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Describe how and why bacteria regulate their nitrogen metabolism.

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Louise Weston Nitrogen Metabolism Describe how and why bacteria regulate their nitrogen metabolism. Nitrogen is essential for all living organisms being a constituent of nucleic acids and amino acids. 79% of the air is N2 however, most organisms can not use nitrogen in this form and must secure it in a fixed form such as NH3. Nitrogen fixing bacteria are known as diazotrophs. Relatively few bacteria fix nitrogen. Of those that do, most are free-living species, some however engage in symbiotic associations with plants where the bacteria provide the pant with fixed nitrogen and the plant provides fixed carbon to the bacteria. In marine environments the bulk of nitrogen fixation is carried out by cyanbacteria. Fixation is energetically costly and so if a fixed source of nitrogen is available it will be used primarily. The levels of such metabolites are sensed through the ntr system described below. In virtually all cells glutamate and glutamine serve as the key nitrogen donors for biosynthetic reactions and there are two major pathways which facilitate the incorporation of nitrogen into glutamate and glutamine. The most important pathway is the glutamine synthetase/glutamate synthetase (GS/ GOGAT) ...read more.


must be phosphorylated to activate transcription. There are two known sources of phosphryl groups for the formation of NRI -P. 1.Phosphyrl groups can be directly transferred from the phosphrylated metabolic intermediate acetyl phosphate. 2. The autophosphorylated form of the glnL/ntrB product known as NRII. NRII binds ATP and becomes autophosphorylated on a histidine residue and on incubation with NRI, this phosporyl group is transferred to an aspartate residue, Asp-54, within the N-terminal domain of NRI. The combination of NRII and the unmodified form of PII when nitrogen is in excess results in the very rapid dephosphorylation of NRI(NtrC)-P, stimulated by ATP. Hence down regulation of the operon. NtrC-P is responsible for the regulation of genes responsible for nitrogen fixation (nif genes) as it relays that nitrogen levels are low. It also regulates the genes required for nitrate and nitrite assimilation (nasFEDCBA) and various other genes implicated in nitrogen regulation. In the fixation process N2 is reduced to ammonium and the ammonium converted to organic form. N2 + 3H2 3NH3 This reaction is thermodynamically favourable ?G = -7.7 Kcal mol -1. However, the kinetic barrier to breaking the nitrogen - nitrogen triple bond that must be overcome, using a catalyst and/or the input of sufficient energy is high. ...read more.


The nitrate and nitrite reductase involved in this process are cytoplasmic enzymes, distinct from those participating in the respiratory process of denitrification. NO3- + 2H+ + 2e- NO2 + H2O nitrate reductase NO2- + 8H+ + 6e- NH4+ + 2H2O nitrite reductase Expression of nas operon encoding these enzymes is also NtrC/?54 regulated as mentioned. nas is also nitrate and nitrite regulated. Nitrate is not used as a source when ammonium levels are high saving the cell providing low potential reductants or additional enzymes. In such bacteriods sym plasmids carry the nif, nod and fix genes (where nod genes are required for the nodule production and maintenance.) Many cyanobacteria are capable of nitrogen fixation, and in some cases this occurs in highly developed cells termed heterocysts that differentiate from the vegetative cell filaments. Both nitrogen fixation and heterocyst development are also subject to ammonium repression. There is evidence for ntr analogous systems in bacreria other than those mentioned. There is however, little evidence of a classical ntr system in grampositive bacteria, nor is there any suggestion of an alternative global regulatory system that characterizes this group. To summerise nitrogen metabolism is imperative for life providing the necessary constituents of living tissue. It is highly regulated to prevent utilization of excess energy, through various systems, with the ntr system being the most common. ...read more.

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