Required for correct biomineralization of the magnetosome, maybe via redox control (PubMed:23889511, PubMed:24020498). May function with MamY, MamZ amd Mms6 in biomineralization (Probable). Binds 2 heme groups per subunit via the 2 magnetochrome (MCR) motifs. Probably interacts with FtsZ-like and MamY proteins. Expressed in exponential phase, peaks about 18 hours (PubMed:20023033, PubMed:24020498). Protein is associated with magnetosomes as they start to develop, maximal protein is detected early then decreases (at protein level) (PubMed:30367002). Second gene in the 4 gene mamXY operon (PubMed:20023033) (Probable). The C-terminus of the protein cannot be tagged, suggesting it is required for function. Slightly reduced magnetic response, produces chains of wild-type magnetite crystals sandwiched between irregularly shaped, small flake-like crystals. Small crystals are in magnetosomes, but some have multiple nucleation sites. Phenotype is exacerbated when grown in NH(4)(+) instead of NO(3)(-) medium. Deletion of the periplasmic nitrate reductase operon also exacerbates the phenotype. Deletion mutants probably make some hematite crystals as well as wild-type magnetite crystals (PubMed:23889511). Loss of magnetic response, considerable decrease in intracellular iron. Magnetosomes are normally organized but the crystals are irregular, smaller superparamagnetic magnetite particles. Increased expression of the downstream genes in the operon (mamZ and ftsZ-like) (PubMed:24020498). Deletion of 4 consecutive genes (mamY, mamX, mamZ, ftsZm) leads to cells with an intermediate magnetic response where magnetosomes have short chains of nearly regularly shaped, cubo-octahedral crystals flanked by small particles with poorly defined morphologies (PubMed:22043287). This bacteria makes up to 60 cubo-octahedral magnetosomes of about 45 nm in diameter which contain membrane-bound crystals of magnetite (Fe(3)O(4)). Belongs to the magnetosome MamX family.