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by the NtcA-dependant activation of pkn41 and pkn42 expression under
low iron conditions (
Cheng, Shi, Latifi, & Zhang, 2006
). These two proteins,
with Ser/Thr kinase and His kinase domains, are essential under low iron or
nitrogen conditions. However, the downstream signal transduction cascade
has not yet been described. The pkn22 operon, including the gene of the
protein kinase Pkn22, is regulated by oxidative stress and iron starvation
(
Latifi, Ruiz, Jeanjean, & Zhang, 2007
;
Xu, Jeanjean, Liu, & Zhang, 2003
)
and might be under the control of NtcA as well (
Latifi, Ruiz, & Zhang,
2009
), a statement, which, however, is not yet further explored.
Another mode of genetic interaction is the strong activation of PfurA
in proheterocysts and heterocysts as a result of NtcA binding to opera-
tors present in the upstream region of furA and alr1690-α-furA (
López-
Gomollon, Hernández, Wolk, Peleato, & Fillat, 2007
). Consistently, the
promoter of furA shows several putative NtcA-binding sites. Thus, furA
expression, and thereby its subsequent regulatory function, is dependant
on the nitrogen status as well (
Hernández, Peleato, Fillat, & Bes, 2004
;
López-Gomollon, Hernández, Pellicer et al., 2007
;
López-Gomollon,
Hernández, Wolk et al., 2007
).
Thus, two modes for the integration of iron and nitrogen starvation
exist. On the one hand, signal transduction cascade components involved
in the response to oxidative stress or iron starvation are linked to nitro-
gen fixation as well. On the other hand, the expression of 'photosynthetic
genes' is regulated by FurA and NtcA. This link is a logical consequence
considering the iron demand of the nitrogenase and the sensitivity of this
enzyme to oxidation leading to a regulatory network able to respond to a
range of environmental conditions. However, the network is not yet fully
understood and described. For instance, one
exbB
gene (see above section
2.4) of
Anabaena
sp. PCC 7120 is strongly induced during nitrogen starva-
tion (
Stevanovic et al., 2012
), which suggests a coupling between nitrogen
starvation and enhanced iron uptake even at the transport level.
4.2. Iron-Dependant Regulation of Transcription
by Antisense RNA
A major regulatory pathway includes the post-transcriptional control of
furA
. The alr1690-α-furA antisense RNA interferes with the furA tran-
script as first documented for
Anabaena
sp. PCC 7120 (
Hernández, Muro-
Pastor et al., 2006
). The cis-α-fur RNA also exists in
M. aeruginosa
PCC
7806 and
Synechocystis
sp. PCC 6803 although the genomic context differs
(
Sevilla et al., 2011
). In
M. aeruginosa
,
fur
appears to be part of a dicistronic
