24
cyanobacteria, including in N 2 fixation and heterocyst differentiation (He et al.
2020; Ionescu et al. 2010).
Some cyanobacteria are unicellular and fix at night, others are filamentous and
fix N only in heterocysts, and others are filamentous and fix under anaerobic conditions (e.g. Leptolyngbya) or aerobically during the day (Trichodesmium) (Thiel
2019). Thus, the regulatory networks differ to coincide with the light-dark cycle and
partially involve a circadian rhythm. The unicellular N 2 -fixing cyanobacteria are
best known from studies of Cyanothece sp. (Aurora and Pakrasi 2008), and more
recently the marine Crocosphaera sp. (Shi et al. 2010). UCYN-A, the unicellular
marine symbiont of a unicellular alga, fixes N 2 and expresses nitrogenase genes during the day, in contrast to the free-living unicellular diazotrophic cyanobacteria.
Interestingly, Cyanothece sp. and Crocosphaera will fix N 2 even under constant
illumination, although the mechanisms are not understood. A comparison of the
daily patterns of whole genome expression showed that cyanobacteria that fix N 2
during the day, Trichodesmium and UCYN-A, have different patterns than the unicellular cyanobacteria Cyanothece and Crocosphaera (Muñoz-Marín et al. 2019).
Amongst the cyanobacteria, the most is known about genetic regulation in
heterocyst- forming species. There are specific cnfR genes to activate 3 different nif
clusters in a cell type specific manner (Thiel 2019). The heterocyst-forming cyanobacterium Dolichospermum (Anabaena) sp. ATCC 29413 has a Mo and a V nitrogenase expressed only in heterocysts, and a third nif2 gene cluster that is expressed in
vegetative cells under anaerobic conditions (Berman-Frank et al. 2003; Thiel 2019).
The organization of the clusters is similar in other cyanobacteria, including unicellular N 2 -fixing cyanobacteria (Thiel 2019). N 2 fixation in Dolichospermum
(Anabaena) begins with the differentiation of the heterocysts involving the regulatory factors NtcA, HetR, NrrA and DevH (Thiel 2019). NtcA activation leads to
expression of a suite of genes involved in N-responses including HetR for heterocyst differentiation (Thiel 2019). HetR and NtcA activate genes only in the cells that
will ultimately be heterocysts, including nitrogenase. The heterocysts, depending on
species, are interspersed with vegetative cells (approximately every 10 cells in
actively fixing Dolichospermum (Anabaena)). This regular arrangement is caused
by the expression of a short peptide, PatS, that is an inhibitor of HetR, produced in
differentiating heterocysts and creates a concentration gradient along the chain of
neighbouring vegetative cells (Herrero et al. 2019).
There has been speculation that there could be post-translational modification
of the Fe protein analogous to the ADP ribosylation in purple bacteria (Herrero
et al. 2001), but it does not involve ribosylation and has never been completely
resolved (Ekman et al. 2013). A proteomic study localized a modification of the
Fe protein in the heterocyst-forming symbiont of Azolla, that is not ADP ribosylation and is small (300-400 Da) but could not identify the modifying group
(Ekman et al. 2013).
2 Fundamentals of N 2 Fixation
cyanobacteria, including in N 2 fixation and heterocyst differentiation (He et al.
2020; Ionescu et al. 2010).
Some cyanobacteria are unicellular and fix at night, others are filamentous and
fix N only in heterocysts, and others are filamentous and fix under anaerobic conditions (e.g. Leptolyngbya) or aerobically during the day (Trichodesmium) (Thiel
2019). Thus, the regulatory networks differ to coincide with the light-dark cycle and
partially involve a circadian rhythm. The unicellular N 2 -fixing cyanobacteria are
best known from studies of Cyanothece sp. (Aurora and Pakrasi 2008), and more
recently the marine Crocosphaera sp. (Shi et al. 2010). UCYN-A, the unicellular
marine symbiont of a unicellular alga, fixes N 2 and expresses nitrogenase genes during the day, in contrast to the free-living unicellular diazotrophic cyanobacteria.
Interestingly, Cyanothece sp. and Crocosphaera will fix N 2 even under constant
illumination, although the mechanisms are not understood. A comparison of the
daily patterns of whole genome expression showed that cyanobacteria that fix N 2
during the day, Trichodesmium and UCYN-A, have different patterns than the unicellular cyanobacteria Cyanothece and Crocosphaera (Muñoz-Marín et al. 2019).
Amongst the cyanobacteria, the most is known about genetic regulation in
heterocyst- forming species. There are specific cnfR genes to activate 3 different nif
clusters in a cell type specific manner (Thiel 2019). The heterocyst-forming cyanobacterium Dolichospermum (Anabaena) sp. ATCC 29413 has a Mo and a V nitrogenase expressed only in heterocysts, and a third nif2 gene cluster that is expressed in
vegetative cells under anaerobic conditions (Berman-Frank et al. 2003; Thiel 2019).
The organization of the clusters is similar in other cyanobacteria, including unicellular N 2 -fixing cyanobacteria (Thiel 2019). N 2 fixation in Dolichospermum
(Anabaena) begins with the differentiation of the heterocysts involving the regulatory factors NtcA, HetR, NrrA and DevH (Thiel 2019). NtcA activation leads to
expression of a suite of genes involved in N-responses including HetR for heterocyst differentiation (Thiel 2019). HetR and NtcA activate genes only in the cells that
will ultimately be heterocysts, including nitrogenase. The heterocysts, depending on
species, are interspersed with vegetative cells (approximately every 10 cells in
actively fixing Dolichospermum (Anabaena)). This regular arrangement is caused
by the expression of a short peptide, PatS, that is an inhibitor of HetR, produced in
differentiating heterocysts and creates a concentration gradient along the chain of
neighbouring vegetative cells (Herrero et al. 2019).
There has been speculation that there could be post-translational modification
of the Fe protein analogous to the ADP ribosylation in purple bacteria (Herrero
et al. 2001), but it does not involve ribosylation and has never been completely
resolved (Ekman et al. 2013). A proteomic study localized a modification of the
Fe protein in the heterocyst-forming symbiont of Azolla, that is not ADP ribosylation and is small (300-400 Da) but could not identify the modifying group
(Ekman et al. 2013).
2 Fundamentals of N 2 Fixation