Microcompartments in Prokaryotes: Carboxysomes and Related Polyhedra

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Abstract
All cyanobacteria and many chemoautotrophs contain poly- hedral inclusion bodies that are bound by a unilamellar protein shell (15, 63). Isolation and enzymatic analysis of the bodies from Halothiobacillus neapolitanus (previously Thiobacillus neapolitanus) revealed that they are filled with ribulose 1,5- bisphosphate carboxylase/oxygenase (RuBisCO); therefore they were given the name "carboxysomes" (59). Subsequent studies of both cyanobacteria and chemoautotrophic bacteria have led to the well-accepted conclusion that the "organelles" or microcompartments function to enhance the catalytic prop- erties of the RuBisCO they contain, although the mechanism of this catalytic enhancement is unclear (51, 65). Localization and characterization of the genes encoding carboxysome com- ponents has underscored the apparent common function of these bodies in carboxysome-containing autotrophic bacteria. More surprising is the finding that a number of heterotrophic prokaryotes harbor genes homologous to those for carboxy- some shell proteins (9, 32, 62). Under proper growth condi- tions, these bacteria produce polyhedral inclusion bodies that are morphologically similar to carboxysomes, although the cells expressing these bodies contain no RuBisCO and do not fix CO2 via the Calvin-Benson-Bassham cycle as a major part of their carbon metabolism. This review evaluates the evidence that relates carboxysome structure to function in the carbon metabolism of autotrophic prokaryotes and examines similar- ities to newly discovered particles found in heterotrophs. The possibility is explored that microcompartmentalization of key metabolic enzymes by carboxysomes and their relatives is a more widely utilized regulatory mechanism in prokaryotes than was previously envisioned.