Magnetospirillum gryphiswaldense uses intracellular chains of membrane-enveloped magnetite crystals, the magnetosomes, to navigate within magnetic fields. The biomineralization of magnetite nanocrystals requires several magnetosome-associated proteins, whose precise functions so far have remained mostly unknown. Here, we analysed the functions of MamX and the Major Facilitator Superfamily (MFS) proteins MamZ and MamH. Deletion of either the entire mamX gene or elimination of its putative haem c-binding magnetochrome domains, and deletion of either mamZ or its C-terminal ferric reductase-like component resulted in an identical phenotype. All mutants displayed WT-like magnetite crystals, flanked within the magnetosome chains by poorly crystalline flake-like particles partly consisting of haematite. Double deletions of both mamZ and its homologue mamH further impaired magnetite crystallization in an additive manner, indicating that the two MFS proteins have partially redundant functions. Deprivation of ΔmamX and ΔmamZ cells from nitrate, or additional loss of the respiratory nitrate reductase Nap from ΔmamX severely exacerbated the magnetosome defects and entirely inhibited the formation of regular crystals, suggesting that MamXZ and Nap have similar, but independent roles in redox control of biomineralization. We propose a model in which MamX, MamZ and MamH functionally interact to balance the redox state of iron within the magnetosome compartment.