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cells of the filament, although at somewhat increased levels in differentiating
cells. On the other hand, the HetR-regulated proximal promoters of
devBCA
(
Camargo et al., 2012
) and
hetC
(
Muro-Pastor, Flores, & Herrero, 2009
), as
well as that producing TSP -271 of
hetR
(
Rajagopalan & Callahan, 2010
), are
activated in differentiating cells. In the case of the
devBCA
proximal promoter,
in spite of the lack of a consensus NtcA-binding site, NtcA binds to DNA
in vitro and, together with 2-oxoglutarate, directly activates transcription
(
Camargo et al., 2012
), with HetR having some role in helping interaction of
NtcA with degenerated NtcA-binding sites. Whether support of activation by
NtcA is the only effect of HetR or whether, in other promoters, it can activate
gene expression in the absence of NtcA should be clarified in the future.
In summary, Class II promoters would be activated throughout the filament
early upon perception of nitrogen stress providing certain level of gene expres-
sion in all the cells, which for genes involved in heterocyst differentiation would
be reinforced later in specific cells in response to a localized increase in NtcA
levels. On the other hand, localized, and in some cases transient, activity of
HetR-dependent promoters would reinforce the spatially localized gene
expression. Thus, the contribution of the two promoter types to direct gene
expression preferentially in the differentiating cells would respond to an ampli-
fication loop resulting from localized positive autoregulation of
ntcA
and
hetR
,
both of which include mutually dependent NtcA- and HetR-dependent
promoters (
Buikema & Haselkorn, 2001
;
Muro-Pastor, Valladares, Flores, &
Herrero, 2002
;
Olmedo-Verd et al., 2006
).
4.3. Other Regulators Co-operating in Gene Activation
Besides NtcA and HetR, other regulators have been identified that have a
positive role on gene activation during heterocyst differentiation although
their spectrum of action is less extensive and less drastic than that of the
principal regulators NtcA and HetR (see
Flores & Herrero, 2010
). The
nrrA
gene encodes a response regulator of the OmpR family that is expressed
from a single Class II NtcA-dependent promoter early upon combined
nitrogen deprivation throughout the filament, although expression at later
times becomes higher in differentiating cells (
Ehira & Ohmori, 2006a
;
Muro-Pastor et al., 2006
). Inactivation of
nrrA
leads to delayed hetero-
cyst differentiation and impaired activation of many nitrogen-regulated
genes, studied globally by microarray analysis in an
nrrA
mutant versus the
wild-type
Anabaena
sp. strain PCC 7120 (
Ehira & Ohmori, 2006a
). A direct
effect of NrrA on induction of the
glgP1
gene involved in glycogen catabo-
lism and
sigE
, a group 2 sigma factor (see below in this section), upon
