Biology Reference
In-Depth Information
to water by heterocyst-specific terminal respiratory oxidases. In
Anabaena
sp. strain PCC 7120, two gene clusters for haeme-copper oxidases,
cox2
(encoding a cytochrome
c
oxidase) and
cox3
(encoding a possible quinol oxi-
dase), are expressed during heterocyst differentiation (
Jones & Haselkorn,
2002
;
Valladares, Herrero, Pils, Schmetterer, & Flores, 2003
;
Valladares,
Maldener, Muro-Pastor, Flores, & Herrero,
2007
). Electrons from H
2
,
produced as a byproduct in the nitrogenase reaction (
Rubio & Ludden,
2008
), can also follow these paths with the concourse of an uptake hydroge-
nase (
Happe, Schütz, & Böhme, 2000
;
Peterson & Wollk, 1978b
). In addition
to the membrane quinone pool and the soluble cytochrome
c
6
, the elec-
tron transport chain involves the activities of the NAD(P)H dehydrogenase
(NDH-1) and cytochrome
b
6
-
f
complexes, in which, as is also the case for
cytochrome
c
oxidase, energy conservation in the form of a proton gradient
takes place. As is well known, the proton gradient results in ATP biosyn-
thesis catalysed by the H
+
-transporting F
0
F
1
ATP synthase. The heterocysts
contain a special arrangement of intracellular membranes that is located
next to the heterocyst neck(s) and has been called the 'honeycomb' (
Lang
& Fay, 1971
) (see
Fig. 8.1
B). Some cytochemical evidence obtained with
Anabaena cylindrica
suggests that respiration can take place in the heterocyst
honeycomb (
Murry, Olafsen, & Benemann, 1981
), and a recent proteomic
analysis of heterocyst membranes from
Nostoc punctiforme
has found that
the intracellular membranes are dominated by photosystem I and ATPase
proteins, whereas the NDH-1 and cytochrome
b
6
-
f
complexes are readily
identified in a 'cell wall' fraction that contains cytoplasmic membranes (
Car-
dona et al., 2009
). Subcellular localization of the bioenergetic complexes of
the heterocyst will merit further investigation.
2.3.3. Nitrogen metabolism
As described above, the heterocyst contains nitrogenase and is the site of
nitrogen fixation in filaments grown under oxic conditions. The nitrogenase
produces ammonia that, as shown by fixation of
13
N-labelled N
2
by whole
filaments, is immediately incorporated as the amido group of glutamine,
from which the label passes to the amino group of glutamate (
Wolk, Thomas,
Shaffer, Austin, & Galonsky, 1976
). This pattern of labelling is consistent with
incorporation of ammonia through the glutamine synthetase-glutamate syn-
thase (GS/GOGAT) cyclic pathway, in which ammonia is added to glutamate
by the ATP-dependent glutamine synthetase and the resulting glutamine
transfers its amido group to 2-oxoglutarate producing two molecules of glu-
tamate in a reaction catalysed by glutamate synthase, which requires reductant
