Biology Reference
In-Depth Information
Since the effects of AT are inhibited in presence of chloramphenicol, it was suggested that AT
might be incorporated into proteins in place of tryptophan in case of
Anabaena
sp. CA due to a
feedback inhibition caused in tryptophan biosynthesis at the level of the fi rst enzyme, anthranilate
synthetase (Bottomley
et al
., 1980). Another explanation provided for the mode of action of AT is
that at concentrations at which AT triggered heterocyst formation, it caused inhibition of GOGAT
in
Anabaen
sp. strain CA, the second enzyme involved in ammonia assimilation. In addition, AT
caused a decline in intracellular glutamate pool, decreased C-phycocyanin content and inhibited
nitrogenase activity leading to nitrogen starvation and triggering more heterocyst differentiation
(Chen
et al
., 1987). Triggering of heterocyst differentiation in presence of MSX is due to a direct or
indirect inhibition of GS activity in
Anabaena
spp. (Stewart and Rowell, 1975). Stacey
et al
. (1979)
confi rmed that AT can relieve the repressive effects of ammonium nitrate in medium by enhancing
heterocyst frequency and nitrogenase synthesis and does not inhibit GS activity as MSX does. So
GS is not solely responsible for the induction of heterocysts and nitrogenase in cyanobacteria.
Adams (1992) also observed the formation of heterocysts in pairs in presence of AT in
A
.
cylindrica
.
According to him though asymmetrical cell divisions occur in case of
A
.
cylindrica
during heterocyst
differentiation at least 10% of these produce daughter cells of the same size and the presence of AT
does not alter the situation and so double heterocyst formation results. The decrease of interheterocyst
interval depended on the duration of incubation in AT and the decrease was directly proportional
to the concentration resulting in multiple heterocysts.
A number of metabolic inhibitors such as thiol inhibitors (mercaptoethanol, p-
mercuribenzoate, iodoacetate and ethyl maleimide) sodium azide and glycine inhibited
heterocyst differentiation in
A
.
ambigua
with a reduction in their frequency (Bahal and
Talpasayi, 1970a,b). Inhibitors of photosynthesis (DCMU), respiration (2,4 dinitrophenol, sodium
azide and iodoacetic acid) effectively inhibited heterocyst differentiation in
A
.
doliolum
. Glucose
reversed the effects of DCMU but not those of 2,4-dinitrophenol (Tyagi, 1973a). Antibiotics such as
chloramphenicol (Talpasayi and Kale, 1967) and streptomycin (Kumar and Kaushik, 1971) inhibited
heterocyst differentiation suggesting the requirement of protein synthesis during heterocyst
differentiation. Actinomycin D (an inhibitor of DNA-dependent RNA synthesis) and mitomycin C
(an inhibitor of DNA-dependent DNA synthesis) failed to inhibit heterocyst differentiation in
A
.
doliolum
(Tyagi, 1973b, 1975). The presence of rifampicin, chloramphenicol and mitomycin
C at concentrations 0.2, 0.4 and 1.0 µg ml
-1
, respectively inhibited proheterocyst and mature
heterocyst formation in
A
.
cylindrica
. Concentrations lower than these permitted the development
of proheterocysts but inhibited the development of mature heterocysts. However, the production of
heterocysts in chains, disturbing the normal pattern, in
Nostoc linckia
due to the presence of rifampicin
(0.1 µg ml
-1
) alone or in combination with 2-4-dichlorophenoxyacetic acid (100 µg ml
-1
) has also been
reported (Tiwari
et al
., 1981). The commitment point is the point at which the commitment of a cell
to differentiate into a heterocyst can not be reversed by the addition of fi xed nitrogen sources or
by incubating in darkness. The commitment points for the nitrogen sources, darkness, rifampicin,
profl avin and fl uorouracil were found to be different (Adams and Carr, 1989). Heterocysts in
A
.
azollae
developed only up to proheterocyst level in presence of tunicamycin (beyond 0.2 µg ml
-1
)
and though heterocyst-specifi c glycolipids are synthesized, they are not transported to form the
laminated layer (Reddy
et al
., 1989). The induction of double and multiple heterocysts with altered
spacing and pattern formation by neopeptone (0.4 µg ml
-1
) has been noted in
A
.
cylindrica
(Sharma,
1984). Ultrastructural studies on heterocysts developed in presence of neopeptone revealed normal
structural features with well developed polar nodules (Sharma and von Hofsten, 1987).
