(CCM) (Price et al. 1998, Badger and Price 2003). Mutants unable to synthesize carboxysomes required high CO2 levels for autotrophic growth (Price and Badger 1989). In an elaborate experiment by Price and Badger (1989), it was demonstrated that expression of human carbonic anhydrase in the cytosol of the Synechoccocus PCC7942 resulted in a massive CO2 leak and an associated high CO2-requiring (HCR) phenotype. It was also demonstrated that by inhibiting the carbonic anhydrase the leak was stopped. Based on these results, they composed a model of cyanobacterial carboxysomes, whereby both carbonic anhydrase and RuBisCO are sequestered in the protein shell of the carboxysome in such a manner that RuBisCO molecules surround a central core composed of carbonic anhydrase. According to their model, bicarbonate as well as CO2 is transported into the cell, however during transport, CO2 is converted to bicarbonate, which is the only carbon species that enters the carboxysome. Once inside the carboxysome, the formation of CO2 from bicarbonate is catalyzed by carbonic anhydrase and CO2 is subsequently fixed by RuBisCO as it diffuses away from the center of the carboxysome. The CO2 leak caused by carbonic anhydrase expression in the cytosol was interpreted to be a result of bicarbonate being converted to CO2 before it entered the carboxysome. Research Overview S. enterica was shown to form polyhedral organelles that resemble carboxysomes during growth on PD ((Bobik et al. 1999)). Although visually similar to the carboxysome, these polyhedral organelles are unlikely to play a role in CO2concentration as S. enterica is not an autotroph and does not express RuBisCO. In several enteric bacteria such as Klebsiella, E. coli, and Salmonella spp. these polyhedral organelles are associated with the B12-dependent degradation of either ethanolamine, PD