[4S04e] 19. Marine Biotechology - Part 3

5:00 PM - 5:30 PM JST(8:00 AM - 8:30 AM UTC)

[4Marine-12-KL] A multiphase CO₂-concentrating mechanism regulated by perception of signals from CO₂ and nutrient availability

○Yusuke Matsuda Matsuda¹ (1. Kwansei Gakuin University (Japan))

Keywords:

diatom,photosynthesis,CO2-concentrating mechanism,environmental response,productivity

Our recent studies revealed a new and unique structural and functional features of the diatom chloroplast, which is the key factor to maintain and control the diatom photosynthetic productivity. At the center of the diatom chloroplast, there is a CO₂-fixing protein body called pyrenoid, which is the condensate of the CO₂-fixing enzyme, Rubisco. Diatom pyrenoid has a thin oval shape with penetrating thylakoid (PPT) membrane at the central axis. This structure plays a critical role in the diatom CO₂-concentrating mechanism (CCM) as a CO₂-evolving machinery for Rubisco. The diatom pyrenoid is shaped by a mesh-like protein sheath composed of newly found PyShell proteins. In the PPT lumen of the PyShell-shaped pyrenoid, there is a specifically localized θ-type carbonic anhydrase (θ-CA), which is another critical component of the CO₂-evolving machinery of the pyrenoid. Under severely limited CO₂ environment, the function of the CO₂-evolving machinery is further strengthened by inducing thylakoidal HCO₃^- channel, bestrophins (BST) and stromal CAs, which respectively play roles to supply accumulated stromal HCO₃^- into the PPT lumen and recapture unfixed CO₂ by rehydration into HCO₃^-, preventing CO₂ from leaking out of the chloroplast. To date, CO₂ response of these CCM components was known be governed by the cAMP-mediated signal transduction system which eventually targets the promoter region by binding bZIP type transcription factor to the CO₂-cAMP responsive element (CCRE), repressing the low CO₂-inducible CCM factors such as BSTs and stromal CAs under high CO₂ environment. As the pyrenoid-based CCM is a system to provide first organic carbon skeletons to the primary metabolisms in the chloroplast, responses of this system to changes in the demand for the organic carbon skeleton are an intriguing issue to be clarified. Indeed, low-CO₂ inducible CCM factors, BST and stromal CA were repressed by limitation of nitrogen, phosphorous, and sulfur even under low CO₂ conditions, while the core factors for the CO₂-evolving machinery, PyShell and luminal θ-CA resiliently expressed in all environments, indicating that very high CCM activity is controlled not only by CO₂ but also by the demand for the organic carbon skeleton for primary metabolisms.