PARENT SESSION
Posters P4C Controling CO₂: Stomates and carbon concentrating mechanisms. Abstracts (631-642)

A novel evolutionary lineage of carbonic anhydrase (-class) is a component of the carboxysome shell. Anthony So¹, George Espie^*,1, Eric Williams², Jessup Shively³, Sabine Heinhorst², Gordon Cannon², ¹ Department of Botany, Mississauga, ON, Canada² Department of Chemistry and Biochemistry, Hattiesburg, MS, USA³ Department of Genetics and Biochemistry, Clemson, SC, USA

ABSTRACT- A significant portion of the total carbon fixed in the biosphere is attributed to the autotrophic metabolism of prokaryotes. In cyanobacteria and many chemolithoautotrophic bacteria, CO₂ fixation is catalyzed by ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) that is packaged in protein microcompartments called carboxysomes. These structures play an integral role in a cellular CO₂-concentrating mechanism and are essential components for autotrophic growth. Here we report that the carboxysomal shell protein, CsoS3, from Halothiobacillus neapolitanus, is a novel carbonic anhydrase (-class CA) that has an evolutionary lineage distinct from those previously recognized in animals, plants, and other prokaryotes. Functional CAs encoded by csoS3 homologues were also identified in the cyanobacteria Prochlorococcus sp. and Synechococcus sp., which dominate the oligotrophic oceans and are major contributors to primary productivity. The location of the carboxysomal CA in the shell suggests that it could supply the active sites of RuBisCO in the carboxysome with the high concentrations of CO2 necessary for optimal RuBisCO activity and efficient carbon fixation in these prokaryotes that are important contributors to the global carbon cycle.

KEY WORDS: - carbonic anhydrase, CO2 - concentrating mechanism, carboxysomes, autotrophic bacteria