Genomics of Salt Acclimation: Synthesis of Compatible Solutes among Cyanobacteria - Advances in Botanical Research - page 39

The involvement of Sps in salt-induced sucrose synthesis has been
verified for the cyanobacterial strains Anabaena sp. PCC 7119 ( Porchia &
Salerno, 1996 ), Nostoc 7120 ( Cumino, Curatti, Giarrocco, & Salerno, 2002 ),
Synechocystis 6803 ( Curatti et al., 1998 ; Hagemann & Marin, 1999 ), and
Synechococcus sp. PCC 7002 ( Cumino, Perez-Cenci, Giarrocco, & Salerno,
2010 ). These studies revealed that Sps is the sucrose synthesis enzyme among
cyanobacteria because purified native or recombinant enzymes showed
sucrose synthesis activities, the corresponding transcripts and proteins are
accumulated under salt-stress conditions, and spsA mutants lost the ability
to accumulate sucrose. The cyanobacterial Sps seems to use UDP-glucose
rather than ADP-glucose as glucosyl donor, because mutation of the ADP-
glucose synthesis enzyme abolished glycogen and GG synthesis but not
sucrose accumulation in Synechocystis 6803 ( Miao, Wu, Wu, & Zhao, 2003 ).
Searching the presently available cyanobacterial genomes (April 2012)
revealed that spsA gene can be found in many but not in all cyanobacterial
strains ( Table 2.1 ). A closer look shows that at least two different subclasses
of Sps proteins exist. First, the Sps protein from Synechocystis 6803, which
represents a biochemical characterized protein with a biochemically active
Sps domain and a biochemically inactive Spp domain ( Lunn et al., 1999 ),
was used in BLAST searches ( Altschul et al., 1997 ). These searches indicated
that beside Synechocystis 6803, only a few other beta-cyanobacteria harbour
proteins of high similarity, while all alpha-cyanobacteria encode those Sps
proteins in their genomes ( Table 2.1 ). During the analysis of the genetic and
biochemical basis for salt-induced sucrose accumulation of heterocystous
strains, two genes were identified coding for Sps proteins, which are shorter
than the Sps from Synechocystis 6803 because the Spp domain is missing
( Cumino et al., 2002 ). Moreover, these proteins showed also lower similari-
ties to SpsA from Synechocystis 6803 than sucrose synthases from filamentous
strains. This second type of Sps is marked by * in Table 2.1 . Additional to
the Anabaena/Nostoc strains, these Sps* proteins are frequently found in
genomes of other beta-cyanobacteria, mostly in two copies. There are only
a few strains, e.g. Nodularia spumigena CCY9414, harbouring genes for the
two Sps types in one genome. Sps* protein-coding genes seem not to exist
in alpha-cyanobacterial genomes.
Interestingly, there are a few cyanobacterial strains with completely
known genomes, e.g. Microcystis aeruginosa NIES-843 or Crocosphaera watsonii
WH 8501, which are virtually free from any Sps-coding genes. For the
latter strains, we showed recently that sucrose is indeed not accumulated
under saline conditions ( Pade, Compaoré, Klähn, Stal, & Hagemann, 2012 ).
However, whether or not those strains are completely unable to make