A Systems-Level Analysis of the Effects of Light Quality on the Metabolism of a Cyanobacterium (original) (raw)
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Unicellular diazotrophic cyanobacteria such as Cyanothece sp. ATCC 51142 (henceforth Cyanothece), temporally separate the oxygen sensitive nitrogen fixation from oxygen evolving photosynthesis not only under diurnal cycles (LD) but also in continuous light (LL). However, recent reports demonstrate that the oscillations in LL occur with a shorter cycle time of ~11 h. We find that indeed, majority of the genes oscillate in LL with this cycle time. Genes that are upregulated at a particular time of day under diurnal cycle also get upregulated at an equivalent metabolic phase under LL suggesting tight coupling of various cellular events with each other and with the cell's metabolic status. A number of metabolic processes get upregulated in a coordinated fashion during the respiratory phase under LL including glycogen degradation, glycolysis, oxidative pentose phosphate pathway, and tricarboxylic acid cycle. These precede nitrogen fixation apparently to ensure sufficient energy and a...
Plant Physiology, 2002
The light state transition regulates the distribution of absorbed excitation energy between the two photosystems (PSs) of photosynthesis under varying environmental conditions and/or metabolic demands. In cyanobacteria, there is evidence for the redistribution of energy absorbed by both chlorophyll (Chl) and by phycobilin pigments, and proposed mechanisms differ in the relative involvement of the two pigment types. We assayed changes in the distribution of excitation energy with 77K fluorescence emission spectroscopy determined for excitation of Chl and phycobilin pigments, in both wild-type and state transition-impaired mutant strains ofSynechococcus sp. PCC 7002 andSynechocystis sp. PCC 6803. Action spectra for the redistribution of both Chl and phycobilin pigments were very similar in both wild-type cyanobacteria. Both state transition-impaired mutants showed no redistribution of phycobilin-absorbed excitation energy, but retained changes in Chl-absorbed excitation. Action spectr...
Photosynthesis Research, 1996
Synechococcus sp. PCC 7942 (Anacystis nidulans R2) contains two forms of the Photosystem II reaction centre protein D1, which differ in 25 of 360 amino acids. DI:I predominates under low light but is transiently replaced by D1:2 upon shifts to higher light. Mutant cells containing only DI:I have lower photochemical energy capture efficiency and decreased resistance to photoinhibition, compared to cells containing Dl:2. We show that when dark-adapted or under low to moderate light, cells with DI:I have higher non-photochemical quenching of PS II fluorescence (higher qN) than do cells with Dl:2. This is reflected in the 77 K chlorophyll emission spectra, with lower Photosystem II fluorescence at 697-698 nm in cells containing DI:I than in cells with Dl:2. This difference in quenching of Photosystem II fluorescence occurs upon excitation of both chlorophyll at 435 nm and phycobilisomes at 570 nm. Measurement of time-resolved room temperature fluorescence shows that Photosystem II fluorescence related to charge stabilization is quenched more rapidly in cells containing DI:I than in those with D1:2. Cells containing D1:1 appear generally shifted towards State II, with PS II down-regulated, while cells with D1:2 tend towards State I. In these cyanobacteria electron transport away from PS II remains non-saturated even under photoinhibitory levels of light. Therefore, the higher activity of D1:2 Photosystem II centres may allow more rapid photochemical dissipation of excess energy into the electron transport chain. D 1:1 confers capacity for extreme State II which may be of benefit under low and variable light. Abbreviations: D 1-t h e atrazine-binding 32 kDa protein of the PS II reaction centre core; D 1:1-the D 1 protein constitutively expressed during acclimated growth in Synechococcus sp. PCC 7942; D 1:2-an alternate form of the D1 protein induced under excess excitation in Synechococcus sp.
Photosynthesis Research, 1995
In this minireview we discuss effects of excitation stress on the molecular organization and function of PS II as induced by high light or low temperature in the cyanobacterium Synechococcus sp. PCC 7942. Synechococcus displays PS II plasticity by transiently replacing the constitutive D1 form (D 1:1) with another form (D 1:2) upon exposure to excitation stress. The cells thereby counteract photoinhibition by increasing D 1 turn over and modulating PS II function. A comparison between the cyanobacterium Synechococcus and plants shows that in cyanobacteria, with their large phycobilisomes, resistance to photoinhibition is mainly through the dynamic properties (D I turnover and quenching) of the reaction centre. In contrast, plants use antenna quenching in the light-harvesting complex as an important means to protect the reaction center from excessive excitation.
