Ferrous iron transport protein B gene ( feoB1) plays an accessory role in magnetosome formation in Magnetospirillum gryphiswaldense strain MSR-1

Abstract

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 (Δ feoB1) 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 Δ feoB1 mutant. Iron content (w/w) of Δ feoB1 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 μM. Transmission electron microscopy revealed that Δ feoB1 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.