Components of the CO2-Concentrating System

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By comparison with C3 plants, Chlamydomonas reinhardtii and other unicellular algae exhibit unusual photosynthetic characteristics. It has been proposed that these characteristics result from the operation of a CO2- concentrating system, which increases the intracellular CO2 concentration to a level which suppresses photorespiration by competitive inhibition of RuBP oxygenase. One of the principal components of the proposed system is a protein which catalyzes the active transport of inorganic carbon across either the plasmalemma or the chloroplast envelope. However, direct, physical evidence for the existence of this transporter, or any other component of the CO2-concentrating system other than carbonic anhydrase, is lacking. Activity of the CO2-concentrating system is under environmental control. Cells grown at limiting CO2 concentrations (air- adapted) show activity, but cells grown at elevated CO2 concentrations (CO2-enriched) show no activity of the CO2- concentrating system. We have identified 7 polypeptides which increase in abundance during adaptation to limiting CO2 concentrations or are specific for cells grown under such conditions. We will continue to extend these observations to comparisons between wild-type and mutants of Chlamydomonas deficient in the CO2-concentrating system to help identify which polypeptides might be involved directly in inorganic carbon accumulation. Using antibodies generated against one or more of these "air-specific" polypeptides, we expect to: (1) determine whether appearance of the "air-specific" polypeptides during "induction" of the CO2-concentrating system correlates with the appearance of the capacity for inorganic carbon accumulation, (2) determine the intracellular location of the corresponding antigens, (3) determine whether Chlamydomonas mutants deficient in the capacity for inorganic carbon accumulation contain polypeptides immunologically cross-reactive with these antibodies, (4) investigate the appearance of translatable mRNA for "air-specific" polypeptides during the "induction" of the CO2-concentrating system. Genes differentially expressed during adaptation of CO2-enriched cells to limiting CO2 will be identified by differential screening of a cDNA library and, if time permits, by molecular cloning of the genes corresponding to the "air-induced" polypeptides. Attainment of all objectives will allow us to better assess the potential for transferring the CO2-concentrating system into higher plants. Some algae can take up carbon dioxide dissolved in water and photosynthesize sugars. How they concentrate the dissolved gas is unknown but could be an important trait to introduce into higher plants. This work investigates the mechanism of uptake and its subsequent enhancement of photosynthesis.***//