Carbonic anhydrase in the marine diatom Thalassiosira weissflogii (Bacillariophyceae)

The zinc metalloenzyme carbonic anhydrase plays a critical role in inorganic carbon acquisition in marine diatoms, thus conferring on zinc a key role in oceanic carbon cycling. As a first step in determining the location and function of carbonic anhydrase (CA) in Bacillariophyceae, we purified and partially sequenced CA from T. weissflogii (Gru) Fryxell et Hasle (TWCA1) and cloned the corresponding cDNA (twca1). The twca1 sequence is different from other known algal carbonic anhydrase genes, and encodes a protein of roughly 34 kDa. The amino terminal amino acids sequenced from purified TWCA1 are 72 residues downstream of the putative starting methionine predicted by twca1. This difference may be due to the presence of a short-lived signal sequence designed to guide the enzyme to the correct cellular location. The absence of any homology between TWCA1 and previously sequenced CAs from Chlorophyceae may indicate either convergent evolution or that carbon acquisition represents a fundamental physiological difference among algal phyla.