in stabilisation of the charge separation in Photosystem II of cyanobacteria through interaction with the Mn cluster.

Responses of a thermophilic Synechococcus isolate from the microbial mat of Octopus Spring to light

Thermophilic cyanobacteria of the genus Synechococcus are major contributors to photosynthetic carbon fixation in the photic zone of microbial mats in Octopus Spring, Yellowstone National Park. Synechococcus OS-B' was characterized with regard to the ability to acclimate to a range of different light irradiances; it grows well at 25 to 200 micromol photons m(-2) s(-1) but dies when the irradiance is increased to 400 micromol photons m(-2) s(-1). At 200 micromol photons m(-2) s(-1) (high light [HL]), we noted several responses that had previously been associated with HL acclimation of cyanobacteria, including cell bleaching, reduced levels of phycobilisomes and chlorophyll, and elevated levels of a specific carotenoid. Synechococcus OS-B' synthesizes the carotenoids zeaxanthin and beta,beta-carotene and a novel myxol-anhydrohexoside. Interestingly, 77-K fluorescence emission spectra suggest that Synechococcus OS-B' accumulates very small amounts of photosystem II relative to that of photosystem I. This ratio further decreased at higher growth irradiances, which may reflect potential photodamage following exposure to HL. We also noted that HL caused reduced levels of transcripts encoding phycobilisome components, particularly that for CpcH, a 20.5-kDa rod linker polypeptide. There was enhanced transcript abundance of genes encoding terminal oxidases, superoxide dismutase, tocopherol cyclase, and phytoene desaturase. Genes encoding the photosystem II D1:1 and D1:2 isoforms (psbAI and psbAII/psbAIII, respectively) were also regulated according to the light regimen. The results are discussed in the context of how Synechococcus OS-B' may cope with high light irradiances in the high-temperature environment of the microbial mat.

Genomic and structural analysis of Syn9, a cyanophage infecting marineProchlorococcusandSynechococcus

Cyanobacteriophage Syn9 is a large, contractile-tailed bacteriophage infecting the widespread, numerically dominant marine cyanobacteria of the genera Prochlorococcus and Synechococcus. Its 177,300 bp genome sequence encodes 226 putative proteins and six tRNAs. Experimental and computational analyses identified genes likely involved in virion formation, nucleotide synthesis, and DNA replication and repair. Syn9 shows significant mosaicism when compared with related cyanophages S-PM2, P-SSM2 and P-SSM4, although shared genes show strong purifying selection and evidence for large population sizes relative to other phages. Related to coliphage T4 - which shares 19% of Syn9's genes - Syn9 shows evidence for different patterns of DNA replication and uses homologous proteins to assemble capsids with a different overall structure that shares topology with phage SPO1 and herpes virus. Noteworthy bacteria-related sequences in the Syn9 genome potentially encode subunits of the photosynthetic reaction centre, electron transport proteins, three pentose pathway enzymes and two tryptophan halogenases. These genes suggest that Syn9 is well adapted to the physiology of its photosynthetic hosts and may affect the evolution of these sequences within marine cyanobacteria.

The response regulator RpaB binds the high light regulatory 1 sequence upstream of the high-light-inducible hliB gene from the cyanobacterium Synechocystis PCC 6803

Cyanobacteria, like other photosynthetic organisms, respond to the potentially damaging effects of high-intensity light by regulating the expression of a variety of stress-responsive genes through regulatory mechanisms that remain poorly understood. The high light regulatory 1 (HLR1) sequence can be found upstream of many genes regulated by high-light (HL) stress in cyanobacteria. In this study, we identify the factor that binds the HLR1 upstream of the HL-inducible hliB gene in the cyanobacterium Synechocystis PCC 6803 as the RpaB (Slr0947) response regulator.

Light-induced energy dissipation in iron-starved cyanobacteria: roles of OCP and IsiA proteins

In response to iron deficiency, cyanobacteria synthesize the iron stress-induced chlorophyll binding protein IsiA. This protein protects cyanobacterial cells against iron stress. It has been proposed that the protective role of IsiA is related to a blue light-induced nonphotochemical fluorescence quenching (NPQ) mechanism. In iron-replete cyanobacterial cell cultures, strong blue light is known to induce a mechanism that dissipates excess absorbed energy in the phycobilisome, the extramembranal antenna of cyanobacteria. In this photoprotective mechanism, the soluble Orange Carotenoid Protein (OCP) plays an essential role. Here, we demonstrate that in iron-starved cells, blue light is unable to quench fluorescence in the absence of the phycobilisomes or the OCP. By contrast, the absence of IsiA does not affect the induction of fluorescence quenching or its recovery. We conclude that in cyanobacteria grown under iron starvation conditions, the blue light-induced nonphotochemical quenching involves the phycobilisome OCP-related energy dissipation mechanism and not IsiA. IsiA, however, does seem to protect the cells from the stress generated by iron starvation, initially by increasing the size of the photosystem I antenna. Subsequently, the IsiA converts the excess energy absorbed by the phycobilisomes into heat through a mechanism different from the dynamic and reversible light-induced NPQ processes.

Marine cyanophages and light

In contrast to the phages of heterotrophic hosts, light can play a key role in all aspects of the life cycle of phages infecting ecologically important marine unicellular cyanobacteria of the genera Synechococcus and Prochlorococcus. Phage adsorption, replication, modulation of the host cell metabolism, and survival in the environment following lysis, all exhibit light-dependent components. The analysis of cyanophage genomes has revealed the acquisition of key photosynthetic genes during the course of evolution, such as those encoding central components of the light harvesting apparatus. These discoveries are beginning to reveal novel features of the interactions between parasite and host that shape the biology of both.

Localization of the small CAB-like proteins in photosystem II

The cyanobacterial small CAB-like proteins (SCPs) consist of one-helix proteins that resemble transmembrane regions of the light-harvesting proteins of plants. To determine whether these proteins are associated with protein complexes in the thylakoid membrane, an abundant member of the SCP family, ScpD, was marked with a His tag, and proteins co-isolating with His-tagged ScpD were identified. These proteins included the major Photosystem (PS) II components as well as FtsH, which is involved in degradation of the PSII complex. To ascertain specific interaction between ScpD and the PSII complex, the His-tagged protein fraction was subjected to two-dimensional blue native/SDS-PAGE. Again, PSII components were co-isolated with ScpD-His, and ScpD-His was found to interact most strongly with CP47. ScpD association was most prominent with the monomeric form of PSII, suggesting ScpD association with PSII that is repaired. Using antibodies that recognize both ScpC and ScpD, we found the ScpC protein, which is very similar in primary structure to ScpD, to also co-isolate with the PSII complex. In contrast, ScpE did not co-isolate with a major protein complex in thylakoids. A fourth member of the SCP family, ScpB, could not be immunodetected, but was found by mass spectrometry in samples co-isolating with ScpD-His. Therefore, ScpB may be associated with ScpD as well. No association between SCPs and PSI could be demonstrated. On the basis of these and other data presented, we suggest that members of the SCP family can associate with damaged PSII and can serve as a temporary pigment reservoir while PSII components are being replaced.

Stimulation of pigment accumulation in Anabaena azollae strains: effect of light intensity and sugars

The influence of high light intensity on the growth and pigment accumulating ability of Anabaena azollae was investigated. A. azollae responded positively to high light intensity (6 klx) and was further evaluated at higher intensities (10 and 15 klx), in the presence of glucose, sucrose and jaggery +/- DCMU. Significant enhancement in phycobiliproteins and carotenoids was observed in the sugar supplemented cultures at high light intensities. SDS-PAGE profiles of whole cell proteins revealed the presence of unique bands in such treatments. Sucrose supplementation induced a 30-90 % increase in carotenoids, phycocyanin and phycoerythrin content at 10 klx. Molecular analysis of the stimulatory and interactive role of sugars on pigment enhancement at high light intensity may aid in better exploitation of cyanobacteria as a source of pigments.

Global gene expression of Prochlorococcus ecotypes in response to changes in nitrogen availability

Nitrogen (N) often limits biological productivity in the oceanic gyres where Prochlorococcus is the most abundant photosynthetic organism. The Prochlorococcus community is composed of strains, such as MED4 and MIT9313, that have different N utilization capabilities and that belong to ecotypes with different depth distributions. An interstrain comparison of how Prochlorococcus responds to changes in ambient nitrogen is thus central to understanding its ecology. We quantified changes in MED4 and MIT9313 global mRNA expression, chlorophyll fluorescence, and photosystem II photochemical efficiency (Fv/Fm) along a time series of increasing N starvation. In addition, the global expression of both strains growing in ammonium-replete medium was compared to expression during growth on alternative N sources. There were interstrain similarities in N regulation such as the activation of a putative NtcA regulon during N stress. There were also important differences between the strains such as in the expression patterns of carbon metabolism genes, suggesting that the two strains integrate N and C metabolism in fundamentally different ways.

