Chemistry Reference
In-Depth Information
FIGURE 19.11
Molecular organisation of magnetosome genes in the magnetosome island (MAI) of all sequenced magnetotactic bacteria.
Different coloured arrows indicate characteristic features of the encoded proteins. The red outline indicates MTB-specific genes, which are
shared by all MTB but have no homologues in nonmagnetic organisms. Diagrams of the representative morphologies are shown. (From
Jogler
&Schuler, 2009
. Copyright 2009 with permission from Annual Reviews.)
predominantly located in several operons of a conserved genomic magnetosome island (
Figure 19.11
)
. Although
a large set of candidate genes has been identified, only a few genes and proteins have been characterised with
respect to their function. The small Mms6 protein, which is a tightly bound constituent of the magnetosome
membrane, affects magnetite crystallisation in vitro. The four small, hydrophobic magnetosome proteins MamG,
MamF, MamD, and MamC are specifically involved in the size control of magnetite crystals. MamJ and MamK
are cytoskeletal elements involved in the assembly of magnetosome chains.
In most MTBs, the mineral core of the magnetosome consists of magnetite (Fe
3
O
4
) although a few synthesise
magnetosomes containing the magnetic iron sulfide greignite (Fe
3
S
4
). The biomineralisation of magnetite
crystals requires the accumulation of large amounts (
4% by dry weight) of iron which is taken up into
precursors which include ferritin and a high-spin ferrous species. The process of biomineralisation involves three
steps (
Figure 19.12
). First, a membrane invagination is derived from the inner membrane, and proteins destined
for the magnetosome are sorted away from cell membrane proteins. The vesicle which forms from this invag-
ination serves as the precursor of the magnetosome membrane. In the second stage, ferrous ions are accumulated
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