Bacterial magnetosomes are membrane-enveloped, nanometer-sized crystals of magnetite, which serve for magnetotactic navigation. All genes implicated in the synthesis of these organelles are located in a conserved genomic magnetosome island (MAI). We performed a comprehensive bioinformatic, proteomic and genetic analysis of the MAI in Magnetospirillum gryphiswaldense. By the construction of large deletion mutants we demonstrate that the entire region is dispensable for growth, and the majority of MAI genes have no detectable function in magnetosome formation and could be eliminated without any effect. Only <25% of the region comprising four major operons could be associated with magnetite biomineralization, which correlated with high expression of these genes and their conservation among magnetotactic bacteria. Whereas only deletion of the mamAB operon resulted in the complete loss of magnetic particles, deletion of the conserved mms6, mamGFDC, and mamXY operons led to severe defects in morphology, size and organization of magnetite crystals. However, strains in which these operons were eliminated together retained the ability to synthesize small irregular crystallites, and weakly aligned in magnetic fields. This demonstrates that whereas the mamGFDC, mms6 and mamXY operons have crucial and partially overlapping functions for the formation of functional magnetosomes, the mamAB operon is the only region of the MAI, which is necessary and sufficient for magnetite biomineralization. Our data further reduce the known minimal gene set required for magnetosome formation and will be useful for future genome engineering approaches.

SummaryMagnetotactic bacteria form chains of intracellular membrane-enclosed, nanometre-sized magnetite crystals for navigation along the earth's magnetic field. The assembly of these prokaryotic organelles requires several specific polypeptides. Among the most abundant proteins associated with the magnetosome membrane of Magnetospirillum gryphiswaldense are MamB and MamM, which were implicated in magnetosomal iron transport because of their similarity to the cation diffusion facilitator family.

SummaryMagnetotactic bacteria synthesize magnetosomes, which are unique organelles consisting of membraneenclosed magnetite crystals. For magnetic orientation individual magnetosome particles are assembled into well-organized chains. The actin-like MamK and the acidic MamJ proteins were previously implicated in chain assembly. While MamK was suggested to form magnetosome-associated cytoskeletal filaments, MamJ is assumed to attach the magnetosome vesicles to these structures. Although the deletion of either mamK in Magnetospirillum magneticum, or mamJ in Magnetospirillum gryphiswaldense affected chain formation, the previously observed phenotypes were not fully consistent, suggesting different mechanisms of magnetosome chain assembly in both organisms. Here we show that in M. gryphiswaldense MamK is not absolutely required for chain formation. Straight chains, albeit shorter, fragmented and ectopic, were still formed in a mamK deletion mutant, although magnetosome filaments were absent as shown by cryo-electron tomography. Loss of MamK also resulted in reduced numbers of magnetite crystals and magnetosome vesicles and led to the mislocalization of MamJ. In addition, extensive analysis of wild type and mutant cells revealed previously unidentified ultrastructural characteristics in M. gryphiswaldense. Our results suggest that, despite of their functional equivalence, loss of MamK proteins in different bacteria may result in distinct phenotypes, which might be due to a species-specific genetic context.