Biology Reference
In-Depth Information
3.7. Metal Sensors and Nitrogen Metabolism
Nitrogen assimilation and biological nitrogen fixation require a large
number of metalloproteins, and changes in the availability of transition
metals pose a particular challenge to the supply of these critical nutri-
ents (
Glass, Wolfe-Simon, & Anbar, 2009
). Nitrogen control in cyano-
bacteria is mediated by NtcA and the signal transduction P
II
protein,
but neither of them are metalloproteins. As mentioned previously, some
key genes involved in nitrogen fixation and heterocyst development are
co-ordinately modulated by NtcA and FurA, the latter also sensing iron
availability.
Molybdenum is a key element in several enzymes involved in nitro-
gen assimilation and fixation, sulphur, and carbon metabolism. Nitroge-
nase requires Mo as part of its metal cofactor, and diazotrophic growth
of
Anabaena variabilis
has shown to be dependent on the presence of Mo
or V, with little growth occurring in their absence (
Herrero, Muro-Pas-
tor, & Flores, 2001
). In
Azotobacter
, Mo represses the synthesis of both V
nitrogenase and nitrogenase-3, and in the absence of Mo, V represses the
synthesis of nitrogenase-3 (
Luque & Pau, 1991
). Molybdenum metabo-
lism and homeostasis are regulated by the molybdate-responsive tran-
scription factor ModE. Orthologues of ModE are widespread amongst
diverse prokaryotes but not ubiquitous, and DNA-binding motifs have
been identified to be quite conserved (
Studholme & Pau, 2003
). Little is
known about Mo-dependent transcriptional regulators in cyanobacteria.
Putative
modE
-like genes have been described in several cyanobacte-
rial genomes (
Nakamura et al., 1998
), but little information is available
in the current literature concerning the role of this regulator in
cyanobacteria.
4. CONCLUSIONS AND PERSPECTIVES
Metalloregulation in cyanobacteria is not restricted to ensuring opti-
mal metal ion homeostasis. The coordination of metal homeostasis with the
response to environmental stresses and central metabolic processes is often
carried out by metalloproteins with regulatory functions. Although most of
the main families involved in these tasks have been characterized in recent
years, further work based on functional genomics and structural biology
remains to be done. The identification of new players, such as potential
metal-binding noncoding RNAs, will provide a more complete picture of
the cyanobacterial metallome.
