Nitrogen Assimilation in Bacteria
Nitrogen is one of the most abundant elements in living matter and, hence, sources of nitrogen are macro-nutrients. Most bacteria can assimilate ammonium, which is the inorganic nitrogen compound that is incorporated into organic material. The ammonium transporter AmtB/Amt1 is ubiquitous in bacteria, permitting scavenging ammonium from the extracellular medium. Once inside the cell, ammonium is generally incorporated into carbon skeletons through the very efficient, but energy-consuming, glutamine synthetase-glutamate synthase cycle. This cycle produces glutamine and glutamate, which distribute nitrogen in cellular anabolism. Because ammonium is not always available, bacteria have evolved the assimilation of many different inorganic and organic nitrogen-containing compounds. Sources of nitrogen alternative to ammonium frequently include nitrate and urea as well as atmospheric nitrogen in the process of nitrogen fixation, carried out only by prokaryotes. More complex organic sources of nitrogen can also be assimilated by many bacteria, and they include several amino acids, nitrogenous bases and even some xenobiotics. Pathways for assimilation of nitrogen sources alternative to ammonium are generally expressed only if the bacteria experience nitrogen deprivation. Regulators that control expression of the genes encoding those pathways include molecular systems as different as the two-component system NtrBC of enterobacteria, the MerR-family transcription factors TnrA and GlnR of Bacillus subtilis and the CRP-family NtcA transcription factor of cyanobacteria. Nonetheless, the C-to-N balance of the cells is generally sensed as levels of 2-oxoglutarate –which accumulates under nitrogen deprivation–, sometimes accompanied by sensing of glutamine –which accumulates under nitrogen sufficiency. Under high C-to-N balance conditions (i.e., nitrogen deficiency), these regulators activate the expression of genes encoding proteins for the assimilation of nitrogen sources alternative to ammonium, but in many cases induction of the genes by the corresponding nitrogen source is necessary as well. 3
| Main Authors: | , , |
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| Format: | capítulo de libro biblioteca |
| Published: |
Academic Press
2019
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| Subjects: | Amino acid catabolism; Ammonium uptake; Cyanophycin; GlnB/GlnK; Glutamine synthetase; Glutamate synthase; Heterocyst differentiation; Nitrate assimilation; Nitrogen control; Nitrogen fixation; Nitrogenase; NtcA; NtrB/NtrC; Respiratory Nif protection; TnrA; Urea assimilation, |
| Online Access: | http://hdl.handle.net/10261/205540 |
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