FeoB2 Functions in magnetosome formation and oxidative stress protection in Magnetospirillum gryphiswaldense strain MSR-1

Abstract

Magnetotactic bacteria (MTB) synthesize unique organelles, the magnetosomes, which are intracellular nanometer-sized, membrane-enveloped magnetite. The biomineralization of magnetosomes involves the uptake of large amounts of iron. However, the iron metabolism of MTB is not well understood. The genome of the magnetotactic bacterium Magnetospirillum gryphiswaldense strain MSR-1 contains two ferrous iron transport genes, feoB1 and feoB2. The FeoB1 protein was reported to be responsible mainly for the transport of ferrous iron and to play an accessory role in magnetosome formation. To determine the role of feoB2, we constructed an feoB2 deletion mutant (MSR-1 ΔfeoB2) and an feoB1 feoB2 double deletion mutant (MSR-1 NfeoB). The single feoB2 mutation did not affect magnetite crystal biomineralization. MSR-1 NfeoB had a significantly lower average magnetosome number per cell (∼65%) than MSR-1 ΔfeoB1, indicating that FeoB2 plays a role in magnetosome formation when the feoB1 gene is deleted. Our findings showed that FeoB1 has a greater ferrous iron transport ability than FeoB2 and revealed the differential roles of FeoB1 and FeoB2 in MSR-1 iron metabolism. Interestingly, compared to the wild type, the feoB mutants showed increased sensitivity to oxidative stress and lower activities of the enzymes superoxide dismutase and catalase, indicating that the FeoB proteins help protect bacterial cells from oxidative stress.

Fig 1

Multiple alignments of the M. gryphiswaldense MSR-1 FeoB1, MSR-1 FeoB2, and E. coli FeoB N termini. FeoB contains a G protein conserved motif (G1 to G5). The multiple alignments were performed using the program ClustalW, and pictures were constructed using the program ESPript 2.2 (http://espript.ibcp.fr/ESPript/ESPript/). Red box, strict identity; yellow box, similarity in a group.

Fig 2

TEM micrographs of wild-type MSR-1 and feoB mutants. (A) Wild type cultured in 80 μM Fe²⁺. (B) MSR-1 ΔfeoB2 cultured in 80 μM Fe²⁺. (C) Wild type cultured in 80 μM Fe³⁺. (D) MSR-1 ΔfeoB2 cultured in 80 μM Fe³⁺. (E) MSR-1 ΔfeoB1 cultured in 80 μM Fe²⁺. (F) MSR-1 NfeoB cultured in 80 μM Fe²⁺. (G) MSR-1 ΔfeoB1 cultured in 80 μM Fe³⁺. (H) MSR-1 NfeoB cultured in 80 μM Fe³⁺.

Fig 3

Intracellular iron contents (% dry weight) of the wild type and feoB mutants. Cells were grown in SLM supplemented with 50 μM ferric citrate and harvested at stationary phase. Iron content was determined by ICP-OES. Experiments were performed in triplicate.

Fig 4

Effect of H₂O₂ on cell growth. Cells were grown in SLM supplemented with various concentrations of H₂O₂, and cell density was measured by spectrophotometry at 600 nm at stationary phase.