Transport and Assimilation of Inorganic Nitrogen in Bacteria
Conrado Moreno-Vivián, Víctor M. Luque-Almagro, Purificación Cabello, M. Dolores Roldán and Francisco Castillo
from: Nitrogen Cycling in Bacteria: Molecular Analysis (Edited by: James W. B. Moir). Caister Academic Press, U.K. (2011)
The incorporation of inorganic nitrogen into cell material is known as nitrogen assimilation. Usually, ammonium is the preferred inorganic nitrogen source for microorganisms. Ammonium assimilation requires the transport of this ion into the cells and its further incorporation into carbon skeletons, mainly through the glutamine synthetase-glutamate synthase pathway. Alternatively, glutamate dehydrogenase may also contribute to ammonium assimilation under certain conditions. Glutamine synthetase is the key enzyme for the regulation of ammonium assimilation; its activity is usually controlled by reversible covalent modification or feedback mechanisms and, at the gene expression level, transcription is often controlled by general nitrogen regulatory systems that vary depending on the organisms. In addition, ammonium transport is also subjected to regulation by carbon and nitrogen availability. Oxidized nitrogen compounds like nitrate and nitrite may be also used as nitrogen sources by many bacteria and archaea. Nitrate assimilation requires nitrate transport into the cells and two enzymes, nitrate and nitrite reductases, which catalyze the two-electron reduction of nitrate to nitrite and the six-electron reduction of nitrite to ammonium, respectively. These assimilatory enzymes are structural and functionally different to respiratory nitrate and nitrite reductases. Control of nitrate assimilation in different organisms may involve distinct regulatory proteins and mechanisms, but usually the process is regulated by nitrate and/or nitrite induction (pathway-specific control) and by ammonium repression (general nitrogen control) read more ...