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“Introduction Carboxysomes are metabolic modules for CO2 fixation that are found in all cyanobacteria and some chemoautotrophic bacteria (Badger and Price CYC202 2003; Cannon et al. 2001; Yeates et al. 2008). They are self-assembling, apparently icosahedral organelles of ~80–150 nm comprised entirely of protein (Schmid et al. 2006) (Fig. 1). Carboxysomes encapsulate a carbonic anhydrase (CA, Price et al. 1992), which converts bicarbonate to carbon dioxide, and most, if not all, cellular ribulose bisphosphate carboxylase oxygenase (RuBisCO) (Cannon and Shively 1983; Lichtle et al. 1995), the enzyme that catalyzes the first step in the Calvin–Benson cycle by

combining CO2 and ribulose-1,5-bisphosphate (RuBP) to form two molecules of 3-phosphoglycerate (3PGA) (Fig. 2). Given that cyanobacteria carry out a large fraction of the total oxygenic photosynthesis on our planet, the carboxysome plays a Akt inhibitor significant role in the Earth’s primary production (Partensky et al. 1999; Whitman et al. 1998). Fig. 1 Transmission electron micrograph of Synechocystis sp. PCC6803 cells showing

three carboxysomes. Image courtesy of Patrick Shih, UC Berkeley Fig. 2 Schematic diagram of a cyanobacterial cell containing a carboxysome and depicting relevant metabolites that cross the G protein-coupled receptor kinase cell membrane and carboxysome shell. The carboxysome-encapsulated reactions are shown. Those related to photorespiration catalyzed by RuBisCO in the presence of oxygen are shown in dashed lines Structural and functional overview Two types of carboxysome have been characterized: the α-carboxysome, which encapsulates Form IA RuBisCO, and the β-carboxysome, which encapsulates Form IB RuBisCO (Badger and Bek 2008; Tabita 1999). α-carboxysomes are found in Prochlorococcus and some marine Synechococcus species as well as in some chemoautotrophic bacteria. The β-carboxysomes are found in all other cyanobacteria, with the exception of an unusual marine species, UCYN-A (Tripp et al. 2010). In addition to differing in the encapsulated form of RuBisCO, α- and β-carboxysomes also differ in gene organization; components of the α-carboxysome are organized into an operon whereas the genes for the β-carboxysome components are generally more dispersed (Fig. 3). Fig. 3 Three examples of carboxysome gene clusters for a β-carboxysome (top) of Synechocystis PCC 6803 and two α-carboxysomes (bottom), from the cyanobacterium Prochlorococcus marinus MED4 and from a chemoautotroph Halothiobacillus neapolitanus.