Biology Reference
In-Depth Information
Stewart, 1977
). Lack of water photolysis would contribute to keeping a
micro-oxic intracellular environment for nitrogenase but pays the price of
loss of photosynthetic CO
2
fixation (
Wolk, 1968
). Indeed, heterocysts lack
ribulose-
bis
-phosphate carboxylase/oxygenase (Rubisco) activity (
Winken-
bach & Wolk, 1973
) and protein (see, e.g. Fig. 4 in
Cardona et al., 2009
).
Nitrogen fixation requires reductant and energy in the form of ATP (
Rubio
& Ludden, 2008
). Carbon fixed in the vegetative cells moves to the het-
erocysts (
Wolk, 1968
), and sucrose has been long supposed to be the trans-
ferred organic substrate (
Wolk et al., 1994
). Recent results have shown that
a heterocyst-specific invertase is indeed needed for diazotrophic growth
corroborating the idea that sucrose is a transferred substrate (
López-Igual,
Flores, & Herrero, 2010
;
Vargas, Nishi, Giarrocco, & Salerno, 2011
). Sugar
catabolism in the heterocyst follows the oxidative pentose phosphate path-
way as suggested by detection of increased levels of glucose 6-phosphate
dehydrogenase and 6-phosphogluconate dehydrogenase (
Lex & Carr, 1974
;
Winkenbach & Wolk, 1973
). Further, a requirement of glucose 6-phos-
phate dehydrogenase for nitrogen fixation has been evidenced by inactiva-
tion of the
zwf
gene in
Nostoc punctiforme
(
Summers, Wallis, Campbell, &
Meeks, 1995
). Other substrates, such as the amino acid alanine, can also be
transferred from vegetative cells to heterocysts, where alanine can be catab-
olized by alanine dehydrogenase and its products enter the tricarboxylic
acid pathway (which in cyanobacteria lacks 2-oxoglutarate dehydrogenase)
providing reductant (
Jüttner, 1983
;
Pernil, Herrero, & Flores, 2010
).
2.3.2. Bioenergetics
A distinct aspect of heterocyst bioenergetics is that the action spectrum
of nitrogen fixation (measured by the acetylene reduction assay) corre-
sponds to that of photosystem I activity (
Fay, 1970
). Because photosystem
I-dependent cyclic electron flow takes place in the heterocysts (
Almon &
Böhme, 1982
), cyclic photophosphorylation is likely important for nitrogen
fixation. On the other hand, the reducing power in the form of NADPH
produced in the oxidation of the sugar transferred from vegetative cells
can be used for N
2
fixation via ferredoxin-NADP
+
oxidoreductase and the
heterocyst-specific ferredoxin, FdxH, that in its reduced state can be a direct
substrate of nitrogenase reductase (
Masepohl, Schölisch, Görlitz, Kutzki, &
Böhme, 1997
;
Razquin et al., 1995
,
1996
). However, NADPH (and NADH)
can also feed electrons into the electron transport chain of heterocyst mem-
branes to provide photosystem I with electrons (that may serve in part
for N
2
fixation;
Lockau, Peterson, Wolk, & Burris, 1978
) and reduce O
2