Genome-wide analysis of light sensing in Prochlorococcus

Prochlorococcus MED4 has, with a total of only 1,716 annotated protein-coding genes, the most compact genome of a free-living photoautotroph. Although light quality and quantity play an important role in regulating the growth rate of this organism in its natural habitat, the majority of known light-sensing proteins are absent from its genome. To explore the potential for light sensing in this phototroph, we measured its global gene expression pattern in response to different light qualities and quantities by using high-density Affymetrix microarrays. Though seven different conditions were tested, only blue light elicited a strong response. In addition, hierarchical clustering revealed that the responses to high white light and blue light were very similar and different from that of the lower-intensity white light, suggesting that the actual sensing of high light is mediated via a blue-light receptor. Bacterial cryptochromes seem to be good candidates for the blue-light sensors. The existence of a signaling pathway for the redox state of the photosynthetic electron transport chain was suggested by the presence of genes that responded similarly to red and blue light as well as genes that responded to the addition of DCMU [3-(3,4-dichlorophenyl)-1,1-N-N'-dimethylurea], a specific inhibitor of photosystem II-mediated electron transport.

Differential expression and localization of early light-induced proteins in Arabidopsis

The early light-induced proteins (Elips) in higher plants are nuclear-encoded, light stress-induced proteins located in thylakoid membranes and related to light-harvesting chlorophyll (LHC) a/b-binding proteins. A photoprotective function was proposed for Elips. Here we showed that after 2 h exposure of Arabidopsis (Arabidopsis thaliana) leaves to light stress Elip1 and Elip2 coisolate equally with monomeric (mLhcb) and trimeric (tLhcb) populations of the major LHC from photosystem II (PSII) as based on the Elip:Lhcb protein ratio. A longer exposure to light stress resulted in increased amounts of Elips in tLhcb as compared to mLhcb, due to a reduction of tLhcb amounts. We demonstrated further that the expression of Elip1 and Elip2 transcripts was differentially regulated in green leaves exposed to light stress. The accumulation of Elip1 transcripts and proteins increased almost linearly with increasing light intensities and correlated with the degree of photoinactivation and photodamage of PSII reaction centers. A stepwise accumulation of Elip2 was induced when 40% of PSII reaction centers became photodamaged. The differential expression of Elip1 and Elip2 occurred also in light stress-preadapted or senescent leaves exposed to light stress but there was a lack of correlation between transcript and protein accumulation. Also in this system the accumulation of Elip1 but not Elip2 correlated with the degree of PSII photodamage. Based on pigment analysis, measurements of PSII activity, and assays of the oxidation status of proteins we propose that the discrepancy between amounts of Elip transcripts and proteins in light stress-preadapted or senescent leaves is related to a presence of photoprotective anthocyanins or to lower chlorophyll availability, respectively.

Cyanobacterial small chlorophyll-binding protein ScpD (HliB) is located on the periphery of photosystem II in the vicinity of PsbH and CP47 subunits

Cyanobacteria contain several genes coding for small one-helix proteins called SCPs or HLIPs with significant sequence similarity to chlorophyll a/b-binding proteins. To localize one of these proteins, ScpD, in the cells of the cyanobacterium Synechocystis sp. PCC 6803, we constructed several mutants in which ScpD was expressed as a His-tagged protein (ScpDHis). Using two-dimensional native-SDS electrophoresis of thylakoid membranes or isolated Photosystem II (PSII), we determined that after high-light treatment most of the ScpDHis protein in a cell is associated with PSII. The ScpDHis protein was present in both monomeric and dimeric PSII core complexes and also in the core subcomplex lacking CP43. However, the association with PSII was abolished in the mutant lacking the PSII subunit PsbH. In a PSII mutant lacking cytochrome b(559), which does not accumulate PSII, ScpDHis is associated with CP47. The interaction of ScpDHis with PsbH and CP47 was further confirmed by electron microscopy of PSII labeled with Ni-NTA Nanogold. Single particle image analysis identified the location of the labeled ScpDHis at the periphery of the PSII core complex in the vicinity of the PsbH and CP47. Because of the fact that ScpDHis did not form any large structures bound to PSII and because of its accumulation in PSII subcomplexes containing CP47 and PsbH we suggest that ScpD is involved in a process of PSII assembly/repair during the turnover of pigment-binding proteins, particularly CP47.

