Orchestral manoeuvres in the light: Crosstalk needed for regulation of the Chlamydomonas carbon concentration mechanism

The inducible carbon concentrating mechanism in Chlamydomonas reinhardtii has been well defined from a molecular and ultrastructural perspective. Inorganic carbon transport proteins, and strategically located carbonic anhydrases deliver CO2 within the chloroplast pyrenoid matrix where Rubisco is packaged. However, there is little understanding of the fundamental signalling and sensing processes leading to CCM induction. While external CO2 limitation has been believed to be the primary cue, the coupling between energetic supply and inorganic carbon demand through regulatory feedback from light harvesting and photorespiration signals, could provide the original CCM trigger. Key questions regarding the integration of these processes are addressed in this review. We consider how the chloroplast functions as a crucible for photosynthesis, importing and integrating nuclear-encoded components from the cytoplasm, and sending retrograde signals to the nucleus to regulate CCM induction. We hypothesise that induction of the CCM is associated with retrograde signals associated with photorespiration and/or light stress. We have also examined the significance of common evolutionary pressures for origins of two co-regulated processes- CCM and photorespiration, in addition to identifying genes of interest involved in transcription, protein folding and regulatory processes which are needed to fully understand the processes leading to CCM induction.

Comparison of the Sulfonamide Inhibition Profiles of the α-Carbonic Anhydrase Isoforms (SpiCA1, SpiCA2 and SpiCA3) Encoded by the Genome of the Scleractinian Coral Stylophora pistillata

The ubiquitous metalloenzymes carbonic anhydrases (CAs, EC 4.2.1.1) are responsible for the reversible hydration of CO2 to bicarbonate (HCO3−) and protons (H+). Bicarbonate may subsequently generate carbonate used in many functional activities by marine organisms. CAs play a crucial role in several physiological processes, e.g., respiration, inorganic carbon transport, intra and extra-cellular pH regulation, and bio-mineralization. Multiple transcript variants and protein isoforms exist in the organisms. Recently, 16 α-CA isoforms have been identified in the coral Stylophora pistillata. Here, we focalized the interest on three coral isoforms: SpiCA1 and SpiCA2, localized in the coral-calcifying cells; and SpiCA3, expressed in the cytoplasm of the coral cell layers. The three recombinant enzymes were heterologously expressed and investigated for their inhibition profiles with sulfonamides and sulfamates. The three coral CA isoforms differ significantly in their susceptibility to inhibition with sulfonamides. This study provides new insights into the coral physiology and the comprehension of molecular mechanisms involved in the bio-mineralization processes, since CAs interact with bicarbonate transporters, accelerating the trans-membrane bicarbonate movement and modulating the pH at both sides of the plasma membranes.

Functionally distinct NAD(P)H dehydrogenases and their membrane localization in Synechocystis sp. PCC6803

The type I NAD(P)H dehydrogenase complex (NDH-1) in cyanobacteria is involved in both respiratory and photosynthetic electron transport processes. NDH-1 is also essential for inorganic carbon transport. It has been postulated that NDH-1-dependent cyclic electron flow around PSI energizes CO2 uptake. The genome information of Synechocystis sp. PCC6803 has enabled us to provide an integrative view of the CO2 concentrating mechanism in this organism. In an attempt to dissect the role of the NDH-1 complex, we have constructed single and double mutants of Synechocystis 6803 by disrupting highly homologous ndhD genes in pairs, and have analysed the growth, CO2 uptake activities, and redox levels of P700 and the plastoquinone pool in these mutants under various conditions. We have also determined the membrane localization of this membrane protein. Our studies have revealed that: (i) mutations in ndh genes lead to inhibition of CO2 uptake, rather than HCO3– uptake; (ii) NDH-1 complexes are localized only in the thylakoid membrane; (iii) there are functionally distinct NDH-1 complexes in Synechocystis #6803. Based on these data, we propose a schematic view of the roles of different NDH-1 complexes in cyanobacteria.