Use of Mutants in Analysis of the CO2-Concentrating Pathway of Chlamydomonas Reinhardtii

In Chlamydomonas reinhardtii and other green algae, a pathway which actively concentrates CO2 at the site of ribulose 1,5-bisphosphate-carbozylase/oxygenase (RUBISCO) is responsible for the suppression of photo-respiration and oxygen inhibition of photosynthesis and for the stimulation of photosynthesis at a low external CO2 concentrations. Increased photosynthesis and reduced photorepiration in Chlamydomonas at air levels of CO2 and O2 are manifested by a high affinity for CO2 in photosynthesis, a nearly maximal photosynthetic rate, absence of O2 inhibition of CO2 fixation, low rates of synthesis of photo respiratory metabolites, and a near-zero CO2 compensation concentration (1,2). The CO2-concentrating pathway of Chlamydomonas is inducible, with induction occurring at air levels of CO2 but not at elevated (1-5%) concentrations of CO2 (2). Biochemical and physiological studies implicated the involvement of at least two components in the pathway, an energy-dependent, saturable inorganic transport process (1,9) and the enzyme carbonic anhydrase (CA) (1,2). Badger et al. (1) suggested that the role of CA in the pathway might be dehydration of transported HCO3 to supple CO2, the substrate of RUBISCO. In order to further characterize the Chlamydomonas CO2-concentrating pathway, we utilized existing, nonphotosynthetic mutants of C. rienhardtii fo study of induction requirements and set out to identify and characterize new mutant strains of C. reinhardtii with defects in the CO2-concentrating pathway itself. This work with Chlamydomonas mutants has helped firmly establish the requirement for photosynthetic competence in the induction of the pathway, unambiguously confirmed that at least two components, CA and HCO3 transport, are involved, and that the principal role of internal CA is dehydration of transported HCO3