Suppression of the lethality of high light to a quadruple HLI mutant by the inactivation of the regulatory protein PfsR in Synechocystis PCC 6803

A regulatory gene, designated pfsR (photosynthesis, Fe homeostasis and stress-response regulator), was discovered by a genetic screen in Synechocystis PCC 6803. Deletion of the gene from a high light-sensitive strain lacking four hli genes (4Xhli) restored viability to the parental strain under high light conditions. The quintuple mutant pfsR-/4Xhli retained photosystem-II and oxygen evolution capacity at levels similar to the wild-type levels under high light conditions. The transcripts of the two bfr genes (encoding bacterioferritin) were found to be constitutively up-regulated, whereas the transcripts of ho1 gene (encoding a heme oxygenase) were greatly down-regulated in high light upon deletion of pfsR. Under intermediate high intensity light, the pfsR deletion strains accumulated carotenoids and chlorophyll a to a significantly higher level than their corresponding parental strains. An exacerbated, transient increase in oxygen evolution during the early hours of high light acclimation and a somewhat increased steady-state level of photosystem-II-mediated oxygen evolution observed in the 4Xhli strain were brought back to the wild-type levels upon deletion of pfsR from the strain. The pfsR deletion mutants were found to be less sensitive to iron limitation under low light conditions and to suffer less lipid peroxidation following exposure to high light. Therefore, inactivation of PfsR resulted in tighter control of iron availability, which in turn reduced oxidative stress during photosynthesis in high light. These studies have revealed a critical role of PfsR in regulation of iron homeostasis and stress response.

Negative control of the high light-inducible hliA gene and implications for the activities of the NblS sensor kinase in the cyanobacterium Synechococcus elongatus strain PCC 7942

The hliA gene of the cyanobacterium Synechococcus elongatus PCC 7942 is known to be upregulated by high-intensity light through the activity of the NblS sensor kinase. In this work it was found that, within the hliA upstream region, changes to the sequence around -30 to -25 (relative to the transcriptional start site) resulted in elevated hliA expression, implicating this region in negative regulation of the gene. Electrophoretic mobility shift assays performed were consistent with a protein binding this region that acts to keep the gene off in lower light. A reduction in gene dosage of nblS in vivo resulted in enhanced hliA expression, suggesting that negative control of hliA is mediated through NblS. An extended version of the high light regulatory 1 (HLR1) motif (previously described in Synechocystis PCC 6803) was identified within the sequence surrounding -30 to -25 of hliA. The extended HLR1 sequence was found upstream of other NblS-controlled genes from S. elongatus and Synechocystis PCC 6803 and upstream of hli genes from a variety of cyanobacterial and related genomes. These results point to the evolutionary conservation of the HLR1 element and its importance in NblS-mediated signaling and yield new insight into NblS-mediated control of gene expression.

Winter acclimation of PsbS and related proteins in the evergreen Arctostaphylos uva-ursi as influenced by altitude and light environment

The evergreen groundcover bearberry (Arctostaphylos uva-ursi [L.] Sprengel) was characterized over two successive years (2002-2004) from both sun-exposed and shaded sites at a montane ponderosa pine and subalpine forest community of 1900- and 2800-m-high altitudes, respectively. During summer, photosynthetic capacities and pre-dawn photosystem II (PSII) efficiency were similarly high in all four populations, and in winter, only the sun-exposed and shaded populations at 2800 m exhibited complete down-regulation of photosynthetic oxygen evolution capacity and consistent sustained down-regulation of PSII efficiency. This photosynthetic down-regulation at high altitude involved a substantial decrease in PSII components [pheophytin, D1 protein, oxygen evolving complex ([OEC)], a strong up-regulation of several anti-early-light-inducible protein (Elip)- and anti-high-light-inducible protein (Hlip)-reactive bands and a warm-sustained retention of zeaxanthin and antheraxanthin (Z + A). PsbS, the protein modulating the rapid engagement and disengagement of Z +A in energy dissipation, exhibited its most pronounced winter increases in the shade at 1900 m, and thus apparently assumes a greater role in providing rapidly reversible zeaxanthin-dependent photoprotection during winter when light becomes excessive in the shaded population, which remains photosynthetically active. It is attractive to hypothesize that PsbS relatives (Elips/Hlips) may be involved in sustained zeaxanthin-dependent photoprotection under the more extreme winter conditions at 2800 m.

Genomic islands and the ecology and evolution of Prochlorococcus

Prochlorococcus ecotypes are a useful system for exploring the origin and function of diversity among closely related microbes. The genetic variability between phenotypically distinct strains that differ by less that 1% in 16S ribosomal RNA sequences occurs mostly in genomic islands. Island genes appear to have been acquired in part by phage-mediated lateral gene transfer, and some are differentially expressed under light and nutrient stress. Furthermore, genome fragments directly recovered from ocean ecosystems indicate that these islands are variable among cooccurring Prochlorococcus cells. Genomic islands in this free-living photoautotroph share features with pathogenicity islands of parasitic bacteria, suggesting a general mechanism for niche differentiation in microbial species.

Action spectrum for expression of the high intensity light-inducible Lhc-like gene Lhl4 in the green alga Chlamydomonas reinhardtii

Lhl4 encodes a distant relative of light-harvesting Chl-a/b proteins in the green alga Chlamydomonas reinhardtii. Lhl4 mRNA markedly accumulated within 30 min after illumination and in proportion to the light intensity up to a fluence rate much higher than that required for photosynthesis. The high intensity light (HL)-induced accumulation of Lhl4 mRNA required continuous illumination, and the mRNA level rapidly decreased when the cells were placed in the dark. HL only slightly stabilized the mRNA, suggesting that the HL-induced expression of the Lhl4 gene is primarily regulated at the level of transcription. Blue light was more effective for inducing Lhl4 gene expression than green or red light, and far-red light had no effect. The action spectrum for Lhl4 gene expression was examined at wavelengths between 325 and 775 nm using the Okazaki Large Spectrograph. The obtained spectrum showed a distinct peak in the blue region (450 nm) and a shoulder in the UV-A region (375 nm). The curve in the spectrum rose steeply in the short wavelength UV region. In addition, we observed two minor peaks in the green (575 nm) and the red (675 nm) regions. The action spectrum suggests that a blue/UV-A light photoreceptor with a flavin-based chromophore participates in the HL response of Lhl4 gene expression. However, the hypersensitivity to near UV-B light suggests the involvement of an unidentified UV light perception system in the expression of the Lhl4 gene.

Winter down-regulation of intrinsic photosynthetic capacity coupled with up-regulation of Elip-like proteins and persistent energy dissipation in a subalpine forest

Overwintering, sun-exposed and photosynthetically inactive evergreens require powerful photoprotection. The goal of this study was to seasonally characterize photosynthesis and key proteins/components involved in electron transport and photoprotection. Maximal photosystem II (PSII) efficiency and photosynthetic capacity, amounts of zeaxanthin (Z), antheraxanthin (A), pheophytin and proteins (oxygen-evolving 33 kDa protein (OEC), PSII core protein D1 and subunit S (PsbS) protein, and members of the early light-inducible protein (Elip) family) were assessed in five conifer species at high altitude and in ponderosa pine (Pinus ponderosa) at moderate altitude during summer and winter. Relative to summer, winter down-regulation of photosynthetic capacity and loss of PSII efficiency at the high-altitude sites were paralleled by decreases in OEC, D1, and pheophytin; massive nocturnal retention of (Z + A) and up-regulation of two to four proteins cross-reactive with anti-Elip antibodies; and no change in PsbS amount. By contrast, ponderosa pine at moderate altitude exhibited no down-regulation of photosynthetic capacity, smaller depressions in PSII efficiency, and less up-regulation of Elip family members. These results support a function for members of the Elip family in the acclimation of sun-exposed needles that down-regulate photosynthesis during winter. A possible role in sustained photoprotection is considered.

Photoprotection in an ecological context: the remarkable complexity of thermal energy dissipation

This review places photoprotection into the context of ecology and species diversity. The focus is on photoprotection via the safe removal - as thermal energy - of excess solar energy absorbed by the light collecting system, which counteracts the formation of reactive oxygen species. An update on the surprisingly complex, multiple variations of thermal energy dissipation is presented, placing these different forms into ecological and genetic contexts. Zeaxanthin-facilitated, flexible thermal dissipation associated with the PsbS protein and controlled by the trans-thylakoid pH gradient apparently occurs ubiquitously in plants, and can become sustained (and thus less flexible) at low temperatures. Long-lived, slow-growing plants with low intrinsic capacities for photosynthesis have greater capacities for this flexible dissipation than short-lived, fast-growing species. Furthermore, potent, but inflexible (zeaxanthin-facilitated) thermal dissipation, prominent in evergreen species under prolonged environmental stress, is characterized with respect to the involvement of photosystem II core rearrangement and/or degradation as well as the absence of control by trans-thylakoid pH and, possibly, PsbS. A role of PsbS-related proteins in photoprotection is discussed.

