Analysis of Chlamydomonas mutants with abnormal expression of CO2 and HCO3- uptake systems

Eukaryotic microalgae such as Chlamydomonas reinhardtii possess an inducible CO2 concentrating mechanism that operates as a very close interaction between pyrenoid-based Rubisco, various carbonic anhydrases (CAs), and inorganic carbon (Ci) transport systems. While external and internal CAs have been characterised to the molecular level, the biochemistry and molecular biology of Ci uptake mechanisms have not been elucidated. Both Ci species, CO2 and HCO3-, are taken up by the cells and chloroplasts during steady-state photosynthesis. After acclimation to limiting Ci, CO2 and HCO3- transport, measured in whole cells or chloroplasts, change their kinetic characteristics from a constitutive low-affinity state to an inducible high-affinity state. In order to learn more about the genes involved in the signal transduction pathway and in the Ci transport systems, we performed insertional mutagenesis using the arg7 gene as a selectable marker. Application of aqueous membrane inlet mass spectrometry allowed discrimination between CO2 and HCO3- uptake. Data is presented on two mutants, M46 and M21, which show severe damage to the constitutive Ci uptake systems and which are unable to induce a high-affinity state. The mutations might be either in the signal transduction pathway or in the transporters themselves. In addition, we present data that shows a very close connection between high-affinity HCO3- uptake and high-affinity NO3- uptake in cells of C. reinhardtii.