Deletion of the ftsZ-like gene results in the production of superparamagnetic magnetite magnetosomes in Magnetospirillum gryphiswaldense

Abstract

Magnetotactic bacteria (MTB) synthesize unique organelles termed "magnetosomes," which are membrane-enclosed structures containing crystals of magnetite or greigite. Magnetosomes form a chain around MamK cytoskeletal filaments and provide the basis for the ability of MTB to navigate along geomagnetic field lines in order to find optimal microaerobic habitats. Genomes of species of the MTB genus Magnetospirillum, in addition to a gene encoding the tubulin-like FtsZ protein (involved in cell division), contain a second gene termed "ftsZ-like," whose function is unknown. In the present study, we found that the ftsZ-like gene of Magnetospirillum gryphiswaldense strain MSR-1 belongs to a 4.9-kb mamXY polycistronic transcription unit. We then purified the recombinant FtsZ-like protein to homogeneity. The FtsZ-like protein efficiently hydrolyzed ATP and GTP, with ATPase and GTPase activity levels of 2.17 and 5.56 mumol phosphorus per mol protein per min, respectively. The FtsZ-like protein underwent GTP-dependent polymerization into long filamentous bundles in vitro. To determine the role of the ftsZ-like gene, we constructed a ftsZ-like mutant (DeltaftsZ-like mutant) and its complementation strain (DeltaftsZ-like_C strain). Growth of DeltaftsZ-like cells was similar to that of the wild type, indicating that the DeltaftsZ-like gene is not involved in cell division. Transmission electron microscopic observations indicated that the DeltaftsZ-like cells, in comparison to wild-type cells, produced smaller magnetosomes, with poorly defined morphology and irregular alignment, including large gaps. Magnetic analyses showed that DeltaftsZ-like produced mainly superparamagnetic (SP) magnetite particles, whereas wild-type and DeltaftsZ-like_C cells produced mainly single-domain (SD) particles. Our findings suggest that the FtsZ-like protein is required for synthesis of SD particles and magnetosomes in M. gryphiswaldense.

FIG. 1.

(a) Diagram of the mamXY cluster of M. gryphiswaldense strain MSR-1; (b) transcriptional analysis of mamXY cluster by RT-PCR. Expected sizes of PCR products are indicated below the arrows. Agarose gel electrophoresis of PCR products is shown at the bottom of each panel. Lanes: RT, RT-PCR; −, negative control with reverse transcriptase omitted; +, positive control with genomic DNA as the template; M, DNA size marker.

FIG. 2.

Detection of ATP and GTP hydrolytic activities of the purified FtsZ-like protein. (A) ATP and GTP hydrolytic activities of FtsZ-like. (B) SDS-PAGE of purified His-FtsZ-like protein. Lane 1, purified His-FtsZ-like protein; lane 2, molecular mass markers of 45 kDa (upper) and 35 kDa.

FIG. 3.

TEM images of FtsZ-like polymers. (a) FtsZ-like in the absence of GTP; (b) well-developed bundle of FtsZ-like in the presence of GTP; (c to f) filamentous bundles of FtsZ-like. Scale bars, 1 μm in panels a and b and 200 nm in panels c to f.

FIG. 4.

TEM images of cells and magnetosomes of M. gryphiswaldense strains. Cells (left column) were viewed by conventional TEM (scale bar, 500 nm; Philips Tecnai F30). Magnetosome chains of whole cells (middle column; scale bar, 100 nm) and crystal lattice of isolated magnetosomes (right column; scale bar, 5 nm) were viewed by high-resolution TEM (JEOL 2010). (a to c) Wild type; (d to f) ΔftsZ-like; (g to i) ΔftsZ-like_C.

FIG. 5.

Low-temperature magnetic (remanence) measurements (left) and room temperature FORC diagrams (right) for wild-type (a), ΔftsZ-like (b), and ΔftsZ-like_C (c) cells.