Unlike MSR-1, AMB-1 contains an extra genomic cluster, termed the magnetotaxis islet (MIS), outside of the MAI region 28. The genes for magnetosome production and chain assembly in MTB, such as mamK, mamJ, and mamY, are arranged into magnetosome gene clusters (MGCs) that are often structured as magnetosome gene islands (MAI) 27. These observations suggest that unknown genetic elements may be needed for species-specific chain organization phenotypes.
In addition, mamY is critical for localizing the chain to the positive curvature of the cell in MSR-1 25 but does not have an impact on chain organization when deleted in AMB-1 26. However, similar deletions in AMB-1 result in subtle defects with magnetosomes still organized as chains 24. When mamJ is deleted in MSR-1, magnetosomes collapse into aggregates in cell 23. The acidic protein MamJ is also a key regulator of chain organization. However, the speed and spatial dynamics of MamK filaments are distinct in each organism 19, 20. While the overall proteomes of AMB-1 and MSR-1 are on average 66% identical, MamK proteins from the two organisms are 90.8% identical at the amino acid level and mamK AMB-1 complements the MSR-1 Δ mamK mutant 22. The actin-like protein MamK is conserved in all characterized MTB and forms a cytoskeleton that specifically regulates the stationary or moving behaviors of magnetosome chains in AMB-1 and MSR-1 19, 20. In contrast, in MSR-1, magnetic crystals are arranged as a continuous chain at the midcell and the divided daughter chains rapidly move from the new poles to the center of the daughter cells after cell division 20, 21. In AMB-1, magnetosomes containing magnetic crystals and empty magnetosomes (EMs) are interspersed to form a chain that is fragmented in appearance, extends from pole to pole in the cell, and remains stationary during the entire cell cycle 19. However, their magnetosome chain organization strategies are distinct. The most widely studied model MTB strains Magnetospirillum magneticum AMB-1 (AMB-1) and Magnetospirillum gryphiswaldense MSR-1 (MSR-1) are closely related Alphaproteobacteria species sharing 96% identity in their 16S rRNA gene sequences 18. The mechanisms leading to distinct chain configurations remain unknown, but may reflect strategies for adaptations to specific biotopes 13. Various and complex magnetosome chains (single- or multi-stranded, continuous, or fragmented) are found in diverse MTB groups 15, 16, 17. To function as an efficient compass needle, individual magnetosomes need to be arranged into a chain. Thus, magnetosome production in MTB is an ideal model system for studying mechanisms of organelle positioning, understanding the evolution of magnetic navigation, and connecting the magnetofossil record to the history of life on Earth. MTB are the simplest and most ancient organism capable of magnetic navigation 12 and fossilized magnetosome chains have been used as robust biosignatures 13, 14. Magnetic navigation is a common behavior in diverse organisms, including bacteria, insects, fish, birds, and mammals 10, 11. Magnetosomes mineralize ferrimagnetic nanoparticles composed of magnetite (Fe 3O 4) and/or greigite (Fe 3S 4) 2, 9, which are used as a compass needle for navigation along the geomagnetic field. A widely studied example of bacterial lipid-bounded organelles is the magnetosome compartment of magnetotactic bacteria (MTB). Similarly, it has been proposed that the carbon storage polyhydroxybutyrate granules associate with nucleoids to mediate segregation during cell division 7, 8. For example, the protein-bounded carbon-fixation organelle, the carboxysome, uses the nucleoid as a scaffold with helper proteins that ensure equal distribution in the cell and proper segregation into daughter cells 6. Many bacteria also produce organelles 2, 3, 4, 5, 6, 7, and actively regulate their placement in the cell. Organelle positioning in eukaryotic cells mainly relies on cytoskeletal and motor proteins 1. Cellular compartmentalization results in the formation of different organelles, which need to be positioned correctly to fulfill their specific functions and ensure proper inheritance throughout cell division 1.
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