Table 2.
Components of the nitrogen assimilation network
| Genea | Proteina,b | Full protein name and/or activity(ies) |
|---|---|---|
| glnA | GS | Glutamine synthetase; catalyzes the synthesis of glutamine |
| glnB | GlnB (PII)c | GlnB protein, signal-transducing protein; GlnB stimulates adenylylation of GS, GlnB-UMP stimulates deadenylylation of GS-AMP, and GlnB stimulates phosphatase activity of NRII |
| glnD | UTased | Uridylyltransferase/uridylyl-removing protein; uridylylates GlnB and GlnK and deuridylylates GlnB-UMP and GlnK-UMP |
| glnE | ATased | Adenylyltransferase/adenylyl-removing protein; adenylylates GS and deadenylylates GS-AMP |
| glnF (ntrA; rpoN) | σ54 (σN) | RNA polymerase sigma factor 54 |
| glnG (ntrC) | NRI (NtrC)e | Nitrogen regulator I/NtrC; NRI-P activates glnALG transcription at the glnAp2 promoter, and NRI represses glnALG transcription at the glnAp1 and glnLp promoters |
| glnL (ntrB) | NRII (NtrB)e | Nitrogen regulator II/NtrB; NRII phosphorylates NRI, and NRII + GlnB or GlnK dephosphorylates NRI-P |
| glnK | GlnKc | GlnK protein, signal-transducing protein; GlnK stimulates adenylylation of GS, GlnK-UMP stimulates deadenylylation of GS-AMP, GlnK stimulates phosphatase activity of NRII, and GlnK inhibits AmtB activity |
| amtB | AmtB | Ammonium transporter AmtB; conducts ammonium across the cytoplasmic membrane and functions as a sensor for extracellular ammonium |
| glnHPQ | GlnHPQ | Glutamine transporter; high-affinity ABC transporter |
| nac | Nac | Nitrogen assimilation control protein |
| crp | Crp | cAMP receptor protein |
| lrp | Lrp | Leucine-responsive regulatory protein |
| gltBDF | GOGAT | Glutamate synthase |
| gdhA | GDH | Glutamate dehydrogenase |
Alternative name(s) is in parentheses.
Historical nomenclature is as follows: PI, ATase; PIIA, GlnB; PIID, GlnB-UMP.
PII and GlnK are PII paralogs, but for clarity, both proteins are named after their gene name throughout this review; thus, PII is called GlnB.
UTase instead of UTase/UR and ATase instead of ATase/AR are used throughout this review to denote these bifunctional/ambiguous enzymes.
We propose the use of NRI and NRII, because they are a complete set of glnA-specific expression regulators (there is no NRIII), which is not suggested by the naming of NtrB and NtrC, because NtrA is a sigma factor that regulates multiple operons, including non-nitrogen-related ones, and is itself not part of the glnALG operon.