Biology Reference
In-Depth Information
et al. (2009). Polyclonal antibodies raised against HetR protein of Anabaena sp. strain PCC 7120 cross
reacted with the HetR protein present in the above fi ve cyanobacteria. het R and patS are present close
to each other and patS gene is located upstream of hetR . In A . platensis , patS encodes a large protein
of 84 amino acids with RGSGR pentapeptide sequence at its C-terminus. The functional nature of
hetR and patS gene sequences of A . platensis (hetR ar and patS ar ) has been tested by introducing the
promoter regions of these genes along with gfp as reporter gene into Anabaena sp. strain PCC 7120
through conjugation. The exconjugants displayed GFP fl uorescence typical of the expression of
these genes in Anabaena sp. strain PCC 7120, with hetR ar expression not restricted to heterocysts but
patS ar expression restricted to heterocysts after a nitrogen step-down. These observations amply
signify that these gene sequences have evolved much before the appearance of heterocyst and their
presence in non-heterocystous members indicates much wider function for HetR and PatS (Zhang
et al ., 2009).
v) Nature of the inhibitor : According to Fogg (1949) the nature of the inhibitor that forms a gradient
on either side of a heterocyst is ammonia or a derivative of it and when the concentration of this
substance falls below a certain critical level such cells tend to differentiate into heterocysts. Wolk (1967)
fi rst suggested that it is the heterocysts that produce the inhibitor that prevented the neighbouring
cells to differentiate into heterocysts. Diffusion of this compound away from heterocysts or its loss
or degradation within vegetative cells results in the establishment of a gradient in the vegetative
cells on either side of the heterocyst. This was explained on the basis that when proheterocysts are
formed they further produce the inhibitory substance and establish gradient on either side of it (Wolk
and Quine, 1975). Since glutamine is the end product of nitrogen fi xation and is transported into
adjacent vegetative cells, glutamine or a derivative of it may be responsible for the pattern formation
(Thomas et al ., 1977). Glutamine or a derivative of it could play such a role has been emphasized
by Wolk (1979, 1982, 1989, 1991). Since proheterocysts are not equipped with the capacity to fi x
nitrogen to establish such a gradient of glutamine, it was suggested that the source of glutamine is
from extensive proteolysis that undergoes in the vegetative cell before becoming a proheterocyst
(Fleming and Haselkorn, 1974). Thiel and Leone (1986) challenged this on the basis that (i) mutants
of A . variabilis with enhanced glutamine uptake showed conversion of most of the glutamine to
glutamate; (ii) the intracellular concentrations of glutamine, glutamate, arginine, ornithine and
citrulline are higher in glutamine grown cells rather than those cells grown in ammonium nitrogen
or N 2 ; (iii) differentiation of heterocyst occurred in glutamine-containing media; (iv) due to high
intracellular concentration of glutamine there is no question of a gradient being established and even
then heterocyst differentiation took place. The effects of the amino acid analogue, AT in inducing
pairs of heterocysts and multiple contiguous heterocysts are explained on the basis of inhibition
of GOGAT activity and the interconnected events leading to a general nitrogen-defi ciency (Chen
et al ., 1987). The observations of Bottomley et al . (1980) on the feedback inhibition by AT of the fi rst
enzyme in the tryptophan biosynthesis pathway and the possibility of AT being incorporated into
proteins that have a regulatory role in heterocyst differentiation point out to the involvement of
tryptophan metabolism in the heterocyst differentiation (Adams, 1992; Bottomley et al ., 1980; Chen et
al ., 1987). Adams (1992) proposed a modifi cation of inhibitor/diffusion model in which all vegetative
cells produce constitutively an inactive inhibitor that is supposed to be activated by a co-inhibitor
produced by a proheterocyst or mature heterocyst. In presence of combined nitrogen sources like
ammonium, the inhibitor is active in all cells and so no differentiation of heterocysts takes place but
in nitrogen-limited cultures, the co-inhibitor produced by proheterocyst or heterocyst will interact
Search WWH ::




Custom Search