Characterization of a Spontaneous Nonmagnetic Mutant ofMagnetospirillum gryphiswaldenseReveals a Large Deletion Comprising a Putative Magnetosome Island

Abstract

Frequent spontaneous loss of the magnetic phenotype was observed in stationary-phase cultures of the magnetotactic bacterium Magnetospirillum gryphiswaldense MSR-1. A nonmagnetic mutant, designated strain MSR-1B, was isolated and characterized. The mutant lacked any structures resembling magnetosome crystals as well as internal membrane vesicles. The growth of strain MSR-1B was impaired under all growth conditions tested, and the uptake and accumulation of iron were drastically reduced under iron-replete conditions. A large chromosomal deletion of approximately 80 kb was identified in strain MSR-1B, which comprised both the entire mamAB and mamDC clusters as well as further putative operons encoding a number of magnetosomeassociated proteins. A bacterial artificial chromosome clone partially covering the deleted region was isolated from the genomic library of wild-type M. gryphiswaldense. Sequence analysis of this fragment revealed that all previously identified mam genes were closely linked with genes encoding other magnetosome-associated proteins within less than 35 kb. In addition, this region was remarkably rich in insertion elements and harbored a considerable number of unknown gene families which appeared to be specific for magnetotactic bacteria. Overall, these findings suggest the existence of a putative large magnetosome island in M. gryphiswaldense and other magnetotactic bacteria.Magnetotactic bacteria are capable of forming magnetosomes, which are specific intracellular structures that enable the cells to orient along magnetic field lines (3, 4, 41). The superior crystalline and magnetic properties of magnetosomes make them potentially useful as a highly ordered biomaterial in a number of applications, e.g., in the immobilization of bioactive compounds, magnetic drug targeting, or as a contrast agent for magnetic resonance imaging (24,41). Recently, the characteristics of bacterial magnetosomes have even been considered for use as biosignatures to identify presumptive Martian magnetofossils (49). Moreover, understanding bacterial magnetosome formation is expected to provide insights into more complex biomineralization systems in higher organisms (19). The biomineralization of magnetosome particles is achieved by a complex mechanism with control over the uptake, accumulation, and precipitation of iron, which, however, is poorly understood at the molecular and biochemical level.The magnetotactic ␣-proteobacterium Magnetospirillum gryphiswaldense microaerobically produces up to 60 cubo-octahedral magnetosomes, which are approximately 45 nm in size and consist of membrane-bounded crystals of the iron mineral magnetite (Fe 3 O 4 ) (34,42). In contrast to most other magnetotactic bacteria, methods for mass culture and genetic manipulation of M. gryphiswaldense are available (17,38,44), which has facilitated its analysis in a number of studies (37,39,40,43).In Magnetospirillum species, the deposition of the mineral particle occurs within a specific compartment, which is provided by the magnetosome membrane ...

Experimental observation of magnetosome chain collapse in magnetotactic bacteria: Sedimentological, paleomagnetic, and evolutionary implications

“…3 and 5, there are hints of this sheath material present -but not commented upon -in several past publications containing TEM images of various magnetotactic bacteria. In particular, Schübbe et al [48] report a spontaneous mutant of M. gryphiswaldense (MRS-1B) that lost an ~80 kb segment of the magnetosome island and produced no magnetosome vesicles or magnetite crystals. However, as seen on their Figure 1a, these mutant cells still retain a clear ghost of organic material running along the length of the cells, where the magnetosome chains should have been.…”

“…The conserved MreB-like protein, named MamK in M. gryphiswaldense, is not bound to the magnetosome membrane in extracted magnetosomes [47]. The sequence similarity of MamK to MreB and its disconnect from the magnetosome has led to speculation that is may be involved in structural control of the magnetosome chain [48]. On the other hand, it is possible that MamK is not involved in the sheath structure and that the sheath is composed of something entirely different, such as a glycoprotein matrix.…”

Magnetosome formation in prokaryotes

“…A spontaneous non-magne totactic mutant of M. gryphiswaldense, which lacks mamB and mamM, as well as numerous other magne tosome membrane proteins and other genes, and which does not biomineralize magnetosomes (discussed below), was found to be deficient in iron uptake 77 . Iron has not been unequivocally shown to be bound and transported by any known CDF protein, although in the yeast S. cerevisiae, the overexpression of two genes that encode mitochondrial membrane proteins affects Fe(II) concentrations in the mitochondria and cytosol 80 .…”

MMS6 Protein Regulates Crystal Morphology during Nano-sized Magnetite Biomineralization in Vivo

“…Transmission Electron Microscopy-mms6 gene is presumably located in an operon with 2 other uncharacterized genes in the magnetosome island (MAI), a genome region containing several genes specific to magnetotactic bacteria (27)(28). To avoid the polar effect, an in-frame gene deletion was carried out for the mms6 gene mutation in this study.…”