Magnetotactic bacteria are widespread aquatic microorganisms that use unique intracellular organelles to navigate along the Earth's magnetic field. These organelles, called magnetosomes, consist of membrane-enclosed magnetite crystals that are thought to help to direct bacterial swimming towards growth-favouring microoxic zones at the bottom of natural waters1. Questions in the study of magnetosome formation include understanding the factors governing the size and redox-controlled synthesis of the nano-sized magnetosomes and their assembly into a regular chain in order to achieve the maximum possible magnetic moment, against the physical tendency of magnetosome agglomeration. A deeper understanding of these mechanisms is expected from studying the genes present in the identified chromosomal ‘magnetosome island’, for which the connection with magnetosome synthesis has become evident2. Here we use gene deletion in Magnetospirillum gryphiswaldense to show that magnetosome alignment is coupled to the presence of the mamJ gene product. MamJ is an acidic protein associated with a novel filamentous structure, as revealed by fluorescence microscopy and cryo-electron tomography. We suggest a mechanism in which MamJ interacts with the magnetosome surface as well as with a cytoskeleton-like structure. According to our hypothesis, magnetosome architecture represents one of the highest structural levels achieved in prokaryotic cells.
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We thank P. Graumann for advice on fluorescence microscopy and F. Widdel for helpful comments. This research was supported by the Max Planck Society and the Biofuture program of the Bundesministerium für Bildung und Forschung. Author Contributions A.S. carried out all genetic and growth experiments and performed fluorescence and TEM microscopy. M.G. carried out cryo-electron tomography and analysis of tomograms. D.F. participated in induction experiments. A.L. participated in three-dimensional visualization. J.M.P. directed cryo-electron tomography, EFTEM experiments and data analysis. D.S. coordinated the study and with A.S. finalized the manuscript.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
Text to accompany the below Supplementary Figures. (DOC 26 kb)
Domain structure of the MamJ protein (PDF 224 kb)
Molecular organization of the mamAB cluster in the wild type and δmamJ (PDF 13 kb)
Cryo-ET of a wild type cell showing a chain of mature magnetosome crystals located adjacent to the cytoplasmic membrane (PDF 1853 kb)
Three-dimensional reconstruction of magnetosome organization along a cytoskeleton-like structure in a wild-type M. gryphiswaldense cell obtained by Cryo-ET. (MOV 9703 kb)
Text to accompany the above Supplementary Video. (DOC 23 kb)
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