K-076. Isolation and Biochemical Characterization of Carboxysomes from the Chemolithoautotrophs Thiomonas intermedia and Thiomicrospira crunogena

B. B. Menon1, Z. Dou1, J. M. Shively2, S. Heinhorst1, G. C. Cannon1;
1Univ. of Southern Mississippi, Hattiesburg, MS, 2Clemson Univ., Clemson, SC.

The polyhedral microcompartments, termed carboxysomes, that exist in many autotrophic bacteria consist of the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) encased within a thin proteinaceous layer (shell). Sequestration of RuBisCO into a simple “organelle” provides a catalytic advantage that clearly enhances the CO2 fixation ability of the bacteria that harbor these microcompartments. The obligate chemolithoautotroph, Halothiobacillus neapolitanus, has served as an excellent model for studying various structure-function aspects of carboxysomes, but little is known about the mechanistic and architectural properties of carboxysomes in other chemoautotrophic species. This study was undertaken to isolate, analyze, and compare the structural and functional characteristics of carboxysomes from Thiomonas intermedia, a facultative autotroph that is typically found in soil, mud, and freshwater or marine sediments, and from Thiomicrospira crunogena, a hydrothermal vent gammaproteobacterium. The carboxysome (cso) operons of these three bacteria are indistinguishable with respect to known carboxysome gene complement and arrangement, and their carboxysomes are very similar in size and shape, as revealed by electron microscopy. The polypeptide compositions of the carboxysome shell, however, while similar, have also revealed some intriguing variations. Current efforts are underway to link these to differences in carboxysome architecture and function.