CDNA CLONING AND GENE-EXPRESSION OF CARBONIC-ANHYDRASE IN CHLAMYDOMONAS-REINHARDTII

cDNA and genes encoding periplasmic carbonic anhydrase (CA) polypeptides of Chlamydomonas reinhardtii have been isolated and characterized. Nucleotide sequence analysis of cDNA clones revealed that the large subunit (35 kDa or 36.5 kDa) and the small subunit (4 kDa) are cotranslated as a precursor polypeptide (41 626 Da) with a NH2-terminal hydrophobic signal peptide of 20 amino acids. The amino acid sequence of Chlamydomonas CA showed 20-22% identity with animal CA isozymes (CAI, CAII, CAIII, and CAVII). Three zinc-liganded histidine residues and those forming the hydrogen-bond network to zinc-bound solvent molecules were highly conserved. No significant sequence similarity was observed between Chlamydomonas CA and chloroplast CAs of spinach and pea. Two copies of structurally related CA genes (CAH1 and CAH2) were tandemly clustered in Chlamydomonas nuclear genome and regulated by external CO2 concentration in a reverse manner. The 5' upstream gene CAH1 encodes the major periplasmic CA whose mRNA level is induced under low CO2 condition in light. Photosynthesis is absolutely required for the accumulation of the CAH1 mRNA. The 3' downstream gene CAH2 is possibly a gene for another periplasmic CA isozyme, which is induced under high CO2 conditions. Light has an inhibitory effect on the accumulation of the CAH2 mRNA.