Expression of a periplasmic β-carbonic anhydrase (CA) gene is positively correlated with HCO3- utilization by the gametophytes of Saccharina japonica (Phaeophyceae, Ochrophyta).

As an ecologically and economically important seaweed, Saccharina japonica has developed strategies to utilize HCO3-. In general, seaweeds have at least three mechanisms for HCO3- acquisition, one of which is the use of extracellular carbonic anhydrases (CA) to convert HCO3- into CO2 for utilization. However, it is unknown which CA in S. japonica performs this function. In order to find the extracellular CA in S. japonica that exerts utilization of HCO3-, in this study, the cloning, characterization, and subcellular localization of Sjβ-CA are described. This enzyme has a full-length cDNA of 1397 bp with a 170-bp 5'-untranslated region (UTR), a 282-bp 3'-UTR, and a 945-bp open reading frame encoding a protein precursor consisting of 314 amino acids which contains a predicted 28-residue signal peptide. Enzyme activity assays showed that the recombinant Sjβ-CA in Escherichia coli possessed CO2 hydration and dehydration activities, thus identifying this gene Sjβ-CA in function. Immunogold electron microscopic observations with the prepared anti-Sjβ-CA polyclonal antibody illustrated that Sjβ-CA was located in periplasmic space of the kelp gametophyte cells. A positive correlation between the gene transcription and the level of exogenous HCO3- utilization was also established. These findings provide a molecular and cellular basis for understanding the mechanism of inorganic carbon absorption of this important kelp. [ABSTRACT FROM AUTHOR]