Tocopherols protect Synechocystis sp. strain PCC 6803 from lipid peroxidation

Tocopherols (vitamin E) are lipid-soluble antioxidants synthesized only by photosynthetic eukaryotes and some cyanobacteria, and have been assumed to play important roles in protecting photosynthetic membranes from oxidative stress. To test this hypothesis, tocopherol-deficient mutants of Synechocystis sp. strain PCC 6803 (slr1736 and slr1737 mutants) were challenged with a series of reactive oxygen species-generating and lipid peroxidation-inducing chemicals in combination with high-light (HL) intensity stress. The tocopherol-deficient mutants and wild type were indistinguishable in their growth responses to HL in the presence and absence of superoxide and singlet oxygen-generating chemicals. However, the mutants showed enhanced sensitivity to linoleic or linolenic acid treatments in combination with HL, consistent with tocopherols playing a crucial role in protecting Synechocystis sp. strain PCC 6803 cells from lipid peroxidation. The tocopherol-deficient mutants were also more susceptible to HL treatment in the presence of sublethal levels of norflurazon, an inhibitor of carotenoid synthesis, suggesting carotenoids and tocopherols functionally interact or have complementary or overlapping roles in protecting Synechocystis sp. strain PCC 6803 from lipid peroxidation and HL stress.

The genome of S-PM2, a "photosynthetic" T4-type bacteriophage that infects marine Synechococcus strains

Bacteriophage S-PM2 infects several strains of the abundant and ecologically important marine cyanobacterium Synechococcus. A large lytic phage with an isometric icosahedral head, S-PM2 has a contractile tail and by this criterion is classified as a myovirus (1). The linear, circularly permuted, 196,280-bp double-stranded DNA genome of S-PM2 contains 37.8% G+C residues. It encodes 239 open reading frames (ORFs) and 25 tRNAs. Of these ORFs, 19 appear to encode proteins associated with the cell envelope, including a putative S-layer-associated protein. Twenty additional S-PM2 ORFs have homologues in the genomes of their cyanobacterial hosts. There is a group I self-splicing intron within the gene encoding the D1 protein. A total of 40 ORFs, organized into discrete clusters, encode homologues of T4 proteins involved in virion morphogenesis, nucleotide metabolism, gene regulation, and DNA replication and repair. The S-PM2 genome encodes a few surprisingly large (e.g., 3,779 amino acids) ORFs of unknown function. Our analysis of the S-PM2 genome suggests that many of the unknown S-PM2 functions may be involved in the adaptation of the metabolism of the host cell to the requirements of phage infection. This hypothesis originates from the identification of multiple phage-mediated modifications of the host's photosynthetic apparatus that appear to be essential for maintaining energy production during the lytic cycle.

Red and far-red light alter the transcript profile in the cyanobacterium Synechocystis sp. PCC 6803: impact of cyanobacterial phytochromes

Cyanobacteria possess genes encoding phytochrome-related proteins. We used a DNA microarray approach to evaluate the impact of the phytochromes Cph1 and Cph2 on red light (R)- and far-red light (FR)-dependent gene expression in the unicellular cyanobacterium Synechocystis sp. PCC 6803. In cells of wild-type and phytochrome mutants, one-fourth of all 3165 annotated putative protein encoding genes was light-responsive. R predominantly enhanced the expression of genes involved in transcription, translation, and photosynthesis, whereas FR upregulated the transcript level of genes known to be inducible by stress. The absence of Cph1 and/or Cph2 altered the light-dependent expression of about 20 genes. Hence, receptor(s) different from the two phytochromes are supposed to trigger the global R/FR alterations of the expression profile.

High-intensity-light-dependent and transient expression of new genes encoding distant relatives of light-harvesting chlorophyll-a/b proteins in Chlamydomonas reinhardtii

We identified four Lhc-like genes (Lhl) encoding proteins that are distant relatives of light-harvesting chlorophyll a/b-binding (LHC) proteins in the green alga Chlamydomonas reinhardtii. Their mRNA levels after transfer from low-intensity light to high-intensity light (HL) were examined and compared with those of Lhc genes encoding LHC proteins of PSII. The transfer caused a decrease in the mRNA level of Lhl3, a homolog of Arabidopsis thaliana Lil3, within 2 h, followed by gradual restoration depending on the intensity of HL. The response was similar to that of Lhc genes. In contrast, the mRNA levels of Lhl1, Lhl2 and Lhl4 significantly increased, reached a maximum within 1 h after the transfer and then rapidly returned to a low level. The intensity of HL little influenced the response of these genes. While the Lhl1 and Lhl2 proteins were homologs of early light-inducible protein (ELIP) and high-light-inducible protein (HLIP), respectively, Lhl4 encoded a novel protein. The HL-induced expression of Lhl4 was most prominent among the genes tested. Studies using various inhibitors indicate that the HL response is not mediated by the redox state of plastoquinone pool or reactive oxygen species, but required de novo protein synthesis in the cytosol.

