Natural nanomagnets

Various organisms possess a genetic program that enables the controlled formation of a mineral. This lecture has been devoted to an analysis of the state of knowledge concerning the biomineralization processes involved in iron oxides contained in magnetotactic bacteria (BMT). These bacteria are microaerobic organisms, requiring a low-oxygen environment to survive. To find a favorable environment for their growth, they have developed an ability called magnetoaerotaxis, i.e. they use magnetic field lines and oxygen gradients to move around. The migration of these bacteria along magnetic field lines and oxygen gradients enables them to reduce a three-dimensional search problem to a one-dimensional one, and thus contributes to the optimized survival of these microorganisms. These BMT are excellent models for identifying the essential biomolecules involved in the genetic control of biomineralization processes. These magnetotactic bacteria orient themselves in the earth's magnetic field thanks to magnetosomes, which are specialized organelles consisting of a membrane enveloping nanometric ferrimagnetic crystals (nanomagnets) of magnetite (_Fe3O4) or greigite (_Fe3S4), depending on the strain. Physico-chemical control of biomineralization is achieved by compartmentalization in phospholipid vesicles constituting the magnetosome membrane, which arises from invagination of the cytoplasmic membrane. The magnetosome membrane is associated with a specific set of proteins belonging to the families: tetratricopeptide p., CDF: cation diffusion facilitating transporter, HtrA-like serine protease, Actin-like protein, generic transporters and other BMT-specific proteins. Analysis of the phenotypes of magnetotactic bacteria from different strains shows convergence on 28 common genes specifically involved in magnetotaxis. Most of the genes involved in magnetosome formation are assembled into operons localized in a genomic island called MAI (MAgnetosome Island), which shares structural and compositional features with several BMTs. The "mini MAI" also displays differences in the content and organization of its genes, which account for the morphological diversity and arrangements of magnetic nanocrystals observed in different BMT strains. The role and main functions of a number of these genes are now well known. For example, MamGFDC regulates the growth of magnetite nanocrystals, while MamK and MamJ control the assembly and bonding of magnetosome chains. Although major advances have been made in identifying the genetic determinants involved in magnetosome synthesis and organization, and in some cases the synthesis of these organelles can be globally described, much progress remains to be made. In particular, in vitro experiments indicate that physico-chemical parameters dominate within the magnetosomal organelle at the time of magnetite formation. For example, iron concentration and pH value have been assessed indirectly on the basis of comparisons between the mechanisms of formation by chemical and biological synthesis. However, direct measurements of pH value, redox potential and iron concentration would be necessary to confirm the hypothesis that mineralization and biomineralization follow similar pathways. From a biological point of view, although the minimum set of genes required for magnetite formation has been identified, the minimum set of genes to regulate magnetite size is not yet fully understood. The genes controlling the number of magnetosome particles have not been identified, and the genes responsible for magnetosome morphology remain completely unknown, partly because the genetic systems associated with bacteria with anisotropic morphologies are not yet established. However, this task is hampered by the fact that a single gene is certainly not solely responsible for a given property; it is probably protein complexes that ultimately do the work. Such complexes are of course more difficult to identify, partly because some genes with redundant functions are hidden, or may be located outside the MAI genomic islands.