Nitrogen assimilation and nitrogen control in cyanobacteria.

Nitrogen sources commonly used by cyanobacteria include ammonium, nitrate, nitrite, urea and atmospheric N(2), and some cyanobacteria can also assimilate arginine or glutamine. ABC (ATP-binding cassette)-type permeases are involved in the uptake of nitrate/nitrite, urea and most amino acids, whereas secondary transporters take up ammonium and, in some strains, nitrate/nitrite. In cyanobacteria, nitrate and nitrite reductases are ferredoxin-dependent enzymes, arginine is catabolized by a combination of the urea cycle and arginase pathway, and urea is degraded by a Ni(2+)-dependent urease. These pathways provide ammonium that is incorporated into carbon skeletons through the glutamine synthetase-glutamate synthase cycle, in which 2-oxoglutarate is the final nitrogen acceptor. The expression of many nitrogen assimilation genes is subjected to regulation being activated by the nitrogen-control transcription factor NtcA, which is autoregulatory and whose activity appears to be influenced by 2-oxoglutarate and the signal transduction protein P(II). In some filamentous cyanobacteria, N(2) fixation takes place in specialized cells called heterocysts that differentiate from vegetative cells in a process strictly controlled by NtcA.