Biology Reference
In-Depth Information
cannot bind to DNA directly and it was suggested that its function in Fe
scavenging may provide protection to the cells. As suggested for
Synechocystis
sp. PCC 6803 (
Wei, Mingjia, Xiufeng, Yang, & Qingyu, 2007
).
These observations point to a fundamental difficulty in annotating these
proteins. On the basis of sequence data alone, it is hard to determine which
group of ferritin protein it belongs to. Proximity matrix analysis of multiple
sequence alignment of ferritin-coding genes from multiple cyanobacterial
strains, for which complete genome sequences are available, indicates a con-
tinuum of protein sequences rather than a clear distinction into three classes.
However, it is important to note that in all the fully sequenced genomes,
represented in Cyanobase, ferritin homologues exist.
Prochlorococcus marinus
MED4 strain with the severely reduced genome size contains one ferritin
homologue. Many of the strains contain multiple ferritin sequences. The
Nostoc punctiforme
ATCC 29133 genome, for example, contains five. Con-
sidering the diverse functions associated to members of the ferritin family,
their presence as gene families should not be surprising.
4. IRON-DEPENDANT GENE REGULATION AND
PHYSIOLOGICAL RESPONSES
The scope of physiological responses to iron limitation is vast. Studies
of iron starvation have uncovered responses ranging from transcriptional
regulation to changes in the quaternary structure of pigment-protein com-
plexes. The regulation at the transcriptional level involves several layers of
signal transduction. The major regulatory system is composed of Fur-type
regulators and antisense RNA. However, as iron starvation is linked to many
other cellular effects, the regulatory systems have a certain overlap to other
stress response pathways. For example, iron limitation causes the accumula-
tion of reactive oxygen species (ROS) by about 10-fold in comparison to
normal growth as shown in
Anabaena
sp. PCC 7120 and
Synechocystis
sp.
PCC 6803 (
Latifi, Jeanjean, Lemeille, Havaux, & Zhang, 2005
). Thus, sepa-
rating the responses to iron limitation and oxidative stress is difficult.
4.1. Regulation of Cyanobacterial Gene Expression by Iron
Fur-mediated regulation was first identified using an
in vitro
coupled tran-
scription translation system by which a function of Fur as Fe(II)-dependant
repressor was established (
Bagg & Neilands, 1987
). As a consequence of
and a prerequisite for the sensing of intracellular iron concentration, Fur
proteins have an affinity for ferrous iron in the range of free cytosolic iron
