23
Upon limitation of availability of fixed N, NtrC is phosphorylated, starting the cascade of N response genes. N response genes, other than nitrogenase, include glutamine synthetase, ammonium transporters, and nitrate reductase. PII is activated/
inactivated by uridylylation/deuridylylation and the active form results in activation
of other proteins such as glutamine synthetase and NtrC by adenylation or phosphorylation. Phosphorylated NtrC activates transcription of nifA (and in
Rhodobacter, anfA), which activates nifHDK. The Mo uptake system (mod genes)
is repressed along with the anf genes by MopAB, when Mo is available and high
affinity uptake or expression of alternative nitrogenases are unnecessary
(Masepohl 2017).
In anaerobes, including clostridia, chlorobi, delta-proteobacteria and
Euryarchaeota (methanogens), the nifI genes encode a divergent PII-like protein,
that is involved in switchoff of dinitrogenase directly (Leigh and Dodsworth 2007).
The nifI genes are usually contiguous with the structural nif genes (Leigh and
Dodsworth 2007).
In proteobacteria, the transcription of the nifHDK genes are regulated in response
to O 2 (particularly in symbionts) and presence of ammonium (Dixon 2004). NifA,
along with the transcription factor sigma-54 (RpoN) act at RpoN promoters, and
plays a major role in activation of transcription of nitrogenase genes, although its
own expression is regulated by other factors, such as NtrB-C (a two component
regulatory system involving phosphorylation) which responds to N availability
(Dixon and Kahn 2004). NifL binds to NifA in the presence of O 2 as part of a complex hierarchical regulatory response that varies amongst species, activated by low
N but inactivated by presence of O 2 . The regulatory cascades involve further gene
products, such as the Fix K and J or RegB-RegA proteins in alpha proteobacteria,
but the intricacies of the regulation and feedback depend on microorganism, whether
it is symbiotic or not, and what the local environmental conditions (e.g. nodules) are
for the cells (Dixon and Kahn 2004). In photosynthetic bacteria like Rhodobacter,
nitrogenase is further rapidly regulated by “switch-off”, a post-translational ADP
ribosylation that responds to ammonium or darkness and prevents electron flow
from the Fe protein. The ADP ribosylation is catalysed by DraT (dinitrogenase
reductase ADP-ribosyl transferase) to inactivate the Fe protein and reactivated by
removal of ADP by DraG (dinitrogenase reductase-activating glycohydrolase).
Cyanobacteria have several differences from N 2 fixation regulation in the noncyanobacteria bacteria. PII, PipX and NtcA are involved in N control, although
NtcA can also regulate other metabolic pathways as well, as either a positive or
negative regulator (Esteves-Ferreira et al. 2018). Similar to other bacteria, N deprivation leads to increased expression of ammonium transporters and expression of
metabolic pathways for alternative N sources such as nitrate, nitrite, urea, arginine
and other N-rich compounds (Herrero et al. 2019). All N 2 -fixing cyanobacteria have
a cnfR gene that senses redox and is involved in regulation, but in heterocyst- forming
strains it is only expressed in heterocysts (Herrero et al. 2019). CnfR works along
with transcription factors to regulate transcription of the nifB promoters that are the
major promoter for the nif clusters, at least in Dolichospermum (Anabaena)
(Thiel 2019). Small RNAs have also been shown to be involved in regulation in
2.5 Cellular Regulation of Nitrogenase
Upon limitation of availability of fixed N, NtrC is phosphorylated, starting the cascade of N response genes. N response genes, other than nitrogenase, include glutamine synthetase, ammonium transporters, and nitrate reductase. PII is activated/
inactivated by uridylylation/deuridylylation and the active form results in activation
of other proteins such as glutamine synthetase and NtrC by adenylation or phosphorylation. Phosphorylated NtrC activates transcription of nifA (and in
Rhodobacter, anfA), which activates nifHDK. The Mo uptake system (mod genes)
is repressed along with the anf genes by MopAB, when Mo is available and high
affinity uptake or expression of alternative nitrogenases are unnecessary
(Masepohl 2017).
In anaerobes, including clostridia, chlorobi, delta-proteobacteria and
Euryarchaeota (methanogens), the nifI genes encode a divergent PII-like protein,
that is involved in switchoff of dinitrogenase directly (Leigh and Dodsworth 2007).
The nifI genes are usually contiguous with the structural nif genes (Leigh and
Dodsworth 2007).
In proteobacteria, the transcription of the nifHDK genes are regulated in response
to O 2 (particularly in symbionts) and presence of ammonium (Dixon 2004). NifA,
along with the transcription factor sigma-54 (RpoN) act at RpoN promoters, and
plays a major role in activation of transcription of nitrogenase genes, although its
own expression is regulated by other factors, such as NtrB-C (a two component
regulatory system involving phosphorylation) which responds to N availability
(Dixon and Kahn 2004). NifL binds to NifA in the presence of O 2 as part of a complex hierarchical regulatory response that varies amongst species, activated by low
N but inactivated by presence of O 2 . The regulatory cascades involve further gene
products, such as the Fix K and J or RegB-RegA proteins in alpha proteobacteria,
but the intricacies of the regulation and feedback depend on microorganism, whether
it is symbiotic or not, and what the local environmental conditions (e.g. nodules) are
for the cells (Dixon and Kahn 2004). In photosynthetic bacteria like Rhodobacter,
nitrogenase is further rapidly regulated by “switch-off”, a post-translational ADP
ribosylation that responds to ammonium or darkness and prevents electron flow
from the Fe protein. The ADP ribosylation is catalysed by DraT (dinitrogenase
reductase ADP-ribosyl transferase) to inactivate the Fe protein and reactivated by
removal of ADP by DraG (dinitrogenase reductase-activating glycohydrolase).
Cyanobacteria have several differences from N 2 fixation regulation in the noncyanobacteria bacteria. PII, PipX and NtcA are involved in N control, although
NtcA can also regulate other metabolic pathways as well, as either a positive or
negative regulator (Esteves-Ferreira et al. 2018). Similar to other bacteria, N deprivation leads to increased expression of ammonium transporters and expression of
metabolic pathways for alternative N sources such as nitrate, nitrite, urea, arginine
and other N-rich compounds (Herrero et al. 2019). All N 2 -fixing cyanobacteria have
a cnfR gene that senses redox and is involved in regulation, but in heterocyst- forming
strains it is only expressed in heterocysts (Herrero et al. 2019). CnfR works along
with transcription factors to regulate transcription of the nifB promoters that are the
major promoter for the nif clusters, at least in Dolichospermum (Anabaena)
(Thiel 2019). Small RNAs have also been shown to be involved in regulation in
2.5 Cellular Regulation of Nitrogenase