Exploring the host range for genetic transfer of magnetic organelle biosynthesis.

Magnetosomes produced by magnetotactic bacteria have great potential for application in biotechnology and medicine due to their unique physicochemical properties and high biocompatibility. Attempts to transfer the genes for magnetosome biosynthesis into non-magnetic organisms have had mixed results. Here we report on a systematic study to identify key components needed for magnetosome biosynthesis after gene transfer. We transfer magnetosome genes to 25 proteobacterial hosts, generating seven new magnetosome-producing strains. We characterize the recombinant magnetosomes produced by these strains and demonstrate that denitrification and anaerobic photosynthesis are linked to the ability to synthesize magnetosomes upon the gene transfer. In addition, we show that the number of magnetosomes synthesized by a foreign host negatively correlates with the guanine-cytosine content difference between the host and the gene donor. Our findings have profound implications for the generation of magnetized living cells and the potential for transgenic biogenic magnetic nanoparticle production.

Complete genome sequencing and annotation of Rhodomicrobium vannielii strain DSM166 suggest affiliation to Rhodomicrobium lacus.

Rhodomicrobium vannielii is a multicellular and differentiating member of the order Hyphomicrobiales in the class Alphaproteobacteria. Here, we report the complete genome of strain DSM166 obtained by PacBio SMRT sequencing. The results suggest that this strain is closely related to Rhodomicrobium lacus.

Experimental analysis of diverse actin-like proteins from various magnetotactic bacteria by functional expression in Magnetospirillum gryphiswaldense.

IMPORTANCE: To efficiently navigate within the geomagnetic field, magnetotactic bacteria (MTB) align their magnetosome organelles into chains, which are organized by the actin-like MamK protein. Although MamK is the most highly conserved magnetosome protein common to all MTB, its analysis has been confined to a small subgroup owing to the inaccessibility of most MTB. Our study takes advantage of a genetically tractable host where expression of diverse MamK orthologs together with a resurrected MamK LUCA and uncharacterized actin-like Mad28 proteins from deep-branching MTB resulted in gradual restoration of magnetosome chains in various mutants. Our results further indicate the existence of species-specific MamK interactors and shed light on the evolutionary relationships of one of the key proteins associated with bacterial magnetotaxis.

Comparing the Colloidal Stabilities of Commercial and Biogenic Iron Oxide Nanoparticles That Have Potential In Vitro/In Vivo Applications.

For the potential in vitro/in vivo applications of magnetic iron oxide nanoparticles, their stability in different physiological fluids has to be ensured. This important prerequisite includes the preservation of the particles' stability during the envisaged application and, consequently, their invariance with respect to the transfer from storage conditions to cell culture media or even bodily fluids. Here, we investigate the colloidal stabilities of commercial nanoparticles with different coatings as a model system for biogenic iron oxide nanoparticles (magnetosomes) isolated from magnetotactic bacteria. We demonstrate that the stability can be evaluated and quantified by determining the intensity-weighted average of the particle sizes (Z-value) obtained from dynamic light scattering experiments as a simple quality criterion, which can also be used as an indicator for protein corona formation.

Functional expression of foreign magnetosome genes in the alphaproteobacterium Magnetospirillum gryphiswaldense.

