Biology Reference
In-Depth Information
A survey of the genomes of
Anabaena
sp. strain PCC 7120,
N
.
punctiforme
ATCC 29133/PCC 73102
and
Synechocystis
sp. strain PCC 6803 for genes encoding C-terminal endopeptidases revealed
hupW
and
hoxW
genes, for the uptake and bidirectional hydrogenases, respectively.
Anabaena
sp.
strain PCC 7120 has a
hoxW
gene (
all0770
) and a
hupW
(
alr1423
) gene for the bidirectional and
uptake hydrogenases, respectively synthesized by it. In
N
.
punctiforme
ATCC 29133 a
hupW
gene
(
c509/r320
) is present for the only uptake hydrogenase present while
Synechocystis
sp. strain PCC
6803 showed a
hoxW
(
slr1876
) for the only bidirectional hydrogenase produced by it (Wünschiers
et al
., 2003). The
hupW
gene is present downstream of the large cluster of genes that encode
Mo-nitrogenase. The unicellular
Gloeothece
sp. ATCC 27152 that fi xes nitrogen aerobically showed
the presence of
hupW
, 184 bp upstream of
hupL
. RT-PCR experiments revealed that
hupSLW
genes are
co-transcribed under nitrogen-fi xing conditions but not in presence of nitrate nitrogen. The presence
of a putative NtcA-binding site in the promoter region of
hupS
around -41.5 bp with respect to tsp is
another signifi cant feature highlighting the role of NtcA in the transcriptional regulation of
hupSLW
(Oliveira
et al
., 2004). The
hypABCDEF
genes are present as an operon as in
Anabaena
sp. strain PCC
7120 and
A
.
variabilis
ATCC 29413 but they are spread out on the genome of
Synechocystis
sp. strain
PCC 6803. The role of
hyp
genes in the latter organism has been identifi ed by the characterization
of deletion and disruptant mutants that did not exhibit any hydrogenase activity (Hoffmann
et
al
., 2006). Signifi cant levels of transcripts for
hypFCDEAB
with
hup
and
hox
genes of hydrogenases
occurred concomitantly under nitrogen-fi xing conditions than under non-nitrogen-fi xing conditions
in
L
.
majuscula
CCAP 1446/4. The involvement of NtcA and LexA in the transcriptional regulation
of
hypFCDEAB
genes has been demonstrated by electrophoretic mobility shift assays (Ferreira
et al
.,
2007). Agervald
et al
. (2008) demonstrated that the
hypABCDEF
genes in
Anabaena
sp. strain PCC
7120 are transcribed as a single operon with tsps at 280 bp and 445 bp upstream of
hypF
and
hypC
,
respectively. This 14-kb region with the tsps of the different
hyp
-genes, the amplifi ed products in
RT-PCR experiments, and the respective primers employed are depicted in Fig. 11 B and C.
15) OTHER FUNCTIONS
Now that the role of heterocyst in nitrogen fi xation has been unequivocally proved, according to
Haselkorn (2010) it is futile to attribute any function to the heterocyst other than nitrogen fi xation.
Probably these 'other functions' listed below have been ascribed to the heterocyst at a time when
the main role in nitrogen fi xation remained obscure. Because the evidences presented in favour of
the germination of heterocyst and its role in akinete formation cannot be completely disregarded
these are considered here. There are at least four other functions attributed to heterocysts. The
heterocysts serve as (i) archaic reproductive structures; (ii) storage cells; (iii) attachment organs and
(iv) structures that regulate akinete formation.
i) Reproductive structures
:
Instances of heterocyst germination have been recorded from specimens
collected from natural habitats as well from pure cultures of certain members. Some of the materials
reported with germinating heterocysts are those of
Nostoc
ellipsosprum
(Bornet and Thuret, 1880),
Anabaena cycadeae
Reinke (Spratt, 1911),
Calothrix
sp. (Steinecke, 1932) and
Rivularia
sp. (Desikachary,
1946). Wolk (1965) reported a germination frequency of 10% in case of
Anabaena cylindrica
grown
presence of glucose and ammonium chloride. Treatment of
Nostoc linckia
with ultraviolet light induced
heterocyst germination (up to 5%) and the percentage of germination increased to about 24% in a
mutant of this organism. Exceptionally a very high percentage (83%) of heterocyst germination has
been recorded in the wild- and its non-sporulating mutant type of
Gloeotrichia ghosei (
Singh and
