18] Inclusions: Carboxysomes

Abstract

This chapter focuses on inclusions in carboxysomes. Inclusions with polygonal profiles (polyhedral bodies) appear to be ubiquitous entities of the vegetative cells and akinetes of cyanobacteria. Heterocysts, which fix nitrogen but not carbon dioxide, do not possess polyhedral bodies. From 3 to 12 (mean number) of these inclusions occur in the nucleoplasmic region of the cell. As observed in thin section, the bodies are 100-900 nm in diameter, have a granular substructure, and are surrounded by a monolayer membrane (shell) 3–4 nm thick. These inclusions have been isolated from Anabaena cylindrica and Chlorogloeopsisfritschii and shown to contain the enzyme ribulose-l,5-bisphosphate carboxylase (RuBisCOase), thus identifying them as carboxysomes as originally described in the autotrophic sulfur bacterium Thiobacillus neapolitanus. In addition to the carboxysomal (particulate) RuBisCOase, the cells possess a soluble form of the enzyme. The mechanism(s) of the formation as well as the loss of carboxysomes is unknown. The disappearance of the inclusions in Thiobacillus intermedius when RuBisCOase is repressed appears to be by dilution (unpublished observations). The function(s) of the carboxysome is also unknown. Several theories have been proposed based on a variety of experimental data, including (l) an active role in carbon dioxide fixation, providing a more favorable environment—for example, reducing oxygen availability or concentrating carbon dioxide, and (2) an inactive role in fixation but preventing excessive turnover of the enzyme.