Carbonic anhydrase is an ancient enzyme widespread in prokaryotes

Carbonic anhydrases catalyze the reversible hydration of CO2 and are ubiqui tous in highly evolved eukaryotes, The recent identification of a third cla ss of carbonic anhydrase (gamma class) in a methanoarchaeon and our present finding that the beta class also extends into thermophilic species from th e Archaea domain led us to initiate a systematic search for these enzymes i n metabolically and phylogenetically diverse prokaryotes. Here we show that carbonic anhydrase is widespread in the Archaea and Bacteria domains, and is an ancient enzyme. The occurrence in chemolithoautotrophic species occup ying deep branches of the universal phylogenetic tree suggests a role for t his enzyme in the proposed autotrophic origin of life. The presence of the beta and gamma classes in metabolically diverse species spanning the Archae a and Bacteria domains demonstrates that carbonic anhydrases have a far mor e extensive and fundamental role in prokaryotic biology than previously rec ognized.