HETEROCYST - Cyanobacteria - page 243

as the best source for supporting acetylene reduction by the isolated heterocysts of Anabaena sp.
strain PCC 7120. While D-erythrose enhanced acetylene reduction by 10 times the rate of control,
the supply of D-glucose, G6P, fructose-1,6-biphosphate and sucrose supported acetylene reduction
by 4-fold (Privalle and Burris, 1984). However, a lot of literature has accumulated on the role of
sucrose synthesis and degradation in the heterocysts to support nitrogen fi xation. The role of sucrose
metabolism in relation to heterocyst differentiation and nitrogen fi xation is presented below.
Sucrose biosynthesis is known to occur in plants and cyanobacteria. Three enzymes are
essential in this respect collectively known as sucrose biosynthesis-related proteins. A two-step
biosynthesis of sucrose is mediated by sucrose phosphate synthase (SPS; UDP-glucose:D-fructose-
6-phosphate 2-α-D-glucosyltransferase; EC 2.4.1.14) and sucrose phosphate phosphatase (SPP;
Sucrose-6-phosphate-phosphohydrolase; EC 3.1.3.24). In the fi rst step, the biosynthesis of sucrose-
6-phosphate takes place in presence of precursors, fructose-6-phosphate and UDP-glucose, energy
for which is derived from the cleavage of uridine diphosphate. In the second terminal step, the
phosphate group from sucrose-6-phosphate is hydrolysed releasing free sucrose. The third enzyme
sucrose synthase (SuS; UDP:glucose-D-fructose-2-α-D-glucosyltransferase; EC 2.4.1.13) synthesizes
sucrose in a single step but this very enzyme can break down sucrose into its subunits. Alkaline/
neutral invertases (A/N-Invs) irreversibly hydrolyze sucrose when there is a high demand for
hexoses. Schilling and Ehrnsperger (1985) were the fi rst to have reported SuS activity in cell-free
extracts from vegetative cells of A . variabilis . Sucrose biosynthesis was mediated by SuS in vegetative
cells and sucrose degradation was attributed to the existence of an A/N-Inv activity in the heterocyst
preparations. A study of the biochemical properties of SuS from Anabaena sp. strain PCC 7119 led to
the identifi cation of two isoforms (Sus-I and SuS-II). SuS-II, a tetramer with a molecular mass of 92
kDa, differed from SuSs of higher plants in substrate specifi city, regulation by metal ions and ATP
and in its N-terminal sequence (Porchia et al ., 1999). The susA gene, encoding Sus-II isoform of SuS,
has been cloned from Anabaena sp. strain PCC 7119 and the deduced amino acid identity with higher
plant enzyme is of the order of 30-40%. The susA gene was expressed in E . coli and the recombinant
protein produced was identical in its biochemical properties with the native enzyme. It is of interest
to know that SuS has a taxonomic signifi cance because of its occurrence in several nitrogen-fi xing
cyanobacterial species and its absence in two of the unicellular non-diazotrophic species investigated
(Curatti et al ., 2000). The involvement of sucrose in the diazotrophic metabolism of Anabaena sp.
strain PCC 7119 has been demonstrated by Curatti et al . (2002) who cloned the sus gene sequences
from Anabaena sp. strain PCC 7119 and Anabaena sp. strain PCC 7120 and designated them as susA
and susB , respectively. The susA gene expression was measured by the transcript abundance in
terms of competitive RT-PCR and the SuSA protein (formerly SS-II) levels. A susA gene disruptant
mutant, LC30 neither showed SuSA protein nor its activity. LC60 strain, an overexpression mutant
of susA , showed enhanced (10-fold) SuSA protein and its activity. There was accumulation of sucrose
in the LC30 cells while it did not accumulate in the LC60 cells suggesting that SuSA is involved in
the cleavage of sucrose in vivo . The wild-type strain, on the other hand, showed no accumulation of
sucrose but showed high invertase activity when compared with the cells grown in presence of nitrate
than in presence of ammonium. Both the mutants performed equally well in nitrate and ammonium
supplemented cultures but in nitrate-free medium LC60 grew poorly despite the differentiation of
heterocysts. Thus a role for SuSA has been ascribed in the diazotrophic metabolism of Anabaena
sp. strain PCC 7119. Curatti et al . (2006) showed that Anabaena sp. strain PCC 7120 possesses susA
sequence besides the susB sequence reported by them earlier (Curatti et al ., 2002). SuSA contributes
to the major extractable SuS activity. Both susA and susB promoter sequences of Anabaena sp. strain
PCC 7120 revealed sequences similar to those present in rbcLS promoter sequences signifying their