A cryptochrome-like protein is involved in the regulation of photosynthesis genes in Rhodobacter sphaeroides

Blue light receptors belonging to the cryptochrome/photolyase family are found in all kingdoms of life. The functions of photolyases in repair of UV-damaged DNA as well as of cryptochromes in the light-dependent regulation of photomorphogenetic processes and in the circadian clock in plants and animals are well analysed. In prokaryotes, the only role of members of this protein family that could be demonstrated is DNA repair. Recently, we identified a gene for a cryptochrome-like protein (CryB) in the α-proteobacterium Rhodobacter sphaeroides. The protein lacks the typical C-terminal extension of cryptochromes, and is not related to the Cry DASH family. Here we demonstrate that CryB binds flavin adenine dinucleotide that can be photoreduced by blue light. CryB binds single-stranded DNA with very high affinity ( K _d∼10⁻⁸ M) but double-stranded DNA and single-stranded RNA with far lower affinity ( K _d∼10⁻⁶ M). Despite of that, no in vitro repair activity for pyrimidine dimers in single-stranded DNA could be detected. However, we show that CryB clearly affects the expression of genes for pigment-binding proteins and consequently the amount of photosynthetic complexes in R. sphaeroides . Thus, for the first time a role of a bacterial cryptochrome in gene regulation together with a biological function is demonstrated.     

A single flavoprotein,AppA, integrates both redox and light signals in Rhodobacter sphaeroides

Anoxygenic photosynthetic proteobacteria exhibit various light responses, including changing levels of expression of photosynthesis genes. However, the underlying mechanisms are largely unknown. We show that expression of the puf and puc operons encoding structural proteins of the photosynthetic complexes is strongly repressed by blue light under semi-aerobic growth in Rhodobacter sphaeroides but not in the related species Rhodobacter capsulatus. At very low oxygen tension, puf and puc expression is independent of blue light in both species. Photosynthetic electron transport does not mediate the blue light repression, implying the existence of specific photoreceptors. Here, we show that the flavoprotein AppA is likely to act as the photoreceptor for blue light-dependent repression during continuous illumination. The FAD cofactor of AppA is essential for the blue light-dependent sensory transduction of this response. AppA, which is present in R. sphaeroides but not in R. capsulatus, is known to participate in the redox-dependent control of photosynthesis gene expression. Thus, AppA is the first example of a protein with dual sensing capabilities that integrates both redox and light signals.     

Distinct leaf developmental and gene expression responses to light quantity depend on blue-photoreceptor or plastid-derived signals,and can occur in the absence of phototropins

Leaf palisade cell development and the composition of chloroplasts respond to the fluence rate of light to maximise photosynthetic light capture while minimising photodamage. The underlying light sensory mechanisms are probably multiple and remain only partially understood. Phototropins (PHOT1 and PHOT2) are blue light receptors regulating responses which are light quantity-dependent and which include the control of leaf expansion. Here we show that genes for proteins in the reaction centres show long-term responses in wild type plants, and single blue photoreceptor mutants, to light fluence rate consistent with regulation by photosynthetic redox signals. Using contrasting intensities of white or broad-band red or blue light, we observe that increased fluence rate results in thicker leaves and greater number of palisade cells, but the anticlinal elongation of those cells is specifically responsive to the fluence rate of blue light. This palisade cell elongation response is still quantitatively normal in fully light-exposed regions of phot1 phot2 double mutants under increased fluence rate of white light. Plants grown at high light display elevated expression of RBCS (for the Rubisco small subunit) which, together with expected down-regulation of LHCB1 (for the photosynthetic antenna primarily of photosystem II), is also observed in phot double mutants. We conclude that an unknown blue light photoreceptor, or combination thereof, controls the development of a typical palisade cell morphology, but phototropins are not essential for either this response or acclimation-related gene expression changes. Together with previous evidence, our data further demonstrate that photosynthetic (chloroplast-derived) signals play a central role in the majority of acclimation responses.     

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.     

不同光质对烟草叶片组织结构及Rubisco羧化酶活性和rbc、rca基因表达的影响

通过对烟草植株覆盖白、红、黄、蓝、紫色滤膜获得不同光质,研究了烟草叶片在7～70d的生长发育期内,不同光质处理对烟叶组织结构特征、核酮糖1,5-二磷酸羧化酶/加氧酶（Rubisco）羧化酶活性、Rubisco基因（rbc）表达及其活化酶（Rca）基因（rca）表达的影响。结果表明,与黄膜处理下生长的烟叶相比,红、蓝、紫膜处理下生长的烟叶有较高的叶片厚度、栅栏组织厚度、海绵组织厚度、栅栏细胞密度和较小的组织空隙率。此外,红、蓝、紫膜处理的叶片有较高的Rubis-co羧化酶活性和净光合速率及较强的rbc和rca基因表达。实验结果表明不同光质对烟草叶片的组织结构特征有显著影响,光质可能通过影响Rubisco羧化酶活性进而影响叶片光合效率,而光质、叶片组织结构和光合效率之间存在某种程度的相互联系。     

