Required for correct biomineralization of the magnetosome; probably transports some form of iron. Partially functionally redundant with MamZ. The first gene in the probable 17 gene mamAB operon. Slight decrease in magnetic response, has about 30% fewer magnetosomes which are about 40% smaller than wild-type. Double mamH-mamZ deletion cells have a poor magnetic response and very few wild-type crystals; most cells have flake-like crystals (PubMed:23889511). Deletion of approximately 80 kb of DNA, including this operon, leads to cells that are non-magnetic, lack internal membrane systems, grow poorly, have reduced mobility and take-up and accumulate iron poorly (PubMed:13129949). 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)). Expression of just the minimal mamAB gene cluster (MGMSRv2__2365 to MGMSRv2__2381), including this gene, is sufficient to form a minimal magnetosome chain with small magnetite particles. Belongs to the major facilitator superfamily. Extended N-terminus.