HETEROCYST - Cyanobacteria - page 244

ammonium-upregulated nature. Further, it was shown that the expression of susA and rbcLS is
developmentally restricted to the vegetative cells under nitrogen-fi xing conditions showing there
by that nitrogen fi xation, carbon fi xation and sucrose metabolism are under the same regulatory net
work (Curatti et al ., 2006). The probable involvement of susA of Anabaena sp. strain PCC 7119 in the
synthesis of polysaccharides from sucrose has been investigated by Curatti et al . (2008). Nitrogen
and carbon sources and light affected the expression of susA gene as they affected the accumulation
of glycogen. A comparison of the wild-type and a susA disruptant mutant revealed that the latter
accumulated lesser amounts of glycogen under nitrogen-fi xing conditions. A strain of Anabaena sp.
strain PCC 7119 constitutively expressing susA showed higher levels of glycogen. Based on these
observations they concluded that SuSA plays an important role in recognizing environmental and
nutritional signals and modulate the expression of susA gene.
Porchia and Salerno (1996) reported for the fi rst time the activities of SPS and SPP in Anabaena
sp. strain PCC 7119. Two species of SPSs, SPS-I and SPS-II have been separated, purifi ed and their
properties studied. These enzymes considerably differed in their properties from those of the higher
plants in having lower molecular mass, broad substrate specifi city, requirement of Mg 2+ and Mn 2+
and in being monomeric in composition. Both SPS-I and SPS-II can utilize fructose-6-phosphate as
their substrate besides UDP-glucose, while the former additionally accepts GDP-glucose and TDP-
glucose as substrates the latter can accept ADP-glucose. Curatti et al . (1998) identifi ed the gene spsA
encoding SPS of Synechocystis sp. strain PCC 6803, a non-nitrogen fi xing unicellular form. Cumino et
al . (2001) compared the biochemical properties of the native SPP from Anabaena sp. strain PCC 7120
and the recombinant SPP produced in E . coli and these were shown to be identical. SPP of Anabaena
sp. strain PCC 7120 differed with the corresponding enzymes from higher plants in its C-terminal
sequence, molecular mass and subunit composition. Sucrose synthesis in the heterocysts of Anabaena
sp. strain PCC 7120 and its probable role in nitrogen fi xation have been elucidated by Cumino et al .
(2007). Their observations confi rmed the operation of a cycle of sucrose synthesis and degradation
and its linking to glycogen accumulation during nitrogen fi xation. The basis of their conclusions are
that (i) sucrose synthesis occurs in light as well as in dark incubated cultures; (ii) total activities of
SPSs and SPPs are higher in nitrogen-fi xing conditions rather than in presence of combined nitrogen
sources; (iii) the increase in the levels of transcripts for spsA and spsB corresponded with increase in
the levels of SPS-A and SPS-B proteins; (iv) the expression of spsA in the vegetative cells and spsB
and sppA in the heterocysts and vegetative cells contributed to sucrose synthesis; (v) the preferred
glucosyl donor for SPS-A is UDP-glucose and for SPS-B it is either UDP-glucose or ADP-glucose; (vi)
transcriptional fusions of optimized version of gfp-mut2 with the promoters of spsA , spsB and sppA
upstream of their tsps at 950 bp, 977 bp and 559 bp respectively, showed the fl uorescence of GFP
in vegetative cells (P - spsA-gfp ) and vegetative cells and heterocysts (P - spsB-gfp and P - sppA-gfp ); (vii) two tsps
at -54 bp and -93 bp have been observed for spsB resulting in the formation of RNA I and RNA II in
nitrate-free, nitrate and ammonium grown cultures but RNA II transcripts were 4-fold more abundant
than that of RNA I in nitrogen-fi xing cultures; (viii) the consensus NtcA-binding site GTA(N 8 )ACA
was found at -39 bp with respect to tspII at -93 bp. The regulation of sucrose metabolism genes
by NtcA has been examined by Marcozzi et al . (2009) who showed that induction of spsB (sucrose
synthesis) gene after nitrogen step-down in Anabaena sp. strain PCC 7120 was abolished in an ntcA -
disruptant mutant with an increase in the activity of SuSA (sucrose cleavage) activity. Both spsB
and susA promoter regions showed interaction with NtcA in vitro at the consensus NtcA-binding
sequences. These observations confi rm that sucrose metabolism and nitrogen metabolism are
transcriptionally under the control of NtcA. Fieulaine et al . (2005) reported the crystal structure of
SPP from Synechocystis sp. strain PCC 6803 that revealed the presence of a core domain consisting