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normally responsible for activating the transcription of nif, nitrogen fixation genes
(Kaneko et al.
2000
; Sullivan et al.
2002
; Uchiumi et al.
2004
; Nukui et al.
2006
;
Nascimento et al.
2012
). The consequence of this somewhat unusual mode of
regulation is that, unlike ACC deaminases from other rhizobia, the
M. loti
ACC
deaminase does not facilitate nodulation but, rather, is expressed within nodules.
The result of this unusual regulation is, in
M. loti
, ACC deaminase may act to
decrease the rate of nodule senescence. This is particularly important because of the
fact that nitrogen fixation, a process that utilizes a very high level of energy in the
form of ATP, could (perhaps inadvertently) activate stress ethylene synthesis
resulting in premature nodule senescence.
2.3.4 Physiological Functions of Siderophores
Iron is essential for almost all life for processes such as respiration and DNA
synthesis. Despite being one of the most abundant elements in the Earth's crust,
the bioavailability of iron in many environments such as the soil is limited by the
very low solubility of the Fe
3+
ion. In the aerobic environment, iron accumulates in
common mineral phases such as iron oxides and hydroxides and hence becomes
inaccessible to organisms. Microbes (e.g. bacteria and fungi) have, therefore,
evolved a strategy to acquire iron by releasing siderophores (Greek: “iron carrier”),
small (generally less than 1,000 molecular weight) high-affinity iron-chelating
compounds, which scavenge iron from the mineral phases by forming soluble
Fe
3+
complexes that can be taken up by active transport mechanisms. Broadly,
siderophores act as solubilizing agents for iron from minerals or organic com-
pounds under conditions of iron starvation (Miethke and Marahiel
2007
;
Indiragandhi et al.
2008
). There are more than 500 different siderophores which
are produced mainly by Gram-positive and Gram-negative bacteria. Siderophores
are highly electronegative and bind Fe (III), preferentially forming a
hexacoordinated complex. The iron ligation groups have been tentatively classified
into three main chemical types: (1) hydroxamate (e.g. aerobactin and ferrichrome),
(2) catecholates/phenolates (e.g. enterobactin) and (3) hydroxyl acids/carboxylates
(e.g. pyochelin). Some siderophores contain more than one of the three iron-
chelating groups (Table
2.6
). Siderophores are, however, usually classified by the
ligands used to chelate the ferric iron. Citric acid can also act as a siderophore. The
wide variety of siderophores may be due to evolutionary pressures placed on
microbes to produce structurally different siderophores (Fig.
2.5
). Siderophores
are important for some pathogenic bacteria for their acquisition of iron. The strict
homeostasis of iron leads to a free concentration of about 10
24
mol/l, and hence,
there are great evolutionary pressures put on pathogenic bacteria to obtain this
metal. For example, the anthrax pathogen
Bacillus anthracis
releases two
siderophores, bacillibactin and petrobactin,
to scavenge ferric iron from iron
proteins.