Journal of Biological Chemistry, 1999
The D1 protein of the photosystem II reaction center is thought to be the most light-sensitive component of the photosynthetic machinery. To understand the mechanisms underlying the light sensitivity of D1, we performed in vitro random mutagenesis of the psbA gene that codes for D1, transformed the unicellular cyanobacterium Synechocystis sp. PCC 6803 with mutated psbA, and selected phototolerant transformants that did not bleach in high intensity light. A region of psbA2 coding for 178 amino acids of the carboxyl-terminal portion of the peptide was subjected to random mutagenesis by low fidelity polymerase chain reaction amplification or by hydroxylamine treatment. This region contains the binding sites for Q B , D2 (through Fe), and P680. Eighteen phototolerant mutants with single and multiple amino acid substitutions were selected from a half million transformants exposed to white light at 320 mol m ؊2 s ؊1. A strain transformed with non-mutagenized psbA2 became bleached under the same conditions. Sitedirected mutagenesis has confirmed that one or more substitutions of amino acids at residues 234, 254, 260, 267, 322, 326, and 328 confers phototolerance. The rate of degradation of D1 protein was not appreciably affected by the mutations. Reduced bleaching of mutant cyanobacterial cells may result from continued buildup of photosynthetic pigment systems caused by changes in redox signals originating from D1.
Cyanobacterial Acclimation to Rapidly Fluctuating Light is Constrained by Inorganic Carbon STATUS1
2005
Acclimation to rapidly fluctuating light, simulating shallow aquatic habitats, is altered depending on inorganic carbon (C i) availability. Under steady light of 50 lmol photons. m À 2. s À 1 , the growth rate of Synechococcus elongatus PCC7942 was similar in cells grown in high C i (4 mM) and low C i (0.02 mM), with induced carbon concentrating mechanisms compensating for low C i. Growth under fluctuating light of a 1-s period averaging 50 lmol photons. m À 2. s À 1 caused a drop in growth rate of 28% AE 6% in high C i cells and 38% AE 8% in low C i cells. In high C i cells under fluctuating light, the PSI/PSII ratio increased, the PSII absorption crosssection decreased, and the PSII turnover rate increased in a pattern similar to highlight acclimation. In low C i cells under fluctuating light, the PSI/ PSII ratio decreased, the PSII absorption cross-section decreased, and the PSII turnover remained slow. Electron transport rate was similar in high and low C i cells but in both was lower under fluctuating than under steady light. After acclimation to a 1-s period fluctuating light, electron transport rate decreased under steady or long-period fluctuating light. We hypothesize that high C i cells acclimated to exploit the bright phases of the fluctuating light, whereas low C i cells enlarged their PSII pool to integrate the fluctuating light and dampen the variation of the electron flux into a rate-restricted C i pool. Light response curves measured under steady light, widely used to predict photosynthetic rates, do not properly predict photosynthetic rates achieved under fluctuating light, and exploitation of fluctuating light is altered by C i status.
Synechocystis sp. strain PCC 6803 is the most widely studied model cyanobacterium, with a well-developed omics level knowledgebase. Like the lifestyles of other cyanobacteria, that of Synechocystis PCC 6803 is tuned to diurnal changes in light intensity. In this study, we analyzed the expression patterns of all of the genes of this cyanobacterium over two consecutive diurnal periods. Using stringent criteria, we determined that the transcript levels of nearly 40% of the genes in Synechocystis PCC 6803 show robust diurnal oscillating behavior, with a majority of the transcripts being upregulated during the early light period. Such transcripts corresponded to a wide array of cellular processes, such as light harvesting, photosynthetic light and dark reactions, and central carbon metabolism. In contrast, transcripts of membrane transporters for transition metals involved in the photosynthetic electron transport chain (e.g., iron, manganese, and copper) were significantly upregulated during the late dark period. Thus, the pattern of global gene expression led to the development of two distinct transcriptional networks of coregulated oscillatory genes. These networks help describe how Synechocystis PCC 6803 regulates its metabolism toward the end of the dark period in anticipation of efficient photosynthesis during the early light period. Furthermore, in silico flux prediction of important cellular processes and experimental measurements of cellular ATP, NADP(H), and glycogen levels showed how this diurnal behavior influences its metabolic characteristics. In particular, NADPH/NADP+ showed a strong correlation with the majority of the genes whose expression peaks in the light. We conclude that this ratio is a key endogenous determinant of the diurnal behavior of this cyanobacterium.