(Ullrich et al. 2005; Richter et al. 2007). The most recently published genome
analysis of Desulfovibrio magneticus RS-1 contains so far the lowest number of
MAI genes common to all sequenced magnetotactic bacterial genomes and thus
gives insights into which genes might be essential for intracellular magnetite
formation. This reduced subset of genes consists of mamA, mamB, mamE, mamK,
mamM, mamO, mamP, mamQ, and mamT. mamA has so far been described in
M. magneticum as required for functional magnetosome vesicle formation (Komeili
et al. 2004), and mamB and mamM are putative iron transporter genes, which will be
discussed later. mamK is involved in magnetosome chain formation as a filamentous actin-like protein (Komeili et al. 2006). The roles of the other MAI genes
remain unclear. Magnetospirillum spp. genomes comprise several further so-called
mam or mms genes (referring to “magnetosome membrane” and “magnetosome
membrane specific”), which are unique to MTB and have been shown to be
somehow involved in magnetite biomineralization, as will be discussed later.
Because not all magnetotactic species are closely related and various transposases
were detected along with MAI genes, it has been proposed that the respective genes
have spread by horizontal gene transfer (Jogler et al. 2009). A further distinct
feature of magnetotactic bacterial genomes is the presence of numerous regulatory
and signaling genes similar to chemotaxis genes that thus might be involved in
magnetotaxis (Jogler et al. 2009; Nakazawa et al. 2009).
1.2.4 Cell Biology
Structurally, MTB possess two cell membranes, the outer and inner membrane
separating periplasm from cytoplasm, as in all bacteria. Magnetosomes form as
invaginations of the inner cell membrane and possibly completely detach from it to
form isolated organelles within the cell (Komeili et al. 2006; Faivre et al. 2007).
Biochemical analysis of the lipid bilayer membrane (Gorby et al. 1988; Gr€ unberg
et al. 2004), electron microscopic (Balkwill et al. 1980; Komeili et al. 2004),
tomographic (Komeili et al. 2006; Scheffel et al. 2006), and M€ ossbauer spectroscopy (Faivre et al. 2007) studies have led to this current model. The lipid composition of both magnetosome and cytoplasmic membrane are virtually identical
(Gorby et al. 1988; Gr€ unberg et al. 2004). The protein content of each, however,
differs dramatically; proteomic analyses showed that the proteins mainly encoded
in the MAI are exclusively found in or attached to the magnetosome membranes of
M. magneticum and M. gryphiswaldense (Gr€ unberg et al. 2004; Matsunaga et al.
2005). The proteins have been partly attributed to known families according to
sequence homologies. Among them are proteins involved in transport such as the
generic transporters and cation diffusion facilitators (CDF) MamB and MamM,
proteases (HtrA-like serine proteases) MamE and MamO, the actin-like filamentous
structural protein MamK, proteins putatively involved in protein–protein
interactions, and complex scaffolding containing PDZ and TPR (tetracopeptide
repeat) domains, and a number of proteins without similarities to any other proteins
8
J. Baumgartner and D. Faivre
analysis of Desulfovibrio magneticus RS-1 contains so far the lowest number of
MAI genes common to all sequenced magnetotactic bacterial genomes and thus
gives insights into which genes might be essential for intracellular magnetite
formation. This reduced subset of genes consists of mamA, mamB, mamE, mamK,
mamM, mamO, mamP, mamQ, and mamT. mamA has so far been described in
M. magneticum as required for functional magnetosome vesicle formation (Komeili
et al. 2004), and mamB and mamM are putative iron transporter genes, which will be
discussed later. mamK is involved in magnetosome chain formation as a filamentous actin-like protein (Komeili et al. 2006). The roles of the other MAI genes
remain unclear. Magnetospirillum spp. genomes comprise several further so-called
mam or mms genes (referring to “magnetosome membrane” and “magnetosome
membrane specific”), which are unique to MTB and have been shown to be
somehow involved in magnetite biomineralization, as will be discussed later.
Because not all magnetotactic species are closely related and various transposases
were detected along with MAI genes, it has been proposed that the respective genes
have spread by horizontal gene transfer (Jogler et al. 2009). A further distinct
feature of magnetotactic bacterial genomes is the presence of numerous regulatory
and signaling genes similar to chemotaxis genes that thus might be involved in
magnetotaxis (Jogler et al. 2009; Nakazawa et al. 2009).
1.2.4 Cell Biology
Structurally, MTB possess two cell membranes, the outer and inner membrane
separating periplasm from cytoplasm, as in all bacteria. Magnetosomes form as
invaginations of the inner cell membrane and possibly completely detach from it to
form isolated organelles within the cell (Komeili et al. 2006; Faivre et al. 2007).
Biochemical analysis of the lipid bilayer membrane (Gorby et al. 1988; Gr€ unberg
et al. 2004), electron microscopic (Balkwill et al. 1980; Komeili et al. 2004),
tomographic (Komeili et al. 2006; Scheffel et al. 2006), and M€ ossbauer spectroscopy (Faivre et al. 2007) studies have led to this current model. The lipid composition of both magnetosome and cytoplasmic membrane are virtually identical
(Gorby et al. 1988; Gr€ unberg et al. 2004). The protein content of each, however,
differs dramatically; proteomic analyses showed that the proteins mainly encoded
in the MAI are exclusively found in or attached to the magnetosome membranes of
M. magneticum and M. gryphiswaldense (Gr€ unberg et al. 2004; Matsunaga et al.
2005). The proteins have been partly attributed to known families according to
sequence homologies. Among them are proteins involved in transport such as the
generic transporters and cation diffusion facilitators (CDF) MamB and MamM,
proteases (HtrA-like serine proteases) MamE and MamO, the actin-like filamentous
structural protein MamK, proteins putatively involved in protein–protein
interactions, and complex scaffolding containing PDZ and TPR (tetracopeptide
repeat) domains, and a number of proteins without similarities to any other proteins
8
J. Baumgartner and D. Faivre