Magnetosomes of magnetotactic bacteria (MTB) consist of structurally perfect, nano-sized magnetic crystals enclosed within vesicles of a proteo-lipid membrane. In species of Magnetospirillum, biosynthesis of their cubo-octahedral-shaped magnetosomes was recently demonstrated to be a complex process, governed by about 30 specific genes that are comprised within compact magnetosome gene clusters (MGCs). Similar, yet distinct gene clusters were also identified in diverse MTB that biomineralize magnetosome crystals with different, genetically encoded morphologies. However, since most representatives of these groups are inaccessible by genetic and biochemical approaches, their analysis will require the functional expression of magnetosome genes in foreign hosts. Here, we studied whether conserved essential magnetosome genes from closely and remotely related MTB can be functionally expressed by rescue of their respective mutants in the tractable model Magnetospirillum gryphiswaldense of the Alphaproteobacteria. Upon chromosomal integration, single orthologues from other magnetotactic Alphaproteobacteria restored magnetosome biosynthesis to different degrees, while orthologues from distantly related Magnetococcia and Deltaproteobacteria were found to be expressed but failed to re-induce magnetosome biosynthesis, possibly due to poor interaction with their cognate partners within multiprotein magnetosome organelle of the host. Indeed, co-expression of the known interactors MamB and MamM from the alphaproteobacterium Magnetovibrio blakemorei increased functional complementation. Furthermore, a compact and portable version of the entire MGCs of M. magneticum was assembled by transformation-associated recombination cloning, and it restored the ability to biomineralize magnetite both in deletion mutants of the native donor and M. gryphiswaldense, while co-expression of gene clusters from both M. gryphiswaldense and M. magneticum resulted in overproduction of magnetosomes. IMPORTANCE We provide proof of principle that Magnetospirillum gryphiswaldense is a suitable surrogate host for the functional expression of foreign magnetosome genes and extended the transformation-associated recombination cloning platform for the assembly of entire large magnetosome gene cluster, which could then be transplanted to different magnetotactic bacteria. The reconstruction, transfer, and analysis of gene sets or entire magnetosome clusters will be also promising for engineering the biomineralization of magnetite crystals with different morphologies that would be valuable for biotechnical applications.

Long-Term Stability, Biocompatibility, and Magnetization of Suspensions of Isolated Bacterial Magnetosomes.

Magnetosomes are magnetic nanoparticles biosynthesized by magnetotactic bacteria. Due to a genetically strictly controlled biomineralization process, the ensuing magnetosomes have been envisioned as agents for biomedical and clinical applications. In the present work, different stability parameters of magnetosomes isolated from Magnetospirillum gryphiswaldense upon storage in suspension (HEPES buffer, 4 °C, nitrogen atmosphere) for one year in the absence of antibiotics are examined. The magnetic potency, measured by the saturation magnetization of the particle suspension, drops to one-third of its starting value within this year-about ten times slower than at ambient air and room temperature. The particle size distribution, the integrity of the surrounding magnetosome membrane, the colloidal stability, and the biocompatibility turn out to be not severely affected by long-term storage.

The transcriptomic landscape of Magnetospirillum gryphiswaldense during magnetosome biomineralization.

BACKGROUND: One of the most complex prokaryotic organelles are magnetosomes, which are formed by magnetotactic bacteria as sensors for navigation in the Earth's magnetic field. In the alphaproteobacterium Magnetospirillum gryphiswaldense magnetosomes consist of chains of magnetite crystals (Fe3O4) that under microoxic to anoxic conditions are biomineralized within membrane vesicles. To form such an intricate structure, the transcription of > 30 specific structural genes clustered within the genomic magnetosome island (MAI) has to be coordinated with the expression of an as-yet unknown number of auxiliary genes encoding several generic metabolic functions. However, their global regulation and transcriptional organization in response to anoxic conditions most favorable for magnetite biomineralization are still unclear. RESULTS: Here, we compared transcriptional profiles of anaerobically grown magnetosome forming cells with those in which magnetosome biosynthesis has been suppressed by aerobic condition. Using whole transcriptome shotgun sequencing, we found that transcription of about 300 of the > 4300 genes was significantly enhanced during magnetosome formation. About 40 of the top upregulated genes are directly or indirectly linked to aerobic and anaerobic respiration (denitrification) or unknown functions. The mam and mms gene clusters, specifically controlling magnetosome biosynthesis, were highly transcribed, but constitutively expressed irrespective of the growth condition. By Cappable-sequencing, we show that the transcriptional complexity of both the MAI and the entire genome decreased under anaerobic conditions optimal for magnetosome formation. In addition, predominant promoter structures were highly similar to sigma factor σ70 dependent promoters in other Alphaproteobacteria. CONCLUSIONS: Our transcriptome-wide analysis revealed that magnetite biomineralization relies on a complex interplay between generic metabolic processes such as aerobic and anaerobic respiration, cellular redox control, and the biosynthesis of specific magnetosome structures. In addition, we provide insights into global regulatory features that have remained uncharacterized in the widely studied model organism M. gryphiswaldense, including a comprehensive dataset of newly annotated transcription start sites and genome-wide operon detection as a community resource (GEO Series accession number GSE197098).

