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functional ability of catalyzing the production and consumption of H
2
both
in vitro
as well as
in vivo
,
the reversible enzymes have been redesignated as bidirectional enzymes in the literature. Based on the
metallocenters, three groups of hydrogenases, i.e. [NiFe] hydrogenases, [FeFe] hydrogenases and [Fe]
hydrogenases have been recognized in prokaryotes. The existence of latter type of hydrogenases has
been described in methane bacteria (Pilak
et al
., 2006). All cyanobacterial hydrogenases are of [NiFe]-
type (Tamagnini
et al
., 2000, 2002, 2005). Nitrogen-fi xing cyanobacteria possess uptake hydrogenase
with the exception of
Synechococcus
sp. BG 043511. In addition, they may also have a bidirectional
hydrogenase and in this respect
Nostoc
sp. strain PCC 73102 is an exception (Tamagnini
et al
., 1997).
The non-nitrogen-fi xing unicellular cyanobacteria generally possess the bidirectional enzyme but
exceptionally
Gloeobacter violaceus
PCC 7421 does not possess this enzyme (Nakamura
et al
., 2003;
Ludwig
et al
., 2006). Min and Sherman (2010) reported the presence of an uptake hydrogenase as well
as a bidirectional hydrogenase in the versatile unicellular nitrogen-fi xing cyanobacterium
Cyanothece
sp. strain ATCC 51142 that showed maximum rates of H
2
production under photosynthetic conditions.
A phylogenetic analysis of hydrogenase genes from fi ve groups of photosynthetic bacteria including
cyanobacteria revelaed the green sulfur bacterium
Chlorofl exus aurantiacus
as the closest ancestor for
vertical transmission of genes into cyanobacteria (Ludwig
et al
., 2006).
Cyanobacterial uptake and bidirectional hydrogenases are nickel-dependent enzymes (Zhang
et
al
., 1984; Almon and Böger, 1984; Pederson
et al
., 1986; Hausinger, 1987). The effect of environmental
factors (light/dark or continuous light regimes, aerobic/anaerobic conditions) and nutrients
such as Ni
2+
on H
2
formation in
A
.
cylindrica
has been investigated (Daday
et al
., 1977; Daday and
Smith, 1983). Ni
2+
(at 0.68 µM) supported maximum (175 nmol h
-1
mg
-1
) H
2
-uptake activity when
compared to Ni
2+
depleted cultures after 5 d growth of
A
.
cylindrica
(Daday
et al
., 1985). Xiankong
et al
. (1984) suggested that nickel may be required for either activation of an uptake hydrogenase
or H
2
formation or in the synthesis of another protein involved in H
2
uptake. On the basis of the
presence of a dark uptake hydrogenase activity and a light-dependent uptake in presence of DCMU
at low O
2
concentration in
Anabaena
strains CA and IF, these workers further concluded that H
2
may
serve as electron donor to both photosynthetic and respiratory electron transport chains. Kumar
and Kumar (1990) studied H
2
evolution by
Anabaena
sp. strain CA in presence of fructose and
erythrose and photosynthetic inhibitors and similarly concluded that the all electrons irrespective
of source must pass through plastoquinone pool before reducing ferredoxin. Photoheterotrophic
cultures (with glucose and DCMU in light) of
Nostoc
sp. strain Cc (symbiont of
Cycas circinalis
)
when supplemented with Ni consumed 10 times more H
2
than control cultures (with DCMU). The
biomass increased with improved nitrogen status underlining the importance of the availability of
Ni as an important factor in establishing symbiosis (Tredici
et al
., 1990). The localization of uptake
hydrogenase in the membranes of heterocysts of
Anabaena
sp. strain PCC 7120 (Peterson and Burris,
1978; Houchins and Burris, 1981b) and other cyanobacteria (Eisbrenner
et al
., 1978; Tel-Or
et al
., 1978)
has been demonstrated. However, the localization of uptake hydrogenase in both vegetative cells
and heterocysts of
N
.
punctiforme
ATCC 29133 and some other symbiotic strains has been shown to
be membrane bound by immunological techniques (Lindblad and Sellstedt, 1990; Rai
et al
., 1992).
Seabra
et al
. (2009) conducted immunolocalization based on antisera prepared from cyanobacterial
hydrogenase enzymes and showed that in
Anabaena
sp. strain PCC 7120 the uptake hydrogenase is
restricted to the heterocysts while in
N
.
punctiforme
ATCC 29133 the uptake hydrogenase was found
in both the vegetative cells and heterocysts. In the non-heterocystous, fi lamentous
Lyngbya majuscula
the uptake hydrogenase was found to be membrane bound. The bidirectional hydrogenase has been
shown to be present in both vegetative cells and heterocysts of
Anabaena
sp. strain PCC 7120 in a
soluble or loosely membrane bound form (Houchins and Burris, 1981b).
