17 research outputs found
Two Antagonistic Clock-Regulated Histidine Kinases Time the Activation of Circadian Gene Expression
No full textThe cyanobacterial circadian pacemaker consists of a three-protein clock—KaiA, KaiB, and KaiC—that generates oscillations in the phosphorylation state of KaiC. Here we investigate how temporal information encoded in KaiC phosphorylation is transduced to RpaA, a transcription factor required for circadian gene expression. We show that phosphorylation of RpaA is regulated by two antagonistic histidine kinases, SasA and CikA, which are sequentially activated at distinct times by the Kai clock complex. SasA acts as a kinase toward RpaA, whereas CikA, previously implicated in clock input, acts as a phosphatase that dephosphorylates RpaA. CikA and SasA cooperate to generate an oscillation of RpaA activity that is distinct from that generated by either enzyme alone and offset from the rhythm of KaiC phosphorylation. Our observations reveal how circadian clocks can precisely control the timing of output pathways via the concerted action of two oppositely acting enzymes.Chemistry and Chemical BiologyMolecular and Cellular BiologyAccepted Manuscrip
Emerging Perspectives on the Mechanisms, Regulation, and Distribution of Light Color Acclimation in Cyanobacteria
No full textABSTRACT Chromatic acclimation (CA) provides many cyanobacteria with the ability to tailor the properties of their lightharvesting
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 CAregulatory
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
Emerging Perspectives on the Mechanisms, Regulation, and Distribution of Light Color Acclimation in Cyanobacteria
No full textDynamical localization of a thylakoid membrane binding protein is required for acquisition of photosynthetic competency
No full textSUMMARYVipp1 is highly conserved and essential for photosynthesis, but its function is unclear as it does not participate directly in light-dependent reactions. We analyzed Vipp1 localization in live cyanobacterial cells and show that Vipp1 is highly dynamic, continuously exchanging between a diffuse fraction that is uniformly distributed throughout the cell and a punctate fraction that is concentrated at high curvature regions of the thylakoid located at the cell periphery. Experimentally perturbing the spatial distribution of Vipp1 by relocalizing it to the nucleoid causes a severe growth defect during the transition from non-photosynthetic (dark) to photosynthetic (light) growth. However, the same perturbation of Vipp1 in dark alone or light alone growth conditions causes no growth or thylakoid morphology defects. We propose that the punctuated dynamics of Vipp1 at the cell periphery in regions of high thylakoid curvature enable acquisition of photosynthetic competency, perhaps by facilitating biogenesis of photosynthetic complexes involved in light-dependent reactions of photosynthesis.</jats:p
Dynamical localization of a thylakoid membrane binding protein is required for acquisition of photosynthetic competency
No full textBridges over Convulsing Waters: the EU aspiring Eastern Partners’ Role in the Regional Governance
No full textThe enlargement of the European Union (EU) to the East in 2004 and 2007 so as to include ten former communist countries and two small Mediterranean islands has triggered new questions on the nature of EU governance. We argue that the accession of Central and Eastern European countries (CEECs) to the EU has affected governance patterns in the EU and beyond. Undeniably, the most recent waves of enlargement have had feed-back effects on Europeanisation mechanisms (Grabbe 2006). Also, the European Neighbourhood Policy (ENP) conditionality attached to the Eastern partners will likely follow similar patterns. The EU is proud of its Enlargement policy, “one of the most successful EU policies”i, and is inclined to extend the enlargement mechanisms to future frameworks as the ENP. Through the example of Ukraine, Moldova and Georgia, and possibly Belarus, we argue that the ENP conditionality contributes to the EU's governance export in the same way the preparations for the fifth Eastern enlargement did. Furthermore, we advance the idea that complying with ENP conditionality may bring EU aspiring Eastern partners closer to accession
Sulfate-Driven Elemental Sparing Is Regulated at the Transcriptional and Posttranscriptional Levels in a Filamentous Cyanobacterium
No full textABSTRACT
Sulfur is an essential nutrient that can exist at growth-limiting concentrations in freshwater environments. The freshwater cyanobacterium
Fremyella diplosiphon
(also known as
Tolypothrix
sp. PCC 7601) is capable of remodeling the composition of its light-harvesting antennae, or phycobilisomes, in response to changes in the sulfur levels in its environment. Depletion of sulfur causes these cells to cease the accumulation of two forms of a major phycobilisome protein called phycocyanin and initiate the production of a third form of phycocyanin, which possesses a minimal number of sulfur-containing amino acids. Since phycobilisomes make up approximately 50% of the total protein in these cells, this elemental sparing response has the potential to significantly influence the fitness of this species under low-sulfur conditions. This response is specific for sulfate and occurs over the physiological range of sulfate concentrations likely to be encountered by this organism in its natural environment.
F. diplosiphon
has two separate sulfur deprivation responses, with low sulfate levels activating the phycobilisome remodeling response and low sulfur levels activating the chlorosis or bleaching response. The phycobilisome remodeling response results from changes in RNA abundance that are regulated at both the transcriptional and posttranscriptional levels. The potential of this response, and the more general bleaching response of cyanobacteria, to provide sulfur-containing amino acids during periods of sulfur deprivation is examined.
</jats:p
Phycoerythrin-specific bilin lyase-isomerase controls blue-green chromatic acclimation in marine Synechococcus.
No full textInternational audienceThe marine cyanobacterium Synechococcus is the second most abundant phytoplanktonic organism in the world's oceans. The ubiquity of this genus is in large part due to its use of a diverse set of photosynthetic light-harvesting pigments called phycobiliproteins, which allow it to efficiently exploit a wide range of light colors. Here we uncover a pivotal molecular mechanism underpinning a widespread response among marine Synechococcus cells known as "type IV chromatic acclimation" (CA4). During this process, the pigmentation of the two main phycobiliproteins of this organism, phycoerythrins I and II, is reversibly modified to match changes in the ambient light color so as to maximize photon capture for photosynthesis. CA4 involves the replacement of three molecules of the green light-absorbing chromophore phycoerythrobilin with an equivalent number of the blue light-absorbing chromophore phycourobilin when cells are shifted from green to blue light, and the reverse after a shift from blue to green light. We have identified and characterized MpeZ, an enzyme critical for CA4 in marine Synechococcus. MpeZ attaches phycoerythrobilin to cysteine-83 of the α-subunit of phycoerythrin II and isomerizes it to phycourobilin. mpeZ RNA is six times more abundant in blue light, suggesting that its proper regulation is critical for CA4. Furthermore, mpeZ mutants fail to normally acclimate in blue light. These findings provide insights into the molecular mechanisms controlling an ecologically important photosynthetic process and identify a unique class of phycoerythrin lyase/isomerases, which will further expand the already widespread use of phycoerythrin in biotechnology and cell biology applications
Thylakoid Membrane Architecture in Synechocystis Depends on CurT, a Homolog of the Granal CURVATURE THYLAKOID1 Proteins
No full textInternational audienc
