THE REGULATION OF CARBONIC-ANHYDRASE AND RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE ACTIVASE BY LIGHT AND CO2 IN CHLAMYDOMONAS-REINHARDTII

We have investigated the regulation of accumulation of ribulose-1,5-bi sphosphate carboxylase/oxygenase activase and the periplasmic carbonic anhydrase (CA) in Chlamydomonas reinhardtii. In algae, the periplasmi c CA is required for efficient CO2 fixation when the CO2 concentration is low. These two proteins are affected differently by the CO2 level in the environment. The steady-state level of the ribulose-1,5-bisphos phate carboxylase/oxygenase activase transcript was only slightly and transiently affected by a reduction in ambient CO2 concentration, wher eas the CA transcript level was strongly induced by air containing amb ient (350 parts per million) CO2 (low CO2) conditions. The transcripts for both proteins showed strong oscillations when the alga was grown under a 12-h light/12-h dark growth regime, with the transcripts encod ing these proteins present just before the onset of the light cycle. T he observation that the CA transcript was made in the dark was surpris ing, since earlier reports indicated that active photosynthesis was re quired for the induction of the periplasmic CA. Further experiments de monstrated that the CA transcript was partially induced under low-CO2 conditions even when the switch to low CO2 was done in the dark. Our r esults suggest that C. reinhardtii might sense the CO2 concentration i n a more direct manner than through C-2 or C-3 cycle intermediates, wh ich has been previously suggested.