Transfer of photosynthesis genes to and from Prochlorococcus viruses

Comparative genomics gives us a new window into phage-host interactions and their evolutionary implications. Here we report the presence of genes central to oxygenic photosynthesis in the genomes of three phages from two viral families (Myoviridae and Podoviridae) that infect the marine cyanobacterium Prochlorococcus. The genes that encode the photosystem II core reaction center protein D1 (psbA), and a high-light-inducible protein (HLIP) (hli) are present in all three genomes. Both myoviruses contain additional hli gene types, and one of them encodes the second photosystem II core reaction center protein D2 (psbD), whereas the other encodes the photosynthetic electron transport proteins plastocyanin (petE) and ferredoxin (petF). These uninterrupted, full-length genes are conserved in their amino acid sequence, suggesting that they encode functional proteins that may help maintain photosynthetic activity during infection. Phylogenetic analyses show that phage D1, D2, and HLIP proteins cluster with those from Prochlorococcus, indicating that they are of cyanobacterial origin. Their distribution among several Prochlorococcus clades further suggests that the genes encoding these proteins were transferred from host to phage multiple times. Phage HLIPs cluster with multicopy types found exclusively in Prochlorocococus, suggesting that phage may be mediating the expansion of the hli gene family by transferring these genes back to their hosts after a period of evolution in the phage. These gene transfers are likely to play a role in the fitness landscape of hosts and phages in the surface oceans.

Consequences of a deletion in dspA on transcript accumulation in Synechocystis sp. strain PCC6803

A sensor histidine kinase of Synechococcus sp. strain PCC7942, designated nblS, was previously identified and shown to be critical for the acclimation of cells to high-light and nutrient limitation conditions and to influence the expression of a number of light-responsive genes. The nblS orthologue in Synechocystis sp. strain PCC6803 is designated dspA (also called hik33). We have generated a dspA null mutant and analyzed global gene expression in both the mutant and wild-type strains under high- and low-light conditions. The mutant is aberrant for the expression of many genes encoding proteins critical for photosynthesis, phosphate and carbon acquisition, and the amelioration of stress conditions. Furthermore, transcripts from a number of genes normally detected only during exposure of wild-type cells to high-light conditions become partially constitutive in the low-light-grown dspA mutant. Other genes for which transcripts decline upon exposure of wild-type cells to high light are already lower in the mutant during growth in low light. These results suggest that DspA may influence gene expression in both a positive and a negative manner and that the dspA mutant behaves as if it were experiencing stress conditions (e.g., high-light exposure) even when maintained at near-optimal growth conditions for wild-type cells. This is discussed with respect to the importance of DspA for regulating the responses of the cell to environmental cues.

Multiple Deletions of Small Cab-like Proteins in the Cyanobacterium Synechocystis sp. PCC 6803

Deletion of the genes for four or five small Cab-like proteins (SCPs) in photosystem (PS) I-less and PS I-less/PS II-less strains of Synechocystis sp. PCC 6803 caused a large decrease in the chlorophyll and carotenoid content of the cells without accumulation of early intermediates in the chlorophyll biosynthesis pathway, suggesting limited chlorophyll availability. The PS II/PS I ratio increased upon deletion of multiple SCPs in a wild type background, similar to what is observed in the presence of subsaturating concentrations of gabaculin, an inhibitor of an early step in the tetrapyrrole biosynthesis pathway. Upon deletion of multiple SCPs, neither 77 K fluorescence emission properties of phycobilisomeless thylakoids from the PS I-less/PS II-less strain nor the energy trapping efficiency of PS II were affected, indicating that under steady-state conditions SCPs do not bind much chlorophyll and do not serve as PS II antenna. Under conditions where protochlorophyllide reduction and thus chlorophyll synthesis were inhibited, chlorophyll disappeared quickly in a mutant lacking all five SCPs. This implies a role of SCPs in stabilization of chlorophyll-binding proteins and/or in reuse of chlorophylls. Under these conditions of inhibited reduction of protochlorophyllide, the accumulation kinetics of this intermediate were greatly altered in the absence of the five SCPs. This indicates an alteration of tetrapyrrole biosynthesis kinetics by SCPs. Based on this and other evidence, we propose that SCPs bind carotenoids and transiently bind chlorophyll, aiding in the supply of chlorophyll to nascent or reassembling photosynthetic complexes, and regulate the tetrapyrrole biosynthesis pathway as a function of the demand for chlorophyll.

