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siderophore when grown under high ( 20 µM) but not under low iron conditions
( 5 µM), the siderophore production pattern here being the reverse of what is normally
observed (Paoletti and Blakemore 1986).
There appear to be at least two iron uptake systems in
Magnetospirillum
gryphiswaldense
(Schüler and Bäuerlein 1996). The iron in magnetite was mostly taken
up as Fe
3+
in an energy-dependent process. Fe
2+
was also taken up by cells but by a slow,
diffusion-like process. Fe
3+
uptake followed Michaelis-Menten kinetics with a K
m
of 3
µM and a V
max
of 0.86 nmol min
−1
(mg dry cell weight)
−1
suggesting that Fe
3+
uptake by
cells of
M. gryphiswaldense
is a low affinity but high velocity transport system. Spent
culture fluid appeared to enhance iron uptake although no evidence for the presence of a
siderophore was found.
Nakamura et al. (1993) found evidence for the involvement of a periplasmic binding
protein called sfuC in the transport of iron by cells of
Magnetospirillum magneticum
strain AMB-1. In this study, siderophores were not detected in spent growth medium.
More recently, this species was reported to produce both hydroxamate and phenolate
siderophores (Calugay et al. 2003). Like cells of
M. magnetotacticum
, those of
M.
magneticum
strain AMB-1 produce siderophores under growth conditions that would be
considered to be at least iron-sufficient, if not iron-rich, for most non-magnetotactic
bacteria. This unusual pattern of siderophore production might be due to the fact that iron
is taken up rapidly and converted to inert magnetite that apparently cannot be used by
cells. Therefore levels of iron available for growth would likely decrease relatively
quickly causing the cells to experience iron-limiting conditions, stimulating siderophore
production.
Recently, we have found that cells of a marine magnetotactic vibrio, strain MV-1,
also synthesize a siderophore that appears to be a hydroxamate type (Dubbels et al.
submitted for publication).The iron concentration pattern of siderophore production is
similar to the
Magnetospirillum
species. We have also found biochemical and molecular
evidence for the presence of a copper-dependent, high affinity Fe uptake system in strain
MV-1 similar to that found in the yeast
Saccharomyces cerevisiae
(Van Ho et al. 2002).
Magnetosome vesicle formation.
The magnetosome membrane in several
Magnetospirillum
species is made up of a lipid bilayer about 3-4 nm thick (Gorby et al.
1988) consisting of phospholipids, fatty acids, some unique proteins and some similar to
those in the cytoplasmic membrane. These similarities between the magnetosome and the
cytoplasmic membranes suggests that the magnetosome membrane vesicle originates
from the cytoplasmic membrane. This may be the reason that magnetosomes in virtually
all magnetotactic bacteria appear to be anchored to the cytoplasmic membrane as
demonstrated by electron microscopy and electron tomography. However, no direct,
unequivocal, evidence for the contiguity of these two membranes has ever been shown.
Nonetheless, current thought is that the magnetosome membrane vesicle is a result of the
invagination and pinching off of the cytoplasmic membrane. It is not clear whether the
vesicle is produced prior to magnetite nucleation and precipitation or whether magnetite
nucleation occurs in the periplasm and invagination of the cytoplasmic membrane occurs
around the developing crystal. There is some evidence for the former as seemingly empty
and partially-filled magnetosome vesicles have been observed in iron-starved cells of
M.
magnetotacticum
(Gorby et al. 1988) and in strain MV-1.
Small GTPases, such as Sar1p, are essential in the budding reaction in the production
of membrane vesicles and vesicle trafficking in eukaryotes (Kirchhausen 2000). Okamura
et al. (2001) identified a 16 kDa protein, Mms16, that shows GTPase activity, in the
magnetosome membrane vesicle of
Magnetospirillum magneticum
strain AMB-1 where
