(Devouard et al. 1998). Unusually large magnetosomes of 250 nm have been
reported for uncultured magnetotactic cocci. These particles are exceptional
because they still exhibit SD properties although their size is outside the typical
SD size range (Lins et al. 2005). It has been speculated that magnetosome crystal
size is restricted by the vesicles themselves, as transmission electron micrographs
of thin sections prepared by cryo-ultramicrotomy suggest that preformed vesicles
are of a similar size as the mature crystals (Komeili et al. 2004).
Furthermore, as shown in M. gryphiswaldense, various membrane proteins seem
to be able to influence size and potentially morphology. A deletion mutant lacking
the whole mamGFDC operon produced magnetite crystals of only 75% the size of
wild-type cells (Scheffel et al. 2008). The resulting crystals were also less regular in
morphology and chain alignment. Thus, the precise roles of the proteins are unclear,
i.e., which effects cause the respective phenotypical outcomes. Complementation
experiments revealed that they might share redundant functions, as single gene
complements led to recovery of wild-type size. Reestablishing the full operon even
caused the particles to exceed their original size. All four proteins account for
around 35% of total protein content in the magnetosome membrane fraction of
M. gryphiswaldense and seem to be located there exclusively. mamG has so far only
been found in spirilla, but mamD, mamF, and mamC are found in all sequenced
genomes of magnetotactic Alphaproteobacteria, although their organization in
operons may differ (Sch€ ubbe et al. 2009). The Deltaproteobacterium D. magneticus
lacks all respective genes (Nakazawa et al. 2009). MamC and MamF are the most
abundant proteins, and the latter has been shown to form highly stable oligomers.
Mam12, the ortholog of MamC in M. magnetotacticum, has also been identified
as a magnetosome membrane protein in this organism (Taoka et al. 2006). MamD
and MamG share similarities with the aforementioned Mms6 protein, as they
also contain leucine–glycine repeats putatively responsible for aggregation of
multimeric complexes.
Mms6 of M. magneticum was shown to exert effects on crystal size and morphology under certain conditions in in vitro syntheses of magnetite using recombinant protein as an additive. In syntheses by coprecipitation of Fe
II and Fe
III with
sodium hydroxide, larger particles of around 30 nm were claimed to be found as
compared to a control without the additive (Arakaki et al. 2003). In another
experiment, magnetite nanoparticles were formed through an oxidative route at
elevated temperature (90
C) (Amemiya et al. 2007). Under these conditions, Mms6
restricted crystallite size and seemed to favor the formation of cuboctahedral rather
than octahedral particles, indicating a possible role as crystal face recognizing
growth regulator. However, these conditions are not physiological and due to the
lack of mms6 mutants so far, it is difficult to draw reliable conclusions.
Changes in magnetsome size and morphology have also been reported for
M. magnetotacticum grown in media supplied with high amounts of various trace
metals such as zinc and nickel, but these changes have not been characterized in
detail (Kundu et al. 2009). It can be speculated that respective metal ions compete
with iron uptake and thus limit its supply for magnetosome mineralization; however, this has not yet been investigated systematically.
1 Magnetite Biomineralization in Bacteria
17
reported for uncultured magnetotactic cocci. These particles are exceptional
because they still exhibit SD properties although their size is outside the typical
SD size range (Lins et al. 2005). It has been speculated that magnetosome crystal
size is restricted by the vesicles themselves, as transmission electron micrographs
of thin sections prepared by cryo-ultramicrotomy suggest that preformed vesicles
are of a similar size as the mature crystals (Komeili et al. 2004).
Furthermore, as shown in M. gryphiswaldense, various membrane proteins seem
to be able to influence size and potentially morphology. A deletion mutant lacking
the whole mamGFDC operon produced magnetite crystals of only 75% the size of
wild-type cells (Scheffel et al. 2008). The resulting crystals were also less regular in
morphology and chain alignment. Thus, the precise roles of the proteins are unclear,
i.e., which effects cause the respective phenotypical outcomes. Complementation
experiments revealed that they might share redundant functions, as single gene
complements led to recovery of wild-type size. Reestablishing the full operon even
caused the particles to exceed their original size. All four proteins account for
around 35% of total protein content in the magnetosome membrane fraction of
M. gryphiswaldense and seem to be located there exclusively. mamG has so far only
been found in spirilla, but mamD, mamF, and mamC are found in all sequenced
genomes of magnetotactic Alphaproteobacteria, although their organization in
operons may differ (Sch€ ubbe et al. 2009). The Deltaproteobacterium D. magneticus
lacks all respective genes (Nakazawa et al. 2009). MamC and MamF are the most
abundant proteins, and the latter has been shown to form highly stable oligomers.
Mam12, the ortholog of MamC in M. magnetotacticum, has also been identified
as a magnetosome membrane protein in this organism (Taoka et al. 2006). MamD
and MamG share similarities with the aforementioned Mms6 protein, as they
also contain leucine–glycine repeats putatively responsible for aggregation of
multimeric complexes.
Mms6 of M. magneticum was shown to exert effects on crystal size and morphology under certain conditions in in vitro syntheses of magnetite using recombinant protein as an additive. In syntheses by coprecipitation of Fe
II and Fe
III with
sodium hydroxide, larger particles of around 30 nm were claimed to be found as
compared to a control without the additive (Arakaki et al. 2003). In another
experiment, magnetite nanoparticles were formed through an oxidative route at
elevated temperature (90
C) (Amemiya et al. 2007). Under these conditions, Mms6
restricted crystallite size and seemed to favor the formation of cuboctahedral rather
than octahedral particles, indicating a possible role as crystal face recognizing
growth regulator. However, these conditions are not physiological and due to the
lack of mms6 mutants so far, it is difficult to draw reliable conclusions.
Changes in magnetsome size and morphology have also been reported for
M. magnetotacticum grown in media supplied with high amounts of various trace
metals such as zinc and nickel, but these changes have not been characterized in
detail (Kundu et al. 2009). It can be speculated that respective metal ions compete
with iron uptake and thus limit its supply for magnetosome mineralization; however, this has not yet been investigated systematically.
1 Magnetite Biomineralization in Bacteria
17