Biology Reference
In-Depth Information
iron-limited cyanobacteria (
Burnap et al., 1993
). Based on that analysis, it
was suggested that CP43′ can be either an additional antenna complex or
a scavenging protein involved in the relocation of chlorophyll. Structural
analysis demonstrated that it forms a ring around trimeric and monomeric
PSI (
Bibby, Nield, & Barber, 2001
;
Boekema et al., 2001
) and thus, it is
assumed that CP43′ forms an alternative antenna complex for PSI, increas-
ing the functional absorption cross-section for PSI. On the other hand, it
was suggested that CP43′ acts as a quencher protecting the photosynthetic
apparatus from photo-inhibitory damages. This suggestion is strengthened
by spectroscopic studies demonstrating fluorescence quenching in CP43′
complexes (
van der Weij-de Wit, Ihalainen, van Grondelle, & Dekker, 2007
).
The presence of
isiA
genes is currently being explored as a biomarker for
iron limitation in oceans (
Richier et al., 2012
).
4.3.2. The Flavodoxin IsiB
The soluble electron transfer protein ferredoxin is required for the transfer
of electrons from PSI to ferredoxin-NADP
+
reductase. The protein con-
tains a [2Fe-2S] cluster as a cofactor and is one of the major iron-containing
proteins in cyanobacterial cells. Consequently, under iron-limiting condi-
tions, the function of this protein is compromised. To avoid malfunction,
these proteins are replaced by flavodoxin under such conditions (
Kouril
et al., 2005
;
Latifi et al., 2005
;
Laudenbach et al., 1988
). The replacement of
ferredoxin by flavodoxin defines the flavodoxin/ferredoxin ratio as a marker
for the iron status of a cyanobacterial cell (
Doucette, Erdner, Peleato, Hart-
mann, & Anderson, 1996
). However, flavodoxins have not been identified
in all cyanobacteria. For example,
Anabaena
ATCC 29211 lacks the poten-
tial to synthesize flavodoxin (
Sandmann, Peleato, Fillat, Lazaro, & Gomez-
Moreno, 1990
). In this cyanobacterium, the ferredoxin content is decreased
at very low iron concentrations (below 1 µM) without replacement.
Flavodoxin is encoded by isiB, downstream of isiA and on the same
operon. It contains a flavin cofactor and, thus, does not require iron. The
functionality and efficiency of flavodoxin is comparable to that of ferre-
doxin. This can be explained by the ferredoxin-like oxidation/reduction
potential of the semiquinone/hydroquinone pair (
Fillat, Edmondson, &
Gomez-Moreno, 1990
;
Fillat, Sandmann, & Gomez-Moreno, 1988
). How-
ever, flavodoxin is not only important under iron-limiting conditions. For
example,
isiB
is constitutively expressed in heterocysts where the flavo-
doxin is thought to function in the transport of electrons to the nitrogenase
(
Sandmann et al., 1990
). Furthermore, it has been shown in
Synechocystis
sp.