Biology Reference
In-Depth Information
NtcA is a protein of c. 220-242 amino acids present and highly con-
served in all cyanobacteria so far analysed that is similar to proteins of the
CRP/FNR family of transcriptional regulators (
Herrero, Muro-Pastor,
& Flores, 2001
;
Luque & Forchhammer, 2008
). NtcA binds DNA as a
dimer at sites with the consensus sequence GTAN
8
TAC, which in acti-
vator sites is frequently centred at c. 41.5 nucleotides upstream from the
TSP of the regulated gene. NtcA sites in this position are accompanied by
a −10 promoter determinant with the consensus sequence TAN3T,
3
T, con-
forming the so-called canonical NtcA-activated promoter (
Herrero et al.,
2001
;
Luque et al., 1994
), which matches the structure of the bacterial
Class II activator-dependent promoters (
Busby & Ebright, 1999
). In some
other NtcA-activated promoters, some of which represent Class I promot-
ers, a single site for NtcA binding is found centred further upstream from
the −41.5 position (see
Busby & Ebright, 1999
;
Luque & Forchhammer,
2008
), whereas in others
more than one NtcA-binding site are present
and/or additional regulatory factors may participate (see sections 4.2 and
4.3 below). In the case of repression by NtcA, the binding site for this
regulator is centred downstream of the −41.5 position and could over-
lap the −10 determinant of the promoter or be located within the gene
(see
Herrero et al., 2001
;
Luque & Forchhammer, 2008
;
Olmedo-Verd
et al., 2008
). Repression by NtcA could be responsible for the exclusive
expression in vegetative cells, or preferential expression in vegetative cells
relative to heterocysts, of genes such as
rbcLS
(
Ramasubramanian et al.,
1994
),
hanA
(encoding a histone-like HU protein;
Khudyakov & Wolk, 1996
)
or
gor
(encoding glutathione reductase;
Jiang, Hellman, Sroga, Bergman, &
Mannervik, 1995
).
NtcA can specifically bind DNA in the absence of effectors (
Luque et al.,
1994
). However, 2-oxoglutarate has been shown to increase NtcA bind-
ing to a number of activated promoters of different cyanobacteria (see, e.g.
Olmedo-Verd et al., 2008
;
Vázquez-Bermúdez, Herrero, & Flores, 2002a
).
This has been recently confirmed with the resolution of the crystal struc-
ture of NtcA from
Anabaena
sp. strain PCC 7120 (
Zhao et al., 2010
) and
the unicellular cyanobacterium
Synechococcus elongatus
(
Llácer et al., 2010
),
which closely resembles that of the CRP protein from
E. coli
. In the case of
Anabaena
, comparison of the structure of the dimeric apoprotein to that in
complex with 2-oxoglutarate showed that this effector produces a change
in the orientation of the two DNA-recognition helices of the NtcA dimer,
which nonetheless could contact DNA in the absence of the effector, with
the effect of enhancing its DNA-binding activity. This is in contrast to the
