Photosynthesis converts the free energy of sunlight into usable chemical energy and thus powers virtually all life on Earth. In plants and algae, photosynthesis takes place in cytoplasmic compartments known as chloroplasts. The thylakoid membrane within chloroplasts harbors photosystems – the pigment-protein complexes that carry out the light harvesting and primary electron transport reactions of photosynthesis. There are two types of photosystems in plants and algae. These are connected in series by a cytochrome b6f complex for a linear electron transport from water to NADP+, producing ATP and NADPH. The energy-rich ATP and electron-rich NADPH are used in the reduction of CO2 to carbohydrates in the Calvin-Benson cycle. Our research seeks to understand the genetic and molecular control mechanisms that ensure safe and efficient use of light energy in plants and algae. Three major research areas are highlighted below.
Role of post-translational modifications in photosystem II function and biogenesis
Photosystem II (PS II) carries out the formidable water-splitting reaction of photosynthesis and thereby extracts electrons and protons necessary for the anabolic reactions of life. Incomplete water-splitting and or inadvertent electron transfer to molecular oxygen generate highly reactive oxygen free radicals that damage key protein subunits in PS II. PS II repair cycle is a highly orchestrated molecular process wherein the damaged PS II subunits are selectively degraded and replaced with newly synthesized copies while the undamaged subunits are simply recycled. How is plant PS II, a massive 1.4 MDa supercomplex embedded in the thylakoid membrane, disassembled for repair and reassembled after repair is an open question. Using biochemical and biophysical approaches we investigate the role of protein phosphorylation and oxidative protein modification in PS II disassembly. Our investigations are expected to provide insight into the molecular mechanisms that ensure the orderly disassembly and repair of plant PS II.
Healing the "sunburn". A ribbon diagram of monomeric plant PS II reaction center core. Oxidative damage (modification) to amino acids is shown as surface projections in darker colors on the polypeptide chains of D1 (red), D2 (yellow), CP47 (green), and CP43 (blue) subunits. Ball and stick representations of donor-side catalytic manganese cluster (purple, red, and green), acceptor-side nonheme iron between the two quinones (brown) , and P680 chlorophyll special pair (green) are also shown.
Structure and function of Chloroplast Sensor Kinase
Plant and algal chloroplasts steadfastly retain Chloroplast Sensor Kinase (CSK), a bacterial-type two-component sensor kinase of cyanobacterial origin. In green algae and plants, CSK is the only chloroplast-targeted sensor kinase. In some non-green algae, CSK is the sole sensor kinase gene in the whole genome. In certain other non-green algae, the CSK gene has survived multiple endosymbiotic events. In cyanobacteria, a CSK orthologue is universal. Despite its deep evolutionary conservation, the functional significance of CSK remains elusive. Our recent research reveals CSK to be an iron-sulfur protein, containing a [3Fe-4S] cluster with which it perceives the redox state of the photosynthetic electron carrier plastoquinone (PQ). We hypothesize that the PQ-redox sensory activity of CSK allows it to integrate diurnal and abiotic signals into regulation of chloroplast gene expression and carbon metabolism. Experiments are currently underway to test this hypothesis. We also seek to solve the structure of oxidized and reduced CSK proteins using cryo-electron microscopy.
A scheme for CSK function in chloroplasts. Abiotic and diurnal signals affect photosynthetic light reactions, which in turn is sensed by CSK. CSK then regulates chloroplast gene expression and carbon metabolism to optimize photosynthesis under the prevailing environmental conditions.
Diatom photosynthesis under changing light and nutrient conditions
Diatoms are a group of mostly aquatic photosynthetic algae with a plastid of red algal origin. While constituting just a fraction of the total biomass on earth, these prolific photosynthesizers fix as much carbon as all rainforests combined. We use the model diatom Phaeodactylum tricornutum to decipher the genetic basis of diatoms’ high photosynthetic efficiency under changing light and nutrient (iron) conditions.
Plastid-localization of Phaeodactylum Chloroplast Response Regulator 1 (CRR1). Far-left panel is a bright field image of a Phaeodactylum cell with a dividing chloroplast; the middle panel, chlorophyll autofluorescence; and far-right panel, green fluorescence from CRR1-GFP fusion protein. (In collaboration with Stefan Zauner and Uwe Maier, Philipps University of Marburg)