7
N
Nitrogen
14.007
Essential: all life
Nitrogen
Environmental and health impacts:
-Major limiting nutrient for agriculture
-Abundant in atmosphere (80% N2), but fixed nitrogen is often scarce
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(1) Nitrogen: An Essential Macronutrient
Nitrogen is required in all cells and is considered one of the macronutrients that are essential for life. Nitrogen is required in amino acids, proteins, nucleic acids (DNA and RNA), and NTPs, which all form the basis of all life on earth.
(2) N Assimilation Proteins Have Less N
Nitrogen limitation can affect protein composition. A single amino acid change in a protein could add up to 3 nitrogen atoms (Gly to Arg). In both S. cerevisiae and E. coli, enzymes for N assimilation are built, on average, using amino acids with fewer N atoms (Baudoiun-Cornu et al., 2000).
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Recent studies in N sparing in C. reinhardtii show similarly that N assimilation-related transporters contain less N in side chains compared with the rest of the proteome. It was found that the proteome induced under N limited conditions contains 6% less N in side chains (Schmollinger et al., 2014).
(3) Organisms in N-Limited Environments are Depleted in N-rich Amino Acids
In marine systems, the surface waters are more typically limited by N rather than C, whereas the converse may be true in deeper waters (Polz & Cordero, 2020). In organisms prevalent in ocean surface waters, the imprint of selection against N-rich amino acids in abundant proteins is readily apparent. Indeed, the predicted proteomes from metagenomic samples of ocean surface waters are depleted in N-rich amino acids when compared to the predicted proteomes associated with the microbial communities in coastal waters (Dittberner et al., 2018; Grzymski and Dussaq, 2012).
(3) Degradation of Ribosomes
Elemental-sparing responses are relatively ineffective in the face of the ubiquitous use of N in cell constituents, so when no external sources can be scavenged and cell growth becomes limited for macronutrients, processes of recycling become incredibly important. Ribosomes represent a major fraction of cell mass in rapidly growing bacteria, and studies in E. coli have revealed that ribosomes are degraded in cells presented with N limitation (Kaplan and Apirion, 1975). Degradation appears to occur during the transition to stationary phase and is correlated with the formation of free ribosomal subunits (Zundel, et al., 2009). It is not clear whether degradation of ribosomes serves primarily to liberate nutrients associated with the abundant rRNA (which is ~50% of the ribosome mass), the ribosomal proteins, or both.
(4) Degradation of Abundant Proteins
In addition to ribosomes, other abundant proteins may be targeted for proteolytic destruction upon nutrient limitation. In mammalian cells, protein degradation mediated by the proteasome provides an important source of amino acids for ongoing protein synthesis when amino acid availability becomes limiting (Vabulas and Hartl, 2005).
(5) Degradation of Phycobilisomes
Photosynthetic organisms take advantage of protein degradation to release macronutrients when they find themselves N limited. In Synechococcus strain DC2, the light-harvesting phycobiliproteins may constitute 50% of total protein. Phycobilisome complexes in Synechococcus can be targeted for degradation in response to either N or S limitation (Collier and Grossman, 1992) and that degradation appears to be a highly regulated and ordered process (Grossman et al., 1993).