Biology Reference
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in the presence of zinc (
Hernández, López-Gomollón et al., 2004
) and it
remains to be analysed whether this supports the multiple modes of FurB
operation or reflects an apo-FurB function of the zinc-specific transcription
factor on its own promoter.
The least is known about the function of FurC. FurC only seems to
be involved in the regulation of the binding activity of FurA and FurB
to corresponding promoter regions, rather than to bind itself to any fur-
type promoter (
Hernández, López-Gomollón et al., 2004
). Its expression is
induced by the addition of H
2
O
2
indicative of an importance in response to
oxidative stress. However, expression in
E. coli
did not lead to an alteration
of the response to oxidative stress (
López-Gomollón et al., 2009
). Thus, its
molecular action is still unknown and remains to be established.
Recent analysis of the expression of furA, furB and furC in
M. aeruginosa
uncovered a differential expression of these genes during exponential and
stationary phase (
Alexova et al., 2011
). Lowering the iron concentration
to 100 nM enforced a downregulation of furA and furB only during the
stationary phase and only in the toxic
M. aeruginosa
sp. PCC 7806. In con-
trast, when the iron concentration was further reduced to 10 nM, a down-
regulation of all three genes was observed in the stationary phase in toxic
form. The differential regulation of Fur proteins in the two strains might be
explained by the importance of Fur for the expression of the microcystin
biosynthesis gene cluster (
Martin-Luna, Sevilla et al., 2006
). In addition,
an upregulation of furB and furC was observed under exponential growth
in both the toxic strain
M. aeruginosa
sp. PCC 7806 and in the nontoxic
strain
M. aeruginosa
sp. PCC 7005, while expression of furA is not affected
under these conditions (
Alexova et al., 2011
). This result is in line with
the suggested regulation of the expression of furB and furC by the redox
state rather than by the iron content and thus, FurA appears to be the main
regulator for iron starvation response.
4.1.3. Communication between the Cyanobacterial
Fur and NtcA Systems
Certain cyanobacterial species, both single cell and filamentous heterocyst-
forming, fix atmospheric nitrogen using nitrogenase - an Fe
7
Mo-containing
enzyme. NtcA is the central transcription factor in the response to nitrogen
limitation in all cyanobacteria. In
Anabaena
sp. PCC 7120, NtcA plays an
important role in the regulation of heterocyst development and nitrogen
fixation (
Nicolaisen, Hahn, & Schleiff, 2009
;
Ohashi et al., 2011
). A coor-
dination in iron acquisition and nitrogen metabolism might be manifested
