Magnetospirillum sp. strain AMB-1 is a Gram-negative α-proteobacterium that synthesizes nano-sized magnetites, referred to as magnetosomes, aligned intracellularly in a chain. The potential of this nano-sized material is growing and will be applicable to broad research areas. It has been expected that genome analysis would elucidate the mechanism of magnetosome formation by magnetic bacteria. Here we describe the genome of Magnetospirillum sp. AMB-1 wild type, which consists of a single circular chromosome of 4967148 bp. For identification of genes required for magnetosome formation, transposon mutagenesis and determination of magnetosome membrane proteins were performed. Analysis of a non-magnetic transposon mutant library focused on three unknown genes from 2752 unknown genes and three genes from 205 signal transduction genes. Partial proteome analysis of the magnetosome membrane revealed that the membrane contains numerous oxidation/reduction proteins and a signal response regulator that may function in magnetotaxis. Thus, oxidation/reduction proteins and elaborate multidomain signaling proteins were analyzed. This comprehensive genome analysis will enable resolution of the mechanisms of magnetosome formation and provide a template to determine how magnetic bacteria maintain a species-specific, nano-sized, magnetic single domain and paramagnetic morphology.

Magnetotactic bacteria are ubiquitous microorganisms that synthesize intracellular magnetite particles (magnetosomes) by accumulating Fe ions from aquatic environments. Recent molecular studies, including comprehensive proteomic, transcriptomic, and genomic analyses, have considerably improved our hypotheses of the magnetosome-formation mechanism. However, most of these studies have been conducted using pure-cultured bacterial strains of α-proteobacteria. Here, we report the whole-genome sequence of Desulfovibrio magneticus strain RS-1, the only isolate of magnetotactic microorganisms classified under δ-proteobacteria. Comparative genomics of the RS-1 and four α-proteobacterial strains revealed the presence of three separate gene regions (nuo and mamAB-like gene clusters, and gene region of a cryptic plasmid) conserved in all magnetotactic bacteria. The nuo gene cluster, encoding NADH dehydrogenase (complex I), was also common to the genomes of three iron-reducing bacteria exhibiting uncontrolled extracellular and/or intracellular magnetite synthesis. A cryptic plasmid, pDMC1, encodes three homologous genes that exhibit high similarities with those of other magnetotactic bacterial strains. In addition, the mamAB-like gene cluster, encoding the key components for magnetosome formation such as iron transport and magnetosome alignment, was conserved only in the genomes of magnetotactic bacteria as a similar genomic island-like structure. Our findings suggest the presence of core genetic components for magnetosome biosynthesis; these genes may have been acquired into the magnetotactic bacterial genomes by multiple gene-transfer events during proteobacterial evolution.

Iron uptake systems were identified by global expression profiling of Magnetospirillum magneticum AMB-1. feo , tpd , and ftr , which encode ferrous iron transporters, were up-regulated under iron-rich conditions. The concomitant rapid iron uptake and magnetite formation suggest that these uptake systems serve as iron supply lines for magnetosome synthesis.

Magnetic particles have been studied with much interest with reference to many engineering applications. Much effort has been devoted to the preparation of nano-sized magnetite particles with well-controlled size and shape. Magnetic bacteria synthesize uniform and nano-sized magnetite particles enveloped by organic lipid membranes. This review discusses recent advances in bacterial magnetite particles. Especially, the tremendous biotechnological potential of bacterial magnetite particles (BMPs) and our recent understanding of mechanism of magnetite particle formation in Magnetospirillum magneticum strain AMB-1 were highlighted.