Biology Reference
In-Depth Information
nitrogen starvation is lower (see, e.g.
Vázquez-Bermúdez, Paz-Yepes, Her-
rero & Flores,
2002b
). Because cyanobacteria lack 2-oxoglutarate dehydro-
genase (
Smith, London, & Stanier, 1967
), the main fate of 2-oxoglutarate in
these organisms is incorporation into glutamate and glutamine (
Vázquez-
Bermúdez, Herrero, & Flores, 2000
). Under conditions of sufficient fixation
of CO
2
, nitrogen deprivation restricts the use of 2-oxoglutarate resulting in its
accumulation in the cells, which has been suggested to indicate a high cellular
carbon-to-nitrogen balance (
Muro-Pastor, Reyes, & Florencio, 2001
). Con-
sistently, addition of 2-oxoglutarate to a unicellular cyanobacterium bearing
a heterologous 2-oxoglutarate permease parallels the effect of a high carbon
supply in the expression of some nitrogen assimilation genes (
Vázquez-Ber-
múdez, Herrero, & Flores, 2003
). A similar approach showed that 2-oxo-
glutarate and a nonmetabolizable analogue of it, 2,2-difluoropentanedioic
acid, can promote heterocyst differentiation in
Anabaena
sp. strain PCC 7120
(
Laurent et al., 2005
;
Li, Laurent, Konde, Bédu, & Zhang, 2003
). These results
are consistent with the notion that a high cellular carbon-to-nitrogen balance
signalled by 2-oxoglutarate triggers heterocyst differentiation. The molecular
basis of this effect relies on the role of 2-oxoglutarate as an effector of the
transcription factor NtcA, which will be described below.
Another protein sensing 2-oxoglutarate levels in cyanobacteria is P
II
, the
glnB
gene product, which is phosphorylated under conditions determining
a high carbon-to-nitrogen balance in the cells (
Forchhammer, 2008
). In
Anabaena
sp. strain PCC 7120, the
glnB
gene is repressed in the heterocysts
(
Paz-Yepes, Flores, & Herrero, 2009
), which nonetheless appear to con-
tain some nonphosphorylated P
II
protein (
Laurent et al., 2004
). An
Ana-
baena
mutant of this gene could be obtained only after overexpression of
downstream genes (
Paz-Yepes et al., 2009
). This mutant was impaired spe-
cifically in diazotrophic growth but could produce heterocysts that lacked
polar (cyanophycin) granules, a phenotype similar to that of an
Anabaena
mutant impaired in dephosphorylation of the P
II
protein (
Laurent et al.,
2004
). As mentioned above, cyanophycin contains arginine, and promotion
of arginine biosynthesis is the best-known function of the P
II
protein in
its dephosphorylated state (
Forchhammer, 2008
). The relevance of this P
II
effect on heterocyst biology remains to be assessed.
3.2. Genes Activated Transiently during Differentiation
with Spatial Specificity
Heterocyst differentiation is the result of a specific program of gene expres-
sion that is established as a last response to the external cue of nitrogen
