Genetic analysis of biomagnetic crystal formation

Magnetospirillum sp. AMB-1 is a freshwater magnetic bacterium which synthesizes intracellular particles of magnetite (Fe₃O₄). A genomic DNA fragment required for synthesis of magnetic particles was previously isolated from a non-magnetic transposon Tn5 mutant. We have determined the complete nucleotide sequence of this fragment. The 2975 bp region contains two putative open reading frames (ORFs). One ORF, designated magA, encodes a polypeptide which is homologous to the cation efflux proteins, the Escherichia coli potassium ion translocating protein, KefC, and the putative Na⁺/H⁺-antiporter, NapA, from Enterococcus hirae. Intracellular localization of the MagA protein was studied using magA - luc fusion proteins. The lue gene was cloned downstream of the magA hydrophilic C-terminal domain. The fusion protein was also detected on the surface of the lipid bilayer covering the magnetic particles. These results suggest that MagA is a membrane-bound protein. Vesicles which contained MagA protein exhibited iron accumulation ability. We consider that the MagA protein is an iron transporter involved in the synthesis of magnetic particles in AMB-1.