A Magnetosome-Based Platform for Flow Biocatalysis.

Biocatalysis in flow reactor systems is of increasing importance for the transformation of the chemical industry. However, the necessary immobilization of biocatalysts remains a challenge. We here demonstrate that biogenic magnetic nanoparticles, so-called magnetosomes, represent an attractive alternative for the development of nanoscale particle formulations to enable high and stable conversion rates in biocatalytic flow processes. In addition to their intriguing material characteristics, such as high crystallinity, stable magnetic moments, and narrow particle size distribution, magnetosomes offer the unbeatable advantage over chemically synthesized nanoparticles that foreign protein "cargo" can be immobilized on the enveloping membrane via genetic engineering and thus, stably presented on the particle surface. To exploit these advantages, we develop a modular connector system in which abundant magnetosome membrane anchors are genetically fused with SpyCatcher coupling groups, allowing efficient covalent coupling with complementary SpyTag-functionalized proteins. The versatility of this approach is demonstrated by immobilizing a dimeric phenolic acid decarboxylase to SpyCatcher magnetosomes. The functionalized magnetosomes outperform similarly functionalized commercial particles by exhibiting stable substrate conversion during a 60 h period, with an average space-time yield of 49.2 mmol L-1 h-1. Overall, our results demonstrate that SpyCatcher magnetosomes significantly expand the genetic toolbox for particle surface functionalization and increase their application potential as nano-biocatalysts.

In vivo Architecture of the Polar Organizing Protein Z (PopZ) Meshwork in the Alphaproteobacteria Magnetospirillum gryphiswaldense and Caulobacter crescentus.

The polar organizing protein Z (PopZ) forms a polar microdomain that is inaccessible to larger macromolecules such as ribosomes, and selectively sequesters proteins crucial for cell cycle control and polar morphogenesis in various Alphaproteobacteria. However, the in vivo architecture of this microdomain has remained elusive. Here, we analyzed the three-dimensional ultrastructural organization of the PopZ network in Magnetospirillum gryphiswaldense and Caulobacter crescentus by Volta phase plate cryo-electron tomography, which provides high spatial resolution and improved image contrast. Our results suggest that PopZ forms a porous network of disordered short, flexible, and branching filaments.

The Complex Transcriptional Landscape of Magnetosome Gene Clusters in Magnetospirillum gryphiswaldense.

Magnetosomes are complex membrane organelles synthesized by magnetotactic bacteria (MTB) for navigation in the Earth's magnetic field. In the alphaproteobacterium Magnetospirillum gryphiswaldense, all steps of magnetosome formation are tightly controlled by >30 specific genes arranged in several gene clusters. However, the transcriptional organization of the magnetosome gene clusters has remained poorly understood. Here, by applying Cappable-seq and whole-transcriptome shotgun RNA sequencing, we show that mamGFDCop and feoAB1op are transcribed as single transcriptional units, whereas multiple transcription start sites (TSS) are present in mms6op, mamXYop, and the long (>16 kb) mamABop. Using a bioluminescence reporter assay and promoter knockouts, we demonstrate that most of the identified TSS originate from biologically meaningful promoters which mediate production of multiple transcripts and are functionally relevant for proper magnetosome biosynthesis. In addition, we identified a strong promoter in a large intergenic region within mamXYop, which likely drives transcription of a noncoding RNA important for gene expression in this operon. In summary, our data suggest a more complex transcriptional architecture of the magnetosome operons than previously recognized, which is largely conserved in other magnetotactic Magnetospirillum species and, thus, is likely fundamental for magnetosome biosynthesis in these organisms. IMPORTANCE Magnetosomes have emerged as a model system to study prokaryotic organelles and a source of biocompatible magnetic nanoparticles for various biomedical applications. However, the lack of knowledge about the transcriptional organization of magnetosome gene clusters has severely impeded the engineering, manipulation, and transfer of this highly complex biosynthetic pathway into other organisms. Here, we provide a high-resolution image of the previously unappreciated transcriptional landscape of the magnetosome operons. Our findings are important for further unraveling the complex genetic framework of magnetosome biosynthesis. In addition, they will facilitate the rational reengineering of magnetic bacteria for improved bioproduction of tunable magnetic nanoparticles, as well as transplantation of magnetosome biosynthesis into foreign hosts by synthetic biology approaches. Overall, our study exemplifies how a genetically complex pathway is orchestrated at the transcriptional level to ensure the balanced expression of the numerous constituents required for the proper assembly of one of the most intricate prokaryotic organelles.

Biocompatibility, uptake and subcellular localization of bacterial magnetosomes in mammalian cells.

Magnetosomes represent biogenic, magnetic nanoparticles biosynthesized by magnetotactic bacteria. Subtle biological control on each step of biomineralization generates core-shell nanoparticles of high crystallinity, strong magnetization and uniform shape and size. These features make magnetosomes a promising alternative to chemically synthesized nanoparticles for many applications in the biotechnological and biomedical field, such as their usage as biosensors in medical diagnostics, as drug-delivery agents, or as contrast agents for magnetic imaging techniques. Thereby, the particles are directly applied to mammalian cells or even injected into the body. In the present work, we provide a comprehensive characterization of isolated magnetosomes as potential cytotoxic effects and particle uptake have not been well studied so far. Different cell lines including cancer cells and primary cells are incubated with increasing particle amounts, and effects on cell viability are investigated. Obtained data suggest a concentration-dependent biocompatibility of isolated magnetosomes for all tested cell lines. Furthermore, magnetosome accumulation in endolysosomal structures around the nuclei is observed. Proliferation rates are affected in the presence of increasing particle amounts; however, viability is not affected and doubling times can be restored by reducing the magnetosome concentration. In addition, we evidence magnetosome-cell interactions that are strong enough to allow for magnetic cell sorting. Overall, our study not only assesses the biocompatibility of isolated magnetosomes, but also evaluates effects on cell proliferation and the fate of internalized magnetosomes, thereby providing prerequisites for their future in vivo application as biomedical agents.

High-Yield Production, Characterization, and Functionalization of Recombinant Magnetosomes in the Synthetic Bacterium Rhodospirillum rubrum "magneticum".

Recently, the photosynthetic Rhodospirillum rubrum has been endowed with the ability of magnetosome biosynthesis by transfer and expression of biosynthetic gene clusters from the magnetotactic bacterium Magnetospirillum gryphiswaldense. However, the growth conditions for efficient magnetite biomineralization in the synthetic R. rubrum "magneticum", as well as the particles themselves (i.e., structure and composition), have so far not been fully characterized. In this study, different cultivation strategies, particularly the influence of temperature and light intensity, are systematically investigated to achieve optimal magnetosome biosynthesis. Reduced temperatures ≤16 °C and gradual increase in light intensities favor magnetite biomineralization at high rates, suggesting that magnetosome formation might utilize cellular processes, cofactors, and/or pathways that are linked to photosynthetic growth. Magnetosome yields of up to 13.6 mg magnetite per liter cell culture are obtained upon photoheterotrophic large-scale cultivation. Furthermore, it is shown that even more complex, i.e., oligomeric, catalytically active functional moieties like enzyme proteins can be efficiently expressed on the magnetosome surface, thereby enabling the in vivo functionalization by genetic engineering. In summary, it is demonstrated that the synthetic R. rubrum "magneticum" is a suitable host for high-yield magnetosome biosynthesis and the sustainable production of genetically engineered, bioconjugated magnetosomes.

Sesbanimide R, a Novel Cytotoxic Polyketide Produced by Magnetotactic Bacteria.

Genomic information from various magnetotactic bacteria suggested that besides their common ability to form magnetosomes, they potentially also represent a source of bioactive natural products. By using targeted deletion and transcriptional activation, we connected a large biosynthetic gene cluster (BGC) of the trans-acyltransferase polyketide synthase (trans-AT PKS) type to the biosynthesis of a novel polyketide in the alphaproteobacterium Magnetospirillum gryphiswaldense Structure elucidation by mass spectrometry and nuclear magnetic resonance spectroscopy (NMR) revealed that this secondary metabolite resembles sesbanimides, which were very recently reported from other taxa. However, sesbanimide R exhibits an additional arginine moiety the presence of which reconciles inconsistencies in the previously proposed sesbanimide biosynthesis pathway observed when comparing the chemical structure and the potential biochemistry encoded in the BGC. In contrast to the case with sesbanimides D, E, and F, we were able to assign the stereocenter of the arginine moiety experimentally and two of the remaining three stereocenters by predictive biosynthetic tools. Sesbanimide R displayed strong cytotoxic activity against several carcinoma cell lines.IMPORTANCE The findings of this study contribute a new secondary metabolite member to the glutarimide-containing polyketides. The determined structure of sesbanimide R correlates with its cytotoxic bioactivity, characteristic for members of this family. Sesbanimide R represents the first natural product isolated from magnetotactic bacteria and identifies this highly diverse group as a so-far-untapped source for the future discovery of novel secondary metabolites.

Induction of Axonal Outgrowth in Mouse Hippocampal Neurons via Bacterial Magnetosomes.

Magnetosomes are membrane-enclosed iron oxide crystals biosynthesized by magnetotactic bacteria. As the biomineralization of bacterial magnetosomes can be genetically controlled, they have become promising nanomaterials for bionanotechnological applications. In the present paper, we explore a novel application of magnetosomes as nanotool for manipulating axonal outgrowth via stretch-growth (SG). SG refers to the process of stimulation of axonal outgrowth through the application of mechanical forces. Thanks to their superior magnetic properties, magnetosomes have been used to magnetize mouse hippocampal neurons in order to stretch axons under the application of magnetic fields. We found that magnetosomes are avidly internalized by cells. They adhere to the cell membrane, are quickly internalized, and slowly degrade after a few days from the internalization process. Our data show that bacterial magnetosomes are more efficient than synthetic iron oxide nanoparticles in stimulating axonal outgrowth via SG.

Identification and elimination of genomic regions irrelevant for magnetosome biosynthesis by large-scale deletion in Magnetospirillum gryphiswaldense.

BACKGROUND: Magnetosome formation in the alphaproteobacterium Magnetospirillum gryphiswaldense is controlled by more than 30 known mam and mms genes clustered within a large genomic region, the 'magnetosome island' (MAI), which also harbors numerous mobile genetic elements, repeats, and genetic junk. Because of the inherent genetic instability of the MAI caused by neighboring gene content, the elimination of these regions and their substitution by a compact, minimal magnetosome expression cassette would be important for future analysis and engineering. In addition, the role of the MAI boundaries and adjacent regions are still unclear, and recent studies indicated that further auxiliary determinants for magnetosome biosynthesis are encoded outside the MAI. However, techniques for large-scale genome editing of magnetic bacteria are still limited, and the full complement of genes controlling magnetosome formation has remained uncertain. RESULTS: Here we demonstrate that an allelic replacement method based on homologous recombination can be applied for large-scale genome editing in M. gryphiswaldense. By analysis of 24 deletion mutants covering about 167 kb of non-redundant genome content, we identified genes and regions inside and outside the MAI irrelevant for magnetosome biosynthesis. A contiguous stretch of ~ 100 kb, including the scattered mam and mms6 operons, could be functionally substituted by a compact and contiguous ~ 38 kb cassette comprising all essential biosynthetic gene clusters, but devoid of interspersing irrelevant or problematic gene content. CONCLUSIONS: Our results further delineate the genetic complement for magnetosome biosynthesis and will be useful for future large-scale genome editing and genetic engineering of magnetosome biosynthesis.

Towards a 'chassis' for bacterial magnetosome biosynthesis: genome streamlining of Magnetospirillum gryphiswaldense by multiple deletions.

BACKGROUND: Because of its tractability and straightforward cultivation, the magnetic bacterium Magnetospirillum gryphiswaldense has emerged as a model for the analysis of magnetosome biosynthesis and bioproduction. However, its future use as platform for synthetic biology and biotechnology will require methods for large-scale genome editing and streamlining. RESULTS: We established an approach for combinatory genome reduction and generated a library of strains in which up to 16 regions including large gene clusters, mobile genetic elements and phage-related genes were sequentially removed, equivalent to ~ 227.6 kb and nearly 5.5% of the genome. Finally, the fragmented genomic magnetosome island was replaced by a compact cassette comprising all key magnetosome biosynthetic gene clusters. The prospective 'chassis' revealed wild type-like cell growth and magnetosome biosynthesis under optimal conditions, as well as slightly improved resilience and increased genetic stability. CONCLUSION: We provide first proof-of-principle for the feasibility of multiple genome reduction and large-scale engineering of magnetotactic bacteria. The library of deletions will be valuable for turning M. gryphiswaldense into a microbial cell factory for synthetic biology and production of magnetic nanoparticles.

Towards standardized purification of bacterial magnetic nanoparticles for future in vivo applications.

Bacterial magnetosomes (MS) are well-defined membrane-enveloped single-domain iron oxide (magnetite) nanoparticles, which are susceptible to genetic and chemical engineering. Additionally, the possibility to manipulate these particles by external magnetic fields facilitates their application in biomedicine and biotechnology, e.g. as magnetic resonance imaging probes or for drug delivery purposes. However, current purification protocols are poorly characterized, thereby hampering standardized and reproducible magnetosome production and thus, reliable testing for in vivo applications. In that context, the establishment of reproducible particle isolation procedures as well as the identification of high quality control parameters and the evaluation of potential cytotoxic effects of purified particles are of major importance. In this study, we characterize a multi-step purification protocol for MS with regard to purity, iron content, size and polydispersity of magnetite particles. In addition, we address potential cytotoxic effects of isolated MS when incubated with mammalian cells. Overall, we provide a detailed overview of the process-structure relationship during the isolation of MS and thus, identify prerequisites for high-yield MS production and their future application in the biomedical and biotechnological field. STATEMENT OF SIGNIFICANCE: Magnetic nanoparticles are of increasing interest for a variety of biomedical and biotechnological applications. Due to their unprecedented material characteristics, bacterial magnetosomes represent a promising alternative to chemically synthesized iron oxide nanoparticles. As applications require well-defined, highly purified and fully characterized nanoparticles, reliable protocols are necessary for efficient and reproducible magnetosome isolation. In our study, we evaluate an improved magnetosome extraction procedure and monitor quality parameters such as particle size distribution, membrane integrity and purity of the suspension by a combination of physicochemical and biochemical methods. Furthermore, the cytotoxicity of the isolated magnetosomes is assessed using different cell lines. In summary, our study helps to establish prerequisites for many real-world applications of magnetosomes in the field of biotechnology and biomedicine.

A bacterial cytolinker couples positioning of magnetic organelles to cell shape control.