Conservation of the photosynthesis gene cluster in Rhodospirillum centenum

Intraspecies and intergenus complementation analysis were utilized to demonstrate that photosynthesis genes are clustered in distantly related purple photosynthetic bacteria. Specifically, we show that the linkage order for genes involved in bacteriochlorophyll and carotenoid biosynthesis in Rhodospirillum centenum are arranged essentially as in Rhodobacter capsulatus and Rhodobacter sphaeroides. In addition, the location and relative distance observed between the puf and puh operons which encode for light harvesting and reaction-centre structural genes are also conserved between these species. Conservation of the photosynthesis gene cluster implies either that there are structural or regulatory constraints that limit rearrangement of the photosynthesis gene cluster or that there may have been lateral transfer of the photosynthesis gene cluster among different species of phototrophic bacteria.     

Concerted changes in gene expression and cell physiology of the cyanobacterium Synechocystis sp. strain PCC 6803 during transitions between nitrogen and light-limited growth

Physiological adaptation and genome-wide expression profiles of the cyanobacterium Synechocystis sp. strain PCC 6803 in response to gradual transitions between nitrogen-limited and light-limited growth conditions were measured in continuous cultures. Transitions induced changes in pigment composition, light absorption coefficient, photosynthetic electron transport, and specific growth rate. Physiological changes were accompanied by reproducible changes in the expression of several hundred open reading frames, genes with functions in photosynthesis and respiration, carbon and nitrogen assimilation, protein synthesis, phosphorus metabolism, and overall regulation of cell function and proliferation. Cluster analysis of the nearly 1,600 regulated open reading frames identified eight clusters, each showing a different temporal response during the transitions. Two large clusters mirrored each other. One cluster included genes involved in photosynthesis, which were up-regulated during light-limited growth but down-regulated during nitrogen-limited growth. Conversely, genes in the other cluster were down-regulated during light-limited growth but up-regulated during nitrogen-limited growth; this cluster included several genes involved in nitrogen uptake and assimilation. These results demonstrate complementary regulation of gene expression for two major metabolic activities of cyanobacteria. Comparison with batch-culture experiments revealed interesting differences in gene expression between batch and continuous culture and illustrates that continuous-culture experiments can pick up subtle changes in cell physiology and gene expression.     

Acclimation to singlet oxygen stress in Chlamydomonas reinhardtii

In an aerobic environment, responding to oxidative cues is critical for physiological adaptation (acclimation) to changing environmental conditions. The unicellular alga Chlamydomonas reinhardtii was tested for the ability to acclimate to specific forms of oxidative stress. Acclimation was defined as the ability of a sublethal pretreatment with a reactive oxygen species to activate defense responses that subsequently enhance survival of that stress. C. reinhardtii exhibited a strong acclimation response to rose bengal, a photosensitizing dye that produces singlet oxygen. This acclimation was dependent upon photosensitization and occurred only when pretreatment was administered in the light. Shifting cells from low light to high light also enhanced resistance to singlet oxygen, suggesting an overlap in high-light and singlet oxygen response pathways. Microarray analysis of RNA levels indicated that a relatively small number of genes respond to sublethal levels of singlet oxygen. Constitutive overexpression of either of two such genes, a glutathione peroxidase gene and a glutathione S-transferase gene, was sufficient to enhance singlet oxygen resistance. Escherichia coli and Saccharomyces cerevisiae exhibit well-defined responses to reactive oxygen but did not acclimate to singlet oxygen, possibly reflecting the relative importance of singlet oxygen stress for photosynthetic organisms.     

Carbon metabolite feedback regulation of leaf photosynthesis and development

Photosynthesis is regulated as a two-way process. Light regulates the expression of genes for photosynthesis and the activity of the gene products (feedforward control). Rate of end-product use down-stream of the Calvin cycle, determined largely by nutrition and temperature, also affects photosynthetic activity and photosynthetic gene expression (feedback control). Whereas feedforward control ensures efficient light use, feedback mechanisms ensure that carbon flow is balanced through the pathways that produce and consume carbon, so that inorganic phosphate is recycled and nitrogen is distributed optimally to different processes to ensure growth and survival. Actual mechanisms are sketchy and complex, but carbon to nitrogen balance rather than carbon status per se is central to understanding carbon metabolite feedback control of photosynthesis. In addition to determining the activity of the metabolic machinery, carbon metabolite feedback mechanisms also regulate photosynthesis at the leaf level through the regulation of leaf development. This review summarizes the current sketchy, but growing, knowledge of the mechanisms through which carbon metabolite feedback mechanisms regulate leaf photosynthesis.