HETEROCYST - Cyanobacteria - page 245

of the active site and a small cap having a glucose-binding site. There are two fl exible hinge tongs
that link the two regions. The expression pattern of spsA and sppA genes of Synechococcus sp. strain
PCC 7002, a marine cyanobacterium, has been reported for the fi rst time and revealed cotranscription
of the genes (Cumino et al ., 2010).
Vargas et al . (2003) reported the occurrence of two A/N-Invs in heterocystous cyanobacteria
and one A/N-Inv in the unicellular forms based on a search of the fully sequenced cyanobacterial
genomes. Anabaena sp. strain PCC 7120 possesses two genes, invA and invB that encode corresponding
proteins with a molecular mass of 53-55 kDa. Lopez-Igual et al . (2010) demonstrated that the
expression of invA and invB occurred under diazotrophic conditions but the abundance of the
transcript and corresponding protein was higher for latter gene. The expression of invA gene was
found uniformly in all vegetative cells as refl ected by the GFP fl uorescence when a transcriptional
fusion construct P - invA-gfp was introduced into Anabaena sp. strain PCC 7120. The expression of invB
was restricted to the developing heterocysts or mature heterocysts suggesting that invB plays an
important role in regulating diazotrophic growth. This is confi rmed by the characteristics of hetR ,
invA and invB mutants. The expression of invB gene was found all along the length of the fi lament
in the the hetR mutant. In the invA mutant the expression of nifHDK was impaired associated
with higher frequency of heterocysts. Loss of Inv-B function in the invB mutant led to impaired
diazotrophic growth.
Thus there are a number of evidences in favour of the existence of intricate relationship between
sucrose synthesis and breakdown, and conversion of sucrose to glycogen and its utilization during
diazotrophic growth in cyanobacteria. Earlier studies of Ernst and Böger (1985) highlighted the
importance of glycogen supported nitrogen fi xation. When A . variabilis ATCC 29413 was subjected
to nitrogen step-down, heterocyst differentiation was preceded by a round of fast accumulation of
glycogen. There is a temporal separation of glycogen accumulation and nitrogen fi xation, the former
occurring in the light and the latter taking place in the dark. The observations of Jensen et al . (1986)
on the high and sustained nitrogen-fi xing capacity of isolated heterocysts, derived from fructose-
grown A . variabilis ATCC 29413, was due to the presence of glycogen granules inside them. Ernst
et al . (1990) reported that a modifi cation of nitrogenase induced by C-limitation could be overcome
due to endogenously stored glycogen and the presence of fructose in presence of air. A positive
regulation of sugar catabolic pathways by a group 2 σ factor SigE (sll1689) in Synechocystis sp. strain
PCC 6803 involving glycolysis, oxidative pentose phosphate pathway and glycogen catabolism may
explain its transcriptional control over these processes (Osnai et al ., 2005).
Cumino et al . (2002) conducted a phylogenetic analysis of sucrose biosynthesis-related
proteins from cyanobacteria ( Anabaena sp. strain PCC 7120, Anabaena sp. strain PCC 7119; N .
punctiforme , Prochlorococcus marinus MED4, P . marinus MIT 9313, Synechococcus sp. strain WH 8102
and Synechocystis sp. strain PCC 6803) and higher plants (rice, maize, potato, Arabidopsis thaliana
and Medicago truncatula ) and indicated that Anabaena SPS maintains a glucosyltransferase domain
that constitutes the minimal cataltytic unit of all SPPs and some SPSs. Lunn (2002) traced the
origin of sucrose synthesis to the proteobacteria or a common ancestor of the proteobacteria and
cyanobacteria. Vargas et al . (2003) conducted a phylogenetic analysis of A/N-Invs of cyanobacteria
and suggested that the origin of higher plant A/N-Invs could be from an orthologous ancestral gene
of a cyanobacteria as a result of endosymbiotic event.
vii) Amino acid transport and diazotrophic growth : A number of amino acids are utilized as
alternative nitrogen sources by cyanobacteria by causing a repression in nitrogenase activity (Neilson
and Larsson, 1980; Vaishampayan, 1982; Rawson, 1985; Spence and Stewart, 1986). However, transport