Frequent spontaneous loss of the magnetic phenotype was observed in stationary-phase cultures of the magnetotactic bacterium Magnetospirillum gryphiswaldense MSR-1. A nonmagnetic mutant, designated strain MSR-1B, was isolated and characterized. The mutant lacked any structures resembling magnetosome crystals as well as internal membrane vesicles. The growth of strain MSR-1B was impaired under all growth conditions tested, and the uptake and accumulation of iron were drastically reduced under iron-replete conditions. A large chromosomal deletion of approximately 80 kb was identified in strain MSR-1B, which comprised both the entire mamAB and mamDC clusters as well as further putative operons encoding a number of magnetosomeassociated proteins. A bacterial artificial chromosome clone partially covering the deleted region was isolated from the genomic library of wild-type M. gryphiswaldense. Sequence analysis of this fragment revealed that all previously identified mam genes were closely linked with genes encoding other magnetosome-associated proteins within less than 35 kb. In addition, this region was remarkably rich in insertion elements and harbored a considerable number of unknown gene families which appeared to be specific for magnetotactic bacteria. Overall, these findings suggest the existence of a putative large magnetosome island in M. gryphiswaldense and other magnetotactic bacteria.Magnetotactic bacteria are capable of forming magnetosomes, which are specific intracellular structures that enable the cells to orient along magnetic field lines (3, 4, 41). The superior crystalline and magnetic properties of magnetosomes make them potentially useful as a highly ordered biomaterial in a number of applications, e.g., in the immobilization of bioactive compounds, magnetic drug targeting, or as a contrast agent for magnetic resonance imaging (24,41). Recently, the characteristics of bacterial magnetosomes have even been considered for use as biosignatures to identify presumptive Martian magnetofossils (49). Moreover, understanding bacterial magnetosome formation is expected to provide insights into more complex biomineralization systems in higher organisms (19). The biomineralization of magnetosome particles is achieved by a complex mechanism with control over the uptake, accumulation, and precipitation of iron, which, however, is poorly understood at the molecular and biochemical level.The magnetotactic ␣-proteobacterium Magnetospirillum gryphiswaldense microaerobically produces up to 60 cubo-octahedral magnetosomes, which are approximately 45 nm in size and consist of membrane-bounded crystals of the iron mineral magnetite (Fe 3 O 4 ) (34,42). In contrast to most other magnetotactic bacteria, methods for mass culture and genetic manipulation of M. gryphiswaldense are available (17,38,44), which has facilitated its analysis in a number of studies (37,39,40,43).In Magnetospirillum species, the deposition of the mineral particle occurs within a specific compartment, which is provided by the magnetosome membrane ...

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Iron uptake and magnetite (Fe3O4) crystal formation could be studied in the microaerophilic magnetic bacteriumMagnetospirillum gryphiswaldense by using a radioactive tracer method for iron transport and a differential light-scattering technique for magnetism. Magnetite formation occurred only in a narrow range of low oxygen concentration, i.e., 2 to 7 μM O2 at 30°C. Magnetic cells stored up to 2% iron as magnetite crystals in intracytoplasmic vesicles. This extraordinary uptake of iron was coupled tightly to the biomineralization of up to 60 magnetite crystals with diameters of 42 to 45 nm.

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Due to their highly regulated biosynthesis, magnetosomes biomineralized by magnetotactic bacteria represent natural magnetic nanoparticles with unique physical and chemical properties. They consist of a magnetite core that is surrounded by a biological membrane and are therefore reminiscent to magnetic “core–shell” nanoparticles. Their usability in many nanotechnological and biomedical applications would be further improved by the introduction of additional catalytic and imaging modalities. Here, a new in vivo strategy is explored for magnetosome display of foreign polypeptides with maximized protein‐to‐particle ratios. Arrays of up to five monomers of the model enzyme glucuronidase GusA plus the additional fluorophore mEGFP are genetically fused as single large hybrid proteins to highly expressed magnetosome protein anchors. In total, about 190 GusA monomers are covalently attached to individual particles. Assuming layers of GusA rows surrounding the particles, the monomers would thus cover up to 90% of the magnetosome surface. The approach generates nanoparticles that exhibit magnetism, fluorescence, and stable catalytic activities, which are stepwise increased with the number of GusA monomers. In summary, multicopy expression of arrayed foreign proteins represents a powerful methodology for the biosynthesis of tailored biohybrid magnetic nanoparticles with several genetically encoded and tunable functionalities.