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In JoVE (1)
Other Publications (4)
Articles by Dylan M. Morris in JoVE
JoVE General
Electron Cryotomography of Bacterial Cells
1Division of Biology, California Institute of Technology - Caltech, 2Howard Hughes Medical Institute, California Institute of Technology - Caltech
We illustrate here how to use electron cryotomography (ECT) to study the ultrastructure of bacterial cells in near-native states, to "macromolecular" (~4 nm) resolution.
Other articles by Dylan M. Morris on PubMed
The Structure of Isolated Synechococcus Strain WH8102 Carboxysomes As Revealed by Electron Cryotomography
Carboxysomes are organelle-like polyhedral bodies found in cyanobacteria and many chemoautotrophic bacteria that are thought to facilitate carbon fixation. Carboxysomes are bounded by a proteinaceous outer shell and filled with ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO), the first enzyme in the CO(2) fixation pathway, but exactly how they enhance carbon fixation is unclear. Here we report the three-dimensional structure of purified carboxysomes from Synechococcus species strain WH8102 as revealed by electron cryotomography. We found that while the sizes of individual carboxysomes in this organism varied from 114 nm to 137 nm, surprisingly, all were approximately icosahedral. There were on average approximately 250 RuBisCOs per carboxysome, organized into three to four concentric layers. Some models of carboxysome function depend on specific contacts between individual RuBisCOs and the shell, but no evidence of such contacts was found: no systematic patterns of connecting densities or RuBisCO positions against the shell's presumed hexagonal lattice could be discerned, and simulations showed that packing forces alone could account for the layered organization of RuBisCOs.
Toward a Biomechanical Understanding of Whole Bacterial Cells
Following decades of research in genetics and biochemistry, the basic metabolism of bacteria is now well understood. In addition to core metabolism, however, bacterial cells also perform a number of mechanical tasks such as maintaining a characteristic shape, moving within their environment, segregating their genome, and dividing. Major advances in imaging technologies including fluorescence light microscopy (fLM) and electron cryotomography (ECT) have provided new insight into the bacterial ultrastructures that accomplish these tasks. It is now clear, for instance, that bacteria are highly organized, possessing cytoskeletons, specifically arranged genomes, internal compartments, and carefully positioned macromolecular machines. These structures and their functions are reviewed here in the form of a progress report toward a complete biomechanical understanding of a generalized bacterial cell. The goal of eventually integrating genetic, biochemical, imaging, and biophysical data into spatially explicit, mechanically predictive models of whole cells is highlighted.
Organization, Structure, and Assembly of Alpha-carboxysomes Determined by Electron Cryotomography of Intact Cells
Carboxysomes are polyhedral inclusion bodies that play a key role in autotrophic metabolism in many bacteria. Using electron cryotomography, we examined carboxysomes in their native states within intact cells of three chemolithoautotrophic bacteria. We found that carboxysomes generally cluster into distinct groups within the cytoplasm, often in the immediate vicinity of polyphosphate granules, and a regular lattice of density frequently connects granules to nearby carboxysomes. Small granular bodies were also seen within carboxysomes. These observations suggest a functional relationship between carboxysomes and polyphosphate granules. Carboxysomes exhibited greater size, shape, and compositional variability in cells than in purified preparations. Finally, we observed carboxysomes in various stages of assembly, as well as filamentous structures that we attribute to misassembled shell protein. Surprisingly, no more than one partial carboxysome was ever observed per cell. Based on these observations, we propose a model for carboxysome assembly in which the shell and the internal RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) lattice form simultaneously, likely guided by specific interactions between shell proteins and RuBisCOs.
Peptidoglycan Remodeling and Conversion of an Inner Membrane into an Outer Membrane During Sporulation
Two hallmarks of the Firmicute phylum, which includes the Bacilli and Clostridia classes, are their ability to form endospores and their "Gram-positive" single-membraned, thick-cell-wall envelope structure. Acetonema longum is part of a lesser-known family (the Veillonellaceae) of Clostridia that form endospores but that are surprisingly "Gram negative," possessing both an inner and outer membrane and a thin cell wall. Here, we present macromolecular resolution, 3D electron cryotomographic images of vegetative, sporulating, and germinating A. longum cells showing that during the sporulation process, the inner membrane of the mother cell is inverted and transformed to become the outer membrane of the germinating cell. Peptidoglycan persists throughout, leading to a revised, "continuous" model of its role in the process. Coupled with genomic analyses, these results point to sporulation as a mechanism by which the bacterial outer membrane may have arisen and A. longum as a potential "missing link" between single- and double-membraned bacteria.