Control of photosynthetic and high-light-responsive genes by the histidine kinase DspA: negative and positive regulation and interactions between signal transduction pathways

We have deleted a gene for a sensor histidine kinase, dspA (or hik33), in the cyanobacterium Synechocystis sp. strain PCC6803. In low and moderate light, the mutant grew slowly under photoautotrophic conditions, with a doubling time of approximately 40 h, and had severely reduced photosynthetic oxygen evolution. When the mutant was maintained in low or moderate light in the presence of glucose, its growth rate was only somewhat lower than that of wild-type cells. However, the mutant was light sensitive and rapidly died in high light. Furthermore, levels of many transcripts encoding genes associated with photosynthesis were altered in the mutant relative to wild-type Synechocystis sp. strain PCC6803 both in low light and following exposure to high light. There was constitutive expression of several high-light-inducible genes, including hli, psbAIII, and gpx2; there was little increased accumulation of sodB mRNA in high light; and the cells failed to accumulate cpcBA and psaAB mRNAs in low light in the presence of glucose, although a normal decline in the levels of these mRNAs was observed during exposure to high light. These results suggest that DspA is involved in controlling sets of photosynthetic and high-light-responsive genes, either directly or indirectly. These and other results, some of which are presented in a companion paper (C.-J. Tu, J. Shrager, R. Burnap, B. L. Postier, and A. R. Grossman, J. Bacteriol. 186:3889-3902, 2004), suggest that DspA acts as a global regulator that helps coordinate cellular metabolism with growth limitations imposed by environmental conditions.

Photosynthetic Electron Transport Controls Expression of the High Light Inducible Gene in the Cyanobacterium Synechococcus elongatus Strain PCC 7942

The hliA gene of Synechococcus, encoding a photoprotective high light inducible polypeptide, is up-regulated by high light (HL) or low intensity blue/UV-A light (BL). hliA expression was found to be up-regulated by KCN in low light (LL) (but not in the dark), and up-regulation in HL, BL, and LL (with KCN) was inhibited by 2,5-dibromo-3-methyl-6-isopropyl-benzoquinone. A working hypothesis is proposed whereby up-regulation is in response to the reduced state of cytochrome b(6)f or a carrier beyond in photosynthesis. Modest up-regulation occurs in LL by treatment with 3-(3,4-dichlorophenyl)-1,1-dimethylurea, but this is related to effects on hliA mRNA stability rather than on transcription.

Light control of hliA transcription and transcript stability in the cyanobacterium Synechococcus elongatus strain PCC 7942

The high-light-inducible proteins (HLIPs) of cyanobacteria are polypeptides involved in protecting the cells from high-intensity light (HL). The hliA gene encoding the HLIP from Synechococcus elongatus strain PCC 7942 is expressed in response to HL or low-intensity blue or UV-A light. In this study, we explore via Northern analysis details of the transcriptional regulation and transcript stability of the hliA gene under various light conditions. Transcript levels of the hliA gene increased dramatically upon a shift to HL or UV-A light to similar levels, followed by a rapid decrease in UV-A light, but not in HL, consistent with blue/UV-A light involvement in early stages of HL-mediated expression. A 3-min pulse of low-intensity UV-A light was enough to trigger hliA mRNA accumulation, indicating that a blue/UV-A photoreceptor is involved in upregulation of the gene. Low-intensity red light was found to cause a slight, transient increase in transcript levels (raising the possibility of red-light photoreceptor involvement), while light of other qualities had no apparent effect. No evidence was found for wavelength-specific attenuation of hliA transcript levels induced by HL or UV-A light. Transcript decay was slowed somewhat in darkness, and when photosynthetic electron transport was inhibited by darkness or treatment with DCMU, there appeared a smaller mRNA species that may represent a decay intermediate that accumulates when mRNA decay is slowed. Evidence suggests that upregulation of hliA by light is primarily a transcriptional response but conditions that cause ribosomes to stall on the transcript (e.g., a shift to darkness) can help stabilize hliA mRNA and affect expression levels.