copper-containing periplasmic protein (ChpA) was identified and speculated to
function as an iron-uptake regulator analogously to the protein Ctr1 in a threecomponent iron-uptake system in Saccharomyces cerevisiae. In this yeast, the
protein is known to provide copper to an Fe
II oxidase which in turn forms a complex
with an iron permease for activation and subsequent iron–siderophore uptake
(Dubbels et al. 2004). Genome sequencing has revealed a number of ferrous and
ferric iron transporters in magnetotactic bacteria that resemble similar proteins
from non-magnetotactic species (Jogler and Sch€ uler 2007; Richter et al. 2007;
Jogler and Sch€ uler 2009). Expression profiling in M. magneticum has shown gene
regulation of several respective transporters based on iron concentrations in the
growth medium (Suzuki et al. 2006). In this organism, ferrous iron transporter
genes, namely, ftr1, tpd, and feoAB are upregulated under microaerobic conditions
with high Fe
II content whereas most ferric iron transporters are downregulated. The
Feo system consists of three components: FeoA, a probably cytoplasmic SH-3
domain protein; FeoB, the inner membrane Fe
II permease possessing an N-terminal
G-protein domain; and only found in Deltaproteobacteria, FeoC, an [Fe-S]-dependent
transcriptional repressor (Cartron et al. 2006). Furthermore, M. magnetotacticum
grown in ferrous iron-rich medium upregulates expression of FeoB1 and the protein
was found to be localized at the cytoplasmic membrane (Taoka et al. 2009).
M. gryphiswaldense seems to behave differently, as feoAB1 gene transcription is
downregulated with higher ferrous iron concentration (Rong et al. 2008). As uptake
in this species apparently does not depend on siderophores, other transporters might
be involved in the process, possibly explaining this difference. However, it was
experimentally demonstrated in M. gryphiswaldense that the Feo system plays at
least an accessory role in iron uptake related to magnetosome formation, since a
deletion mutant (DfeoB1) forms smaller and fewer magnetite particles (Rong et al.
2008). A cytoplasmic ATPase identified in M. magneticum was shown to be
involved in ferrous iron uptake, most probably by energizing membrane
transporters such as FeoB. Its target transporter, however, is still unknown (Suzuki
et al. 2007). Disruption of a fur-like gene in M. gryphiswaldense resulted in low
intracellular iron levels and inhibition of magnetosome formation, thereby
indicating its involvement in biomineralization-related iron uptake (Yijun et al.
2007). Fur (ferric uptake regulator) controls iron homeostasis in bacteria and was
shown in Escherichia coli to act by regulating gene expression according to
intracellular iron concentrations (Escolar et al. 1999). It can bind Fe
II and the
resulting complex acts as a transcriptional repressor for genes related to iron uptake.
When intracellular iron concentrations are low, the Fe
II –Fur complex dissociates
and thereby loses its ability to bind DNA. The respective genes are then transcribed
and expressed.
Final iron transport into magnetosomes is poorly understood. In M. magneticum,
a possible proton/iron antiporter protein, MagA, has been described as being
involved in the process. The protein was detected at both cell and magnetosome
membrane (Nakamura et al. 1995). However, no experimental evidence from other
MTB exists to confirm an essential role of MagA in magnetite biomineralization.
Proteomic analysis has also identified two cation diffusion facilitators located
12
J. Baumgartner and D. Faivre
function as an iron-uptake regulator analogously to the protein Ctr1 in a threecomponent iron-uptake system in Saccharomyces cerevisiae. In this yeast, the
protein is known to provide copper to an Fe
II oxidase which in turn forms a complex
with an iron permease for activation and subsequent iron–siderophore uptake
(Dubbels et al. 2004). Genome sequencing has revealed a number of ferrous and
ferric iron transporters in magnetotactic bacteria that resemble similar proteins
from non-magnetotactic species (Jogler and Sch€ uler 2007; Richter et al. 2007;
Jogler and Sch€ uler 2009). Expression profiling in M. magneticum has shown gene
regulation of several respective transporters based on iron concentrations in the
growth medium (Suzuki et al. 2006). In this organism, ferrous iron transporter
genes, namely, ftr1, tpd, and feoAB are upregulated under microaerobic conditions
with high Fe
II content whereas most ferric iron transporters are downregulated. The
Feo system consists of three components: FeoA, a probably cytoplasmic SH-3
domain protein; FeoB, the inner membrane Fe
II permease possessing an N-terminal
G-protein domain; and only found in Deltaproteobacteria, FeoC, an [Fe-S]-dependent
transcriptional repressor (Cartron et al. 2006). Furthermore, M. magnetotacticum
grown in ferrous iron-rich medium upregulates expression of FeoB1 and the protein
was found to be localized at the cytoplasmic membrane (Taoka et al. 2009).
M. gryphiswaldense seems to behave differently, as feoAB1 gene transcription is
downregulated with higher ferrous iron concentration (Rong et al. 2008). As uptake
in this species apparently does not depend on siderophores, other transporters might
be involved in the process, possibly explaining this difference. However, it was
experimentally demonstrated in M. gryphiswaldense that the Feo system plays at
least an accessory role in iron uptake related to magnetosome formation, since a
deletion mutant (DfeoB1) forms smaller and fewer magnetite particles (Rong et al.
2008). A cytoplasmic ATPase identified in M. magneticum was shown to be
involved in ferrous iron uptake, most probably by energizing membrane
transporters such as FeoB. Its target transporter, however, is still unknown (Suzuki
et al. 2007). Disruption of a fur-like gene in M. gryphiswaldense resulted in low
intracellular iron levels and inhibition of magnetosome formation, thereby
indicating its involvement in biomineralization-related iron uptake (Yijun et al.
2007). Fur (ferric uptake regulator) controls iron homeostasis in bacteria and was
shown in Escherichia coli to act by regulating gene expression according to
intracellular iron concentrations (Escolar et al. 1999). It can bind Fe
II and the
resulting complex acts as a transcriptional repressor for genes related to iron uptake.
When intracellular iron concentrations are low, the Fe
II –Fur complex dissociates
and thereby loses its ability to bind DNA. The respective genes are then transcribed
and expressed.
Final iron transport into magnetosomes is poorly understood. In M. magneticum,
a possible proton/iron antiporter protein, MagA, has been described as being
involved in the process. The protein was detected at both cell and magnetosome
membrane (Nakamura et al. 1995). However, no experimental evidence from other
MTB exists to confirm an essential role of MagA in magnetite biomineralization.
Proteomic analysis has also identified two cation diffusion facilitators located
12
J. Baumgartner and D. Faivre