LIGHT-INDUCED CARBONIC-ANHYDRASE EXPRESSION IN CHLAMYDOMONAS-REINHARDTII

The effect of external inorganic carbon concentration and light on carbonic anhydrase (CA) protein accumulation and steady-state mRNA levels were examined in Chlamydomonas reinhardtii. When photoautotrophically grown high-CO2 cells were transferred to low-CO2 conditions, they exhibited a significant accumulation of the 2.0-kilobase CA transcript after 1 hour with the maximum level reached after 2 hours. An increase in the accumulation of the 37-kilodalton CA monomer was observed after 2-hour exposure to air. Cells allowed to adapt to air levels of CO2 in the dark showed neither an increase in CA mRNA abundance nor in the accumulation of the enzyme. Similarly, addition of 10 micromole 3-(3,4-dichlorophenyl)-1,1-dimethylurea immediately after transferring high-CO2 cells to low-CO2 condition did not cause an increase in CA transcript abundance and enzyme accumulation, suggesting that photosynthesis is absolutely required in the regulation of CA transcript abundance. In addition to the photosynthesis-dependent process, a blue light stimulated mechanism is also involved in CA transcript regulation. Experiments with transcription inhibitors confirmed the notion that the gene for carbonic anhydrase enzyme is encoded by the nuclear genome and that the induction of the enzyme depends heavily on the transcription of the CA gene.