Biology Reference
In-Depth Information
from these sites is directly regulated by NrrA. In the
nrrA
mutant, the transcripts from the above
corresponding tsps are either absent or reduced after nitrogen step-down. Since transcription of
hetR
at tsp at -728bp is dependent on NtcA, it may be that NtcA binding to this tsp is mediated by
NrrA. Further, when NrrA is overexpressed, it leads to up-regulation of HetR synthesis. Ehira and
Ohmori (2006b) explained the sequence of events as follows: nitrogen limitation causes increased
production of NrrA stimulated by NtcA and in turn NrrA induces the expression of
hetR
resulting
in heterocyst differentiation.
10) SIGNALLING MOLECULES
Two signalling molecules calcium and 2-oxoglutarate (2-OG) have been identifi ed that transmit
signals for the need to differentiate heterocysts to fulfi ll diazotrophic growth.
i) Calcium
:
Calcium is required for a number of physiological processes in cyanobacteria. These
include the dependence of PSII activities of
Anacystis nidulans
in
vitro
(England and Evans, 1983)
and
in vivo
(Becker and Brand, 1985), phosphate uptake by
A
.
nidulans
and
Oscillatoria limnetica
(Kerson
et al
., 1984) and in the protection of nitrogenase in
Gleocapsa
sp. 1430/3 (Falah Hamadi and
Gallon, 1981) and
Gloeothece
sp. ATCC 27152 from O
2
damage (Gallon and Falah Hamadi, 1984).
The requirement of calcium as a macronutrient for aerobic nitrogen fi xation by two species of
Anabaena
(
Anabaena
sp. and
Anabaena
sp. ATCC 33047) and its requirement in traces when grown
in microaerophilc conditions, prompted Rodríguez
et al.
(1990) to suggest that calcium has role
in conferring protection to nitrogenase present in heterocysts. A calcium-mediated regulation of
heterocyst frequency and nitrogen fi xation has been demonstrated in
Nostoc
sp. PCC 6720 (ATCC
27895) by Smith
et al
. (1987) who showed that a decrease of calcium concentration (0.01 mM to 0.1
mM) stimulated heterocyst production from 5% to 9% whereas at calcium concentrations normally
employed in growth media (0.1 to 1.0 mM) the heterocyst frequency did not fl uctuate (remained at
5%). In presence of calcium ionophore, compound A23187 (calcimycin; in between 0.01 µM to 0.5
µM), that facilitates calcium uptake, and 0.1 to 1.0 mM Ca
2+
, the frequency of heterocysts increased
from 7.0% to 10.5%. This signifi es that Ca
2+
and A23187 showed synergistic effect. A reduction in the
frequency of heterocysts has been noted by the inclusion of calcium channel inhibitor, lanthanium
(La
3+
).
45
Ca
2+
-uptake experiments showed that an increase in intracellular Ca
2+
occurred under
low illumination probably due to a decreased rate of effl ux of Ca
2+
and in presence of A23187, the
intracellular Ca
2+
increased. Intracellular
45
Ca
2+
and heterocyst frequency in
Nostoc
sp. PCC 6720 have
been shown to be negatively correlated with incident light (Smith and Wilkins, 1988).
The cellular Ca
2+
concentration is modulated by a special calcium-binding protein known as
calmodulin (calcium modulated protein) which interacts with cellular proteins that are to be activated
by the release of calcium. Calmodulin and calcium-mediated regulation of cellular processes has
thus received attention not only in eukaryotes but in prokaryotes as well (Onek and Smith, 1992).
Isolation and characterization of calmodulin-like proteins from cyanobacteria have been successful.
Pettersson and Bergman (1989) put forward evidences for the presence of calmodulin-like protein
in vegetative cells and heterocysts of three species of
Anabaena
. The immunogold particles are more
densely distributed in the central portion of vegetative cells while in heterocysts they are evenly
distributed. These studies revealed the presence of a polypeptide of about 17 kDa that interacted
with the antiserum raised against spinach calmodulin. Onek
et al
. (1992) studied the properties of
calmodulin-like protein from
Nostoc
sp. PCC 6720 and showed that the 21 kDa protein possessed
characteristics of eukaryotic calmodulin and activated pea NAD kinase
in vitro
in a Ca
2+
-requiring
reaction. The intracellular free Ca
2+
([Ca
2+
]
i
), in response to increasing extracellular Ca
2+
during heat