Magnetotactic bacteria maneuver within the geomagnetic field by means of intracellular magnetic organelles, magnetosomes, which are aligned into a chain and positioned at midcell by a dedicated magnetosome-specific cytoskeleton, the "magnetoskeleton." However, how magnetosome chain organization and resulting magnetotaxis is linked to cell shape has remained elusive. Here, we describe the cytoskeletal determinant CcfM (curvature-inducing coiled-coil filament interacting with the magnetoskeleton), which links the magnetoskeleton to cell morphology regulation in Magnetospirillum gryphiswaldense Membrane-anchored CcfM localizes in a filamentous pattern along regions of inner positive-cell curvature by its coiled-coil motifs, and independent of the magnetoskeleton. CcfM overexpression causes additional circumferential localization patterns, associated with a dramatic increase in cell curvature, and magnetosome chain mislocalization or complete chain disruption. In contrast, deletion of ccfM results in decreased cell curvature, impaired cell division, and predominant formation of shorter, doubled chains of magnetosomes. Pleiotropic effects of CcfM on magnetosome chain organization and cell morphology are supported by the finding that CcfM interacts with the magnetoskeleton-related MamY and the actin-like MamK via distinct motifs, and with the cell shape-related cytoskeleton via MreB. We further demonstrate that CcfM promotes motility and magnetic alignment in structured environments, and thus likely confers a selective advantage in natural habitats of magnetotactic bacteria, such as aquatic sediments. Overall, we unravel the function of a prokaryotic cytoskeletal constituent that is widespread in magnetic and nonmagnetic spirilla-shaped Alphaproteobacteria.

Genome-Wide Identification of Essential and Auxiliary Gene Sets for Magnetosome Biosynthesis in Magnetospirillum gryphiswaldense.

Magnetotactic bacteria (MTB) stand out by their ability to manufacture membrane-enclosed magnetic organelles, so-called magnetosomes. Previously, it has been assumed that a genomic region of approximately 100 kbp, the magnetosome island (MAI), harbors all genetic determinants required for this intricate biosynthesis process. Recent evidence, however, argues for the involvement of additional auxiliary genes that have not been identified yet. In the present study, we set out to delineate the full gene complement required for magnetosome production in the alphaproteobacterium Magnetospirillum gryphiswaldense using a systematic genome-wide transposon mutagenesis approach. By an optimized procedure, a Tn5 insertion library of 80,000 clones was generated and screened, yielding close to 200 insertants with mild to severe impairment of magnetosome biosynthesis. Approximately 50% of all Tn5 insertion sites mapped within the MAI, mostly leading to a nonmagnetic phenotype. In contrast, in the majority of weakly magnetic Tn5 insertion mutants, genes outside the MAI were affected, which typically caused lower numbers of magnetite crystals with partly aberrant morphology, occasionally combined with deviant intracellular localization. While some of the Tn5-struck genes outside the MAI belong to pathways that have been linked to magnetosome formation before (e.g., aerobic and anaerobic respiration), the majority of affected genes are involved in so far unsuspected cellular processes, such as sulfate assimilation, oxidative protein folding, and cytochrome c maturation, or are altogether of unknown function. We also found that signal transduction and redox functions are enriched in the set of Tn5 hits outside the MAI, suggesting that such processes are particularly important in support of magnetosome biosynthesis.IMPORTANCE Magnetospirillum gryphiswaldense is one of the few tractable model magnetotactic bacteria (MTB) for studying magnetosome biomineralization. So far, knowledge on the genetic determinants of this complex process has been mainly gathered using reverse genetics and candidate approaches. In contrast, nontargeted forward genetics studies are lacking, since application of such techniques in MTB has been complicated for a number of technical reasons. Here, we report on the first comprehensive transposon mutagenesis study in MTB, aiming at systematic identification of auxiliary genes necessary to support magnetosome formation in addition to key genes harbored in the magnetosome island (MAI). Our work considerably extends the candidate set of novel subsidiary determinants and shows that the full gene complement underlying magnetosome biosynthesis is larger than assumed. In particular, we were able to define certain cellular pathways as specifically important for magnetosome formation that have not been implicated in this process so far.