Nitrogen Assimilation and C/N Balance Sensing
Ignacio Luque and Karl Forchhamer
from: The Cyanobacteria: Molecular Biology, Genomics and Evolution (Edited by: Antonia Herrero and Enrique Flores). Caister Academic Press, U.K. (2008)
Cyanobacteria inhabit nearly all illuminated environments on Earth and play key roles in the carbon and nitrogen cycle of the biosphere. Generally, cyanobacteria are able to utilize a variety of inorganic and organic sources of combined nitrogen, like nitrate, nitrite, ammonium, urea or some amino acids. Several cyanobacterial strains are also capable of diazotrophic growth. Genome sequencing has provided a large amount of information on the genetic basis of nitrogen metabolism and its control in different cyanobacteria. Comparative genomics, together with functional studies, has led to a significant advance in this field over the past years. The first part of this review will summarize the current knowledge on the physiology, biochemistry and genetics of the utilization of inorganic and organic nitrogen sources. The second part of the review will turn towards the sophisticated mechanisms cyanobacteria have evolved to tune nitrogen metabolism to the environmental conditions. 2-oxoglutarate has turned out to be the central signalling molecule reflecting the carbon/nitrogen balance of cyanobacteria. Central players of nitrogen control are the global transcriptional factor NtcA, which controls the expression of many genes involved in nitrogen metabolism, as well as the PII signalling protein, which fine-tunes cellular activities in response to changing C/N conditions. These two proteins are sensors of the cellular 2-oxoglutarate level and have been conserved in all cyanobacteria. In contrast, the adaptation to nitrogen starvation involves heterogeneous responses in different strains read more ...