P-II family proteins are regulators of nitrogen metabolism, with a length of
~110 amino acid residues, found throughout the bacteria and most archaea. P-II
proteins sense cellular 2-oxoglutarate as an indicator for nitrogen limitation
and bind and respond to ATP. The family is named after the P-II protein (gene
glnB), a bacterial protein important for the control of glutamine synthetase
[1,2,3]. In nitrogen-limiting conditions, when the ratio of glutamine to
2-ketoglutarate decreases, P-II is uridylylated on a tyrosine residue to form
P-II-UMP. P-II-UMP allows the deadenylation of glutamine synthetase (GS), thus
activating the enzyme. Conversely, in nitrogen excess, P-II-UMP is
deuridylated and then promotes the adenylation of GS. P-II also indirectly
controls the transcription of the GS gene (glnA) by preventing NR-II (ntrB) to
phosphorylate NR-I (ntrC) which is the transcriptional activator of glnA. Once
P-II is uridylylated, these events are reversed. The tyrosine which is
uridylated is located in the T-loop in the central part of the protein.
Not all P-II family proteins have this site for sensing cellular glutamine
. In cyanobacteria, P-II seems to be phosphorylated on a serine residue
rather than being uridylated. It has a different set of interacting proteins.
In methanogenic archaebacteria, the nitrogenase iron protein gene (nifH) is
followed by two open reading frames highly similar to the eubacterial P-II
protein . These proteins could be involved in the regulation of nitrogen
In the red alga, Porphyra purpurea, there is a glnB homolog encoded in the
Other proteins highly similar to glnB are:
- Bacillus subtilis protein nrgB .
- Escherichia coli glnK .
- Plant P-II protein, which is localized to the chloroplast .
We developed two signature patterns for P-II protein. The first one is a
conserved stretch (in eubacteria) of six residues which contains the
uridylated tyrosine, the other is derived from a conserved region in the C-terminal part of the P-II protein. We also developed a profile that covers the
whole P-II protein.
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