Received November 10, 2008
Accepted December 5, 2008
Thylakoid Lumen Carbonic Anhydrase (CAH3) Mutation Suppresses Air-dier Phenotype of LCIB Mutant in Chlamydomonas reinhardtii
Deqiang Duanmu , Yingjun Wang , and Martin H. Spalding *
Department of Genetics, Development and Cell Biology, Iowa State University, Ames, Iowa 50011
* Corresponding author; email: mspaldin{at}iastate.edu.
An active CO2 concentrating mechanism (CCM) is induced when Chlamydomonas reinhardtii acclimates to limiting inorganic carbon (Ci), either low-CO2 (air level; 
0.04% CO2) or very low-CO2 (0.01% CO2) conditions. A mutant, ad1, which is defective in the limiting-CO2-inducible, plastid localized LCIB, can grow in high-CO2 or very low-CO2 conditions but dies in low-CO2, indicating a deficiency in a low-CO2-specific Ci uptake and accumulation system. In this study, we identified two ad1 suppressors that can grow in low-CO2 but die in very low-CO2. Molecular analyses revealed that both suppressors have mutations in the CAH3 gene, which encodes a thylakoid lumen localized carbonic anhydrase. Photosynthetic rates of low-CO2 acclimated suppressors under acclimation CO2 concentrations were more than two fold higher than ad1, apparently resulting from a more than 20 fold increase in the intracellular concentration of Ci as measured by direct Ci uptake. However, photosynthetic rates of very low-CO2 acclimated cells under acclimation CO2 concentrations were too low to support growth in spite of a significantly elevated intracellular Ci concentration. We conclude that LCIB functions downstream of CAH3 in the CCM and probably plays a role in trapping CO2 released by CAH3 dehydration of accumulated Ci. Apparently dehydration by the chloroplast stromal carbonic anhydrase CAH6 of the very high internal Ci caused by the defect in CAH3 provides Rubisco sufficient CO2 to support growth in low-CO2 acclimated cells, but not in very low-CO2 acclimated cells, even in the absence of LCIB.
