To investigate the role of ferrous iron transport (Feo) systems in magnetosome formation, the gene for protein FeoB (feoB1), encoding 704 amino acids, was cloned from magnetotactic bacterium Magnetospirillum gryphiswaldense strain MSR-1. feoB1 constitutes a putative operon with feoA1, and the interval between the two genes is 36 base pairs. A feoB1-deficient mutant (DeltafeoB1) was constructed, and compared with wild-type in terms of iron uptake, iron content and functional complementation. Ferrous iron and ferric iron uptake in wild-type were respectively 1.8-fold and 1.3-fold higher than in the DeltafeoB1 mutant. Iron content (w/w) of DeltafeoB1 mutant was enhanced only slightly as extracellular iron concentration (either ferrous or ferric citrate) increased, whereas iron content of wild-type increased about 2-fold as extracellular iron concentration rose from 20 to 80 microM. Transmission electron microscopy revealed that DeltafeoB1 cells grown with either ferrous or ferric citrate produced fewer magnetosomes, with smaller diameter, compared to wild-type cells. Assay of feoAB1 promoter-lacZ transcriptional fusions indicated that the feoAB1 putative operon was downregulated when MSR-1 cells were grown under iron-rich condition. Magnetosome formation was reduced but not abolished in the feoB1 mutant, indicating that FeoB1 protein plays a significant role in this process. Other iron transport systems are presumed to be involved in iron uptake in MSR-1.