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nitrogen deprivation has also been described (
Ehira & Ohmori, 2011
).
These results are consistent with the idea that NrrA has a role in the general
response to nitrogen deprivation. Because the expression of
hetR
is impaired
in the
nrrA
mutant, and because purified NrrA protein binds in vitro to
sequences upstream the −728/−696 TSPs of
hetR
, it has been proposed
that the NtcA effect on activation of
hetR
transcription from the −728 TSP
takes place indirectly through NrrA (
Ehira & Ohmori, 2006b
).
The
Anabaena
sp. strain PCC 7120
pipX
gene, encoding a 92-amino
acid protein, is activated in cells differentiating into heterocysts, at inter-
mediate-to-late stages of the process, and its inactivation leads to a low
nitrogenase activity and impaired diazotrophic growth, which result from
the impaired expression of late heterocyst genes (e.g. the
cox2
,
cox3
and
nif-
HDK
operons) (
Valladares et al., 2011
). Taking into account the report of
the crystal structure of PipX from
Synechococcus elongatus
in complex with
NtcA, consisting of one NtcA dimer and two PipX monomers (
Llácer
et al., 2010
),
Anabaena
PipX may be a co-activator of NtcA reinforcing
NtcA-dependent activation of gene expression specifically during the late
steps of heterocyst differentiation.
The
hetP
gene encodes a product without recognizable homology and
its expression increases localized to proheterocysts (
Higa & Callahan, 2010
).
Ectopic overexpression of this gene leads to some degree of heterocyst dif-
ferentiation in the absence of a functional
hetR
gene, suggesting a direct
function of HetP downstream of HetR. Other regulatory elements, includ-
ing response regulators, histidine kinases, serine/threonine kinases, HstK
kinases (proteins with both a serine/threonine kinase domain and a his-
tidine kinase domain) and CRP homologues, participate in specific steps
of heterocyst maturation such as the synthesis and deposition of the Hgl
and Hep layers of the heterocyst envelope (
Flores & Herrero, 2010
;
Kumar,
Mella-Herrera, & Golden, 2010
).
In
Anabaena
sp. strain PCC 7120, NtcA has been shown to activate tran-
scription in vitro promoted by an RNAP including the principal sigma factor,
SigA, both at Class II activated promoters (
Valladares et al., 2008
) and at one
HetR-regulated promoter (
Camargo et al., 2012
). However, this cyanobacte-
rium bears, besides
sigA
, other eleven
putative RNAP sigma factor-encoding
genes (
Aldea, Mella-Herrera, & Golden, 2007
). Of these, two group 2 sigma
factor genes,
sigC
and
sigE
, and one group 4 gene,
sigG
, are upregulated in
differentiating cells at 4 h, 16 h and 9 h, respectively, after nitrogen stepdown
(
Aldea et al., 2007
), and inactivation of
sigE
results in delayed heterocyst
differentiation and impaired expression of
nifH
(
Mella-Herrera, Neunuebel,
