Characterization and expression analysis of genes encoding a and b carbonic anhydrases in Arabidopsis

Abstract

Carbonic anhydrases (CAs) are Zn-containing metalloenzymes that catalyse the reversible hydration of CO2 .W e investigated the aCA and bCA families in Arabidopsis, which contain eight aCA (AtaCA1-8) and six bCA genes (AtbCA1-6). Analyses of expressed sequence tags (ESTs) from The Arabidopsis Information Resource (TAIR) database indicate that all the bCA encoding sequences, but only three of the AtaCA, are expressed. Using semiquantitative PCR experiments, functional CA genes were more strongly expressed in green tissue, but strong expression was also found in roots for bCA3, bCA6 and aCA2. Two aCA genes were shown to respond to the CO2 environment, while the others were unresponsive. Using the green fluorescent reporter protein gene fused with cDNA sequences coding for bCAs, we provided evidence that bCAs were targeted to specific subcellular compartments: bCA1 and bCA5 were targeted to the chloroplast, bCA2 and bCA3 to the cytosol, bCA4 to the plasma membrane and bCA6 to the mitochondria. The targeting and the pattern of gene expression suggest that CA isoforms play specific roles in subcellular compartments, tissues and organs. The data indicate that other CA isoforms than the well-characterized bCA1 may contribute to the CO2 transfer in the cell to the catalytic site of ribulose 1·5bisphosphate carboxylase/oxygenase (Rubisco).