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possess additional genes encoding proteins that may play similar functions,
despite lacking the typical motifs generally found in PCP, as it was unveiled
by the synteny analysis performed here. The differences anticipated for the
mechanisms of EPS assembly and export in cyanobacteria may be related to
its unique cellular envelope that combines features of both Gram-negative
and Gram-positive bacteria (
Hoiczyk & Hansel, 2000
), and/or to the com-
plexity of the polysaccharidic polymers produced. Concerning the
opx
, the
selective forces that acted upon them were different, favouring, in general,
the loss of the
opxB
and the duplication of
opxA
. Interestingly, despite the
lower number of
pcp
and
opx
copies present on the smaller cyanobacte-
rial genomes, including those of
Prochlorococcus
spp.,
Synechococcus
and
Ther-
mosynechococcus
, no straightforward correlation between genome size and
copy number is observed. The number of gene copies rather seems to be
correlated with the morphological group, with the heterocystous strains
possessing more
pcp
genes, whereas
opx
are generally more abundant in
the filamentous cyanobacteria. Despite this general trend,
Gloeobacter
and
Lyngbya
possess a high number of homologues of both genes, and thus, it
is probable that during evolution, these two strains were exposed to par-
ticular environmental conditions that selected the maintenance of this large
number. No direct correlation was also observed between the number/
phylogenetic relationships of
opx
and
pcp
genes with the strains' habitat and
diazotrophic capacity. For instance, the same number of
pcp
homologues was
found in the thermophilic
Thermosynechococcus
, the freshwater
Microcystis
,
and the marine
Cyanothece
sp. CCY 0110. Similarly, the same number of
opx
was present in the non-N
2
fixing
Microcystis
and the diazotrophic
N. punc-
tiforme
. Furthermore, no correlation was found with the ability to produce
efficiently EPS. This outcome is not unexpected, since most cyanobacteria
produce some type of EPS in the form of a well-defined sheath, a thick
capsule, mucilage or released polysaccharides (RPS), but it remains to be
known whether all of these structures are assembled following the same
pathway.
Synechococcus elongatus
constitutes a fascinating case since, although
it lacks identifiable PCP and OPX proteins and never forms a well-defined
sheath, produces mucilage (
Castenholz, 2001
).
As more genome sequences and detailed genome annotations become
available, more information can be used to complement these findings. The
work presented here contributes to a better understanding of the evolution
of the EPS assembly and export machinery in cyanobacteria, pinpointing
specific evolutionary events and probable functions. However, there is still
a long way to go in order to have a robust reconstruction of the evolution
