[Carboxysomes of the thermophilic hydrogen bacterium Pseudomonas thermophila]

The technique of electron microscopy was used to detect the presence of paracrystal hexagonal inclusions in the cells and spheroplasts of the thermophilic hydrogen bacterium Pseudomonas thermophila. The connection of the inclusions with DNA threads can easily be seen on photomicrographs of the spheroplasts. The cells were disintegrated by freezing and thawing and the resultant homogenates were centrifuged in a sucrose density gradient; up to 80% of the activity of ribulose-1,5-diphosphate carboxylase (RDP carboxylase, EC 4.1.1.39) was found in the fraction of particles containing hexagonal paracrystal inclusions (carboxysomes). Their granular content and the tendency to diffuse were seen at a magnification x100,000. The percentage of the insoluble enzyme was higher in cells in the stationary growth phase than in growing cells. Only 25% of the enzyme activity was detected in the particles after the cells treated with lysozyme had been subjected to osmotic shock. A possible role of carboxysomes in cells as a compartment storing RDP carboxylase is discussed.