Biology Reference
In-Depth Information
ferrisiderophore is transported back into the host cell via specific trans-
porters on the cell surface (See section
2.3
).
The secretion of siderophores was first established by a chrome azurol
sulfonate (CAS) assay (
Schwyn & Neilands, 1987
). This assay is based on
the transfer of ferric iron complexed with CAS to siderophores, which is
accompanied by a shift from blue to yellow. To test for respective hydrox-
amate- and catecholate-type siderophores, colorimetric measurements were
developed by
Arnow (1937)
and Csaky (1948) methods that are still widely
used today. The tyrosinase from the tiger moth
Arctia caja
converts catecho-
late siderophores to melanin, which can be determined colorimetrically
(
Arnow, 1937
). Similarly, hydroxamates can be oxidized to nitrite in an ace-
tic environment and nitrite can be visualized by means of a colour reaction
with sulphanilic acid and α-naphthylamine (
Csáky et al., 1948
).
In the past 60 years, many cyanobacteria were tested for their ability
to produce and to utilize siderophores.
Estep, Armstrong, and van Baalen
(1975)
as well as
Murphy, Lean, and Nalewajko (1976)
pioneered the work
by demonstrating that siderophores, known to be secreted by other bacteria,
can also be detected after growing cyanobacteria in iron-depleted media.
McKnight and Morel (1979)
could measure hydroxamate-type sidero-
phores in seven different species. The more complex catecholate-type che-
lator was originally isolated from
Oscillatoria tenius
(
Brown & Trick, 1992
).
Wilhelm and Trick (1994)
demonstrated that hydroxamate-type as well as
catecholate-type siderophores can be found in the supernatant of grow-
ing cyanobacteria cultures (
Table 3.1
). Thus, different cyanobacterial classes
and species produce a variety of hydroxamate-type siderophores (
Goldman,
Lammers, Berman, & Sanders-Loehr, 1983
).
The hydroxamate-type siderophore schizokinen is a derivative of
citric acid and chelates iron via two α-hydroxamate groups and one
α-hydroxy-carboxylate group (Fig.
3
.3A;
Simpson & Neiland, 1976
). It was
initially characterized in the Gram-positive bacterium
Bacillus megaterium
(
Mullis, Pollack, & Neilands, 1971
) and afterwards in
Anabaena
sp. PCC
6411 (
Simpson & Neiland, 1976
) and
Anabaena
sp. PCC 7120 (
Lammers &
Sanders-Loehr, 1982
). The function of schizokinen is thought to be twofold.
On the one hand, it is part of the iron acquisition strategy (in
Anabaena
sp.
PCC 7120, for example,
Lammers & Sanders-Loehr, 1982
) and on the other
hand, schizokinen - as well as other siderophores - is able to complex cop-
per (
McKnight & Morel, 1979
,
1980
). This activity was found to be required
for alleviating copper toxicity (
Clarke, Stuart, & Sanders-Loehr, 1987
).
