Characterization and expression analysis of genes encoding α and β carbonic anhydrases in Arabidopsis

Carbonic anhydrases (CAs) are Zn-containing metalloenzymes that catalyse the reversible hydration of CO2. We investigated the alpha CA and beta CA families in Arabidopsis, which contain eight alpha CA (At alpha CA1-8) and six beta CA genes (At beta CA1-6). Analyses of expressed sequence tags (ESTs) from The Arabidopsis Information Resource (TAIR) database indicate that all the beta CA encoding sequences, but only three of the At alpha CA, are expressed. Using semi-quantitative PCR experiments, functional CA genes were more strongly expressed in green tissue, but strong expression was also found in roots for beta CA3, beta CA6 and alpha CA2. Two alpha CA 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 beta CAs, we provided evidence that beta CAs were targeted to specific subcellular compartments: beta CA1 and beta CA5 were targeted to the chloroplast, beta CA2 and beta CA3 to the cytosol, beta CA4 to the plasma membrane and beta CA6 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 beta CA1 may contribute to the CO2 transfer in the cell to the catalytic site